Since I returned to the air at the end of December 2012 I have been using QRP with a newly-purchased and assembled KX3. My antenna is not a real antenna, just an L-network tuner connected to the aluminum eaves trough (you might call it a rain gutter) on my 2-story house. The neighbours might notice a wire running the 1 meter from around the upper-floor window casement to the eaves trough, if they look very very carefully. This is a stealth antenna on steroids.
With my primary interest in DX it was only natural that I'd try to work some despite knowing that it would be difficult. Doable but difficult. Apart from one VE and several W/K stations all my contacts have been DX.
As I continued my casual operating with this minimalist station the countries slowly added up. As the total grew I began to wonder if I could reach 100, the threshold for DXCC. I was not obsessed by the idea, only curious if I could do it, at least before the warm weather arrived and I could proceed with building a real antenna. By "real" I mean an arrangement of conductors designed to effectively work as an antenna rather than a random conductor co-opted and coerced into behaving as one.
I am happy (and exhausted) to report that I have done it. Last week I logged country number 100 -- OA -- and so I have tentatively qualified for DXCC, assuming I actually go ahead and make an application using these criteria (NB: my actual total is over 300), which is not in my plans. It's enough to know I've done it. This goes to show that with a little perseverance, and luck, just how attainable DXCC can be. If I can do it in 4 months with QRP and a no-antenna antenna pretty much anyone can do so.
My most productive band was 20, closely followed by 30 and 15. Both 10 and 40 were well behind. The main reasons 10 was poor were the lower availability of openings and the need for a really low radiation angle. I also suspect, like many tuners, my little MFJ L-network is consuming most of the power, and on 10 meters that was usually 3 watts. Any more and the KX3 folded back due to the excessive RF getting into all the cables. It's due to pretty much having my temporary shack inside the antenna.During periods when conditions were poor (geomagnetic disturbance) I could work almost nothing.
Apart from the early days, the countries did not come easy. There were ample "VE3?" queries from some rare and not-so-rare DX that never did make it into the log. There were a few good catches, including: J5, A7, XT, RI (South Shetland). There are many more common ones I did not log, and these included KL7, ZL and JA. Then there are the frustratingly loud ones that never hear me, such as D3, 5N, and 5U. I did once get a "VE3?" from the D3. Most DXpeditions were out of the question since only rarely can I get through when even so much as one other station is calling. Pile-ups are a no-go. Yet I did occasionally try, sometimes for the practice and sometimes hoping to get lucky.
All 100 contacts were on CW. I did make a few SSB DX contacts. The wider bandwidth (and therefore 8 to 10 db lower SNR) was not friendly to my puny signal.
With this small victory I can proceed with a lighter heart to construct a proper station, including a room dedicated to the shack and actual antennas. My real objective in this DXCC pursuit had been whether the rekindling of my interest in hamming would last. It has.
How speedily station construction will go I am not sure since like for many hams the radio is more of a winter pursuit. Summer is for antenna raising and maintenance, but mostly for non-radio activities. As things stand, the new shack is drywalled and ready for painting. I have enough material on hand to put up simple masts and antennas. If I can pick up a few tower sections to ease construction and maintenance of suitable supports I will do so. One way or another I plan to be fully equipped by the fall.
I may stick with QRP for a while yet. At least it won't be so painful with a real antenna.
Sunday, April 28, 2013
Tuesday, April 23, 2013
Basic Site Planning
I have slowed my antenna design plans a little, which is why I've had less to say on the topic recently. The reason is that I am somewhat constrained with what I will use to mount the antenna. Well, not only that but also a variety of related tasks such as building my new shack and dealing with non-radio matters.
The shack-building has gone slow because of the warming weather. I'd rather be outdoors than mucking with drywall in the basement. However I must finish that space since I need to move out of the spare upstairs bedroom. I also have no intention of broaching the walls on the second floor to bring in coax. On the plus side the basement area I'm using was designed for a shack from the start, including a couple of 240 VAC outlets for the amplifiers I no longer have nor intend to buy.
Getting back to mounted antennas at some height, my plans have changed. I was never comfortable with the idea of using the roof as a support -- although it is a good spot -- because it's simply too visible (see picture in this post). There might also be some friction with the city for much the same reason. The policy on this matter was new to me since I've been out of the loop for so many years.
Since I also have no intention of installing a tower (at least not for the foreseeable future) my options are more constrained. The diagram below shows my major options.
The drawing is not to scale or accurate (my house has a more complicated layout). It is only intended to show relevant structures in approximate positions. The direction we're looking is west.
Option A is rejected, as I've said. In addition to the visibility issue, especially if I put up delta loops, there is a lesser concern with guying the mast. I do have a plan for that which would suffice nicely for a fibreglass mast plus wire loop, since the wind load would be low. However a yagi is out of the question! I am more concerned that the structure would appear fragile to others even if it is in fact very robust.
Option B is my second choice for where to mount the antennas for 20, 15 and 10 (and maybe 17) meters. This would be either a mast or short "TV" type tower secured to the house below the roof line and anchored to the ground without using any concrete. The antennas themselves would stand clear of the roof although the bottom of any delta loop would be below the roof apex. The eaves are about 6 meters above grade and the roof apex is about 8 meters. The roof pitch is not nearly as steep as shown. The aluminum eaves trough that runs along the south roof line is my present antenna.
There are important considerations when it comes to attached a mast or tower to a building. I do know how I would go about it but will not get into that here. It would make a good subject for a future post. One advantage I have is that my house was custom built so I not only know the structural details, I also have many photographs of the raw frame that I took during construction, including the exact spots where the brackets would be placed.
The base and bracket position has been selected so that the mast/tower does not block any windows (important!). The base will be close to the basement shack. I have a spot selected to open the wall to install a conduit for the coax cables that is convenient, secure and out of the way of accidental encounters.
Options C and D are for masts, or short towers plus masts for 40 and 30 meters. Since I concluded earlier that I am favouring delta loops which come close to the ground (~3 meters up) their supports must be placed in the away from the house and deck. My property is deep (200') but relatively narrow (50'). Fortunately it is long in the north-south direction (350°-170°) so it is easy enough to position a loop broadside to Europe. I did this very thing in the 1980s.
What has changed is the vegetation. I drew the two trees nearest to the C and D to their approximately correct heights. Both were much shorter in 1992. Even though both were decapitated in the 1998 ice storm they have since grown well past those heights. Neither is robust enough to support an antenna but the lower trunks are ideal supports for guy lines. They are each close to the property edges (east and west sides) and almost exactly opposite each other. The space in between is a septic field, which is not the place for a tower base. My old tower was close to D so even the concrete had to be broken out of the bedrock to meet DOE regulations for the new septic system.
Option C would place the north end of the delta loops somewhat close to the house but still well out of the reach of eager fingers. The guy line for the south end of the loop would go a tree or something else farther to the south.
Option D places the loops further away from the house, which is desirable. The problem is that there are two other, larger trees to the south of D (not shown in the diagram). Their trunks are just outside my property lines and they branch out over my property so that they nearly touch at a height of about 10 meters. The squirrels love the arrangement (which didn't exist 20 years ago), using it to practice their acrobatics as they bravely jump the space from one flimsy outer branch to another. They almost always make it.
For a ham these trees are a problem. They are large and old, regularly shedding debris that would surely impact any delta loop or inverted vee that dared to traverse that space. There is also a real risk that one of the trees will come down in future years. Option D is out.
So that's the tentative plan: high band antennas at B and low band antennas at C. The choice for high band antennas is still in flux for a variety of reasons. I'll get to those reasons in a coming post. Regardless of how it goes I will make sure to write about the 20-15-10 delta loop array I've designed. I just don't yet know if I'll build it.
The shack-building has gone slow because of the warming weather. I'd rather be outdoors than mucking with drywall in the basement. However I must finish that space since I need to move out of the spare upstairs bedroom. I also have no intention of broaching the walls on the second floor to bring in coax. On the plus side the basement area I'm using was designed for a shack from the start, including a couple of 240 VAC outlets for the amplifiers I no longer have nor intend to buy.
Getting back to mounted antennas at some height, my plans have changed. I was never comfortable with the idea of using the roof as a support -- although it is a good spot -- because it's simply too visible (see picture in this post). There might also be some friction with the city for much the same reason. The policy on this matter was new to me since I've been out of the loop for so many years.
Since I also have no intention of installing a tower (at least not for the foreseeable future) my options are more constrained. The diagram below shows my major options.
The drawing is not to scale or accurate (my house has a more complicated layout). It is only intended to show relevant structures in approximate positions. The direction we're looking is west.
Option A is rejected, as I've said. In addition to the visibility issue, especially if I put up delta loops, there is a lesser concern with guying the mast. I do have a plan for that which would suffice nicely for a fibreglass mast plus wire loop, since the wind load would be low. However a yagi is out of the question! I am more concerned that the structure would appear fragile to others even if it is in fact very robust.
Option B is my second choice for where to mount the antennas for 20, 15 and 10 (and maybe 17) meters. This would be either a mast or short "TV" type tower secured to the house below the roof line and anchored to the ground without using any concrete. The antennas themselves would stand clear of the roof although the bottom of any delta loop would be below the roof apex. The eaves are about 6 meters above grade and the roof apex is about 8 meters. The roof pitch is not nearly as steep as shown. The aluminum eaves trough that runs along the south roof line is my present antenna.
There are important considerations when it comes to attached a mast or tower to a building. I do know how I would go about it but will not get into that here. It would make a good subject for a future post. One advantage I have is that my house was custom built so I not only know the structural details, I also have many photographs of the raw frame that I took during construction, including the exact spots where the brackets would be placed.
The base and bracket position has been selected so that the mast/tower does not block any windows (important!). The base will be close to the basement shack. I have a spot selected to open the wall to install a conduit for the coax cables that is convenient, secure and out of the way of accidental encounters.
Options C and D are for masts, or short towers plus masts for 40 and 30 meters. Since I concluded earlier that I am favouring delta loops which come close to the ground (~3 meters up) their supports must be placed in the away from the house and deck. My property is deep (200') but relatively narrow (50'). Fortunately it is long in the north-south direction (350°-170°) so it is easy enough to position a loop broadside to Europe. I did this very thing in the 1980s.
What has changed is the vegetation. I drew the two trees nearest to the C and D to their approximately correct heights. Both were much shorter in 1992. Even though both were decapitated in the 1998 ice storm they have since grown well past those heights. Neither is robust enough to support an antenna but the lower trunks are ideal supports for guy lines. They are each close to the property edges (east and west sides) and almost exactly opposite each other. The space in between is a septic field, which is not the place for a tower base. My old tower was close to D so even the concrete had to be broken out of the bedrock to meet DOE regulations for the new septic system.
Option C would place the north end of the delta loops somewhat close to the house but still well out of the reach of eager fingers. The guy line for the south end of the loop would go a tree or something else farther to the south.
Option D places the loops further away from the house, which is desirable. The problem is that there are two other, larger trees to the south of D (not shown in the diagram). Their trunks are just outside my property lines and they branch out over my property so that they nearly touch at a height of about 10 meters. The squirrels love the arrangement (which didn't exist 20 years ago), using it to practice their acrobatics as they bravely jump the space from one flimsy outer branch to another. They almost always make it.
For a ham these trees are a problem. They are large and old, regularly shedding debris that would surely impact any delta loop or inverted vee that dared to traverse that space. There is also a real risk that one of the trees will come down in future years. Option D is out.
So that's the tentative plan: high band antennas at B and low band antennas at C. The choice for high band antennas is still in flux for a variety of reasons. I'll get to those reasons in a coming post. Regardless of how it goes I will make sure to write about the 20-15-10 delta loop array I've designed. I just don't yet know if I'll build it.
Wednesday, April 17, 2013
Power and Contests
You notice some strange things when you have a puny station as It is not easy to be heard with 10 watts and an eaves trough as a poor excuse for an antenna . Even hearing DX is often a challenge on the high bands.
There are two edge cases I've noticed with this station. One is the DX contest stations that are practically beacons on the bands, day in and day out, irrespective of contests. While I don't know any particulars, many seem to rotate operators into this contest mode of operation as a form of training. They sit on a frequency calling CQ and working any and all callers for hours on end. These are undoubtedly superior stations with excellent antenna systems and high power (QRO).
The antenna systems are easy to guess at since these stations are getting through when others in their areas are far weaker on this end of the path. While I can be less sure of the power they are running it is surely a long, long way from QRP. A strong bit of evidence is that even when quite strong here most of these stations do not hear me when I call. Antennas are reciprocal but power is not.
The second case is the weak DX station. My gut reaction is to simply pass them by, reasoning that if I can barely hear them they will surely never hear me. I've proved this many times by tossing in a call and hearing them simply resend their CQ. Not always: sometimes they do hear me and I log a new one. A recent surprise of this sort was working a ZA last week on 20 meters.
It is likely that the ones that do hear me are either running QRP like me, resulting in reciprocity of results, or they have a lower level of noise than I do here. You simply never know. It costs little to make one call and if the weak DX doesn't come back to me or anyone else to just keep spinning the dial. Sometimes I try a few tricks before giving up.
Which brings me back to power and its purpose. Those stations with the big antennas and perhaps even bigger power may seem a bit overindulgent. It is reasonable to ask whether the power is needed when the antennas are so good. Let's also be honest and recognize that not only are a number of these stations running high power, they may be running just a wee bit over the legal limit in their countries.
For the casual operator or even serious DXer with a fantastic antenna farm it is arguably excessive to overdo the power, even if it is kept legal. Breaking through the pile-up for a new one is a proper use of power, but not so much at other times. Every ham makes a judgment call on what suits them. There is no one right answer.
Contests are a different matter. Power has a purpose that has nothing whatsoever to do with pile-up busting. What would seem excessive in other circumstances becomes justifiable in a contest.
Contest stations in the high-power category want to use QRO to maximum advantage. You cannot do this by search-and-pounce (aka hunt-and-peck) as commonly practiced by smaller stations. Sure, you'll work each QSO faster (higher rate) with power but the strategy of search-and-pounce is unproductive other than to add new multipliers
To consistently get big contact totals you must have others call you; that is, create long runs. To run stations you will be the one sitting on a frequency and calling CQ. Others find you and respond. Your success is determined by several factors:
This behaviour is similar to the factory trawlers with their giant nets sweeping the ocean currents. They will pick up a lot of unwanted fish (the by-catch). They will readily admit this is unfortunate but will also see it as justifiable since they are getting more of what they want.
I just shrug and move on when they don't hear me. There is no reason to feel offended.
There are two edge cases I've noticed with this station. One is the DX contest stations that are practically beacons on the bands, day in and day out, irrespective of contests. While I don't know any particulars, many seem to rotate operators into this contest mode of operation as a form of training. They sit on a frequency calling CQ and working any and all callers for hours on end. These are undoubtedly superior stations with excellent antenna systems and high power (QRO).
The antenna systems are easy to guess at since these stations are getting through when others in their areas are far weaker on this end of the path. While I can be less sure of the power they are running it is surely a long, long way from QRP. A strong bit of evidence is that even when quite strong here most of these stations do not hear me when I call. Antennas are reciprocal but power is not.
The second case is the weak DX station. My gut reaction is to simply pass them by, reasoning that if I can barely hear them they will surely never hear me. I've proved this many times by tossing in a call and hearing them simply resend their CQ. Not always: sometimes they do hear me and I log a new one. A recent surprise of this sort was working a ZA last week on 20 meters.
It is likely that the ones that do hear me are either running QRP like me, resulting in reciprocity of results, or they have a lower level of noise than I do here. You simply never know. It costs little to make one call and if the weak DX doesn't come back to me or anyone else to just keep spinning the dial. Sometimes I try a few tricks before giving up.
Which brings me back to power and its purpose. Those stations with the big antennas and perhaps even bigger power may seem a bit overindulgent. It is reasonable to ask whether the power is needed when the antennas are so good. Let's also be honest and recognize that not only are a number of these stations running high power, they may be running just a wee bit over the legal limit in their countries.
For the casual operator or even serious DXer with a fantastic antenna farm it is arguably excessive to overdo the power, even if it is kept legal. Breaking through the pile-up for a new one is a proper use of power, but not so much at other times. Every ham makes a judgment call on what suits them. There is no one right answer.
Contests are a different matter. Power has a purpose that has nothing whatsoever to do with pile-up busting. What would seem excessive in other circumstances becomes justifiable in a contest.
Contest stations in the high-power category want to use QRO to maximum advantage. You cannot do this by search-and-pounce (aka hunt-and-peck) as commonly practiced by smaller stations. Sure, you'll work each QSO faster (higher rate) with power but the strategy of search-and-pounce is unproductive other than to add new multipliers
To consistently get big contact totals you must have others call you; that is, create long runs. To run stations you will be the one sitting on a frequency and calling CQ. Others find you and respond. Your success is determined by several factors:
- The higher your power for a given antenna more stations will hear you.
- Big signals are more attractive than small ones. If you don't believe it, try it. Tune the band. If you're like most hams you will be attracted to calling the louder stations. It just seems so much easier than digging for the weak ones. Many a rare DX station with a small signal gets few callers.
- Create elbow room. This one is more controversial. Bands are crowded during a contest and stations are continually jockeying for space. Stations that also want to start a run have to squeeze in where there simply isn't any room. Conflict is inevitable. No one wants to sidle up adjacent to a mega-station since they will not be heard or will be overlooked (see previous point). Power helps the contester hold a frequency, and thus makes it easier to run up the contacts.
This behaviour is similar to the factory trawlers with their giant nets sweeping the ocean currents. They will pick up a lot of unwanted fish (the by-catch). They will readily admit this is unfortunate but will also see it as justifiable since they are getting more of what they want.
I just shrug and move on when they don't hear me. There is no reason to feel offended.
Saturday, April 13, 2013
The Multibander Within
When any antenna is shortened by means of loading elements it tends to become a purely single band antenna. This should not be surprising since the loading elements are specifically tuned to the band of interest, and not for any of the harmonically-related bands. For example, a full-length 40 meters dipole (vertical or horizontal) would be useful at 15 meters with modest tuning.
If we go back to the 40 meter linear-loaded vertical I recently looked at you will find that is truly single band. The following SWR scan covering 40 through 10 meters makes that fact clear.The antenna is mounted 2 meters above average ground.
However that isn't the end of the story. Any conductor can be tuned with a suitable matching network between the transmission line and the feed point; that is, transformed from one impedance to another. We can of course do this in the shack provided that the additional transmission line loss due to high SWR is within reason. From the chart above it should be evident that the SWR at other bands is not within reason.
A matching network at the feed point adds complexity, cost and (most emphatically) loss, but it can be done. However it isn't always exceptionally difficult. The present situation is an interesting case study in that regard. The reason is that the antenna is 13 meters tall, which is ~43 feet. Many hams have heard that a vertical of this height is easy to match on multiple bands with a simple network. It is also a subject of controversy for reasons that only some hams can see.
This linear-loaded antenna isn't quite a simple 43-footer since it has already been loaded to resonate on 40 meters. Even so it does show evidence of other resonances, resonances which are not especially evident from the above SWR chart. The actual R and X values do indicate some promise, though you'd have to plot them on a Smith chart to see that.
I "eyeballed" the values and decided to, metaphorically speaking, roll the dice. In EZNEC I connected a λ/2 length (calculated at 7.1 MHz) of open-wire transmission line. This is equivalent to making a full circle around the Smith chart, bringing the impedance right back to what it is at the feed point, though only on 40 meters (but is exact only for zero-loss line). At every other frequency there will be a net reactance that will transform the impedance.
After only a few minutes of fiddling with values in EZNEC I settled on 300Ω open-wire line, and I fed the antenna near the bottom rather than at the centre. The shift downward changes the values of E and I (and phase) at the feed point, which shifts the impedance (Z = E / I). Here is the resulting SWR chart from 7 to 30 MHz.
Notice how we've made visible some otherwise hidden easy matches. This was done no more sophisticated a matching network than a hunk of open-wire line. This is a bit like that trick in old detective movies where the hero lightly rubs a pencil lead over the criminal's notepad to highlight the depressions, and the message, from the previously removed top sheet.
Some of the SWR dips are useful, such as on 30, 17 and 12 meters, but not all. That isn't too bad for a roll of the dice, and it shows the matching potential of the 43-foot vertical. However, please keep in mind this is only a thought experiment not a serious antenna design.
The pattern on a multi-band antenna is always a challenge. The pattern of any antenna with an electrical length much longer than λ/2 is never simple, and can get quirky, with lobes and nulls in various directions, as you go ever higher in frequency.
For an especially bad example, let's look at how this antenna performs on 17 meters. With our "magical" feed line addition there is an attractive resonance near 18.1 MHz that draws our interest.
Ground losses are high at -9 db, which is considerably worse than on 40 meters; the pattern is asymmetric; and, there is also substantial radiation at higher angles, which does not suit my focus on DX. These figures become clear when we have EZNEC show us the currents on this antenna. Notice the position of the current maxima and minima: the bulk of the antenna's radiation comes from the bottom loading wire. This does not help us get a decent pattern.
So, again, don't be deceived by low SWR. The pattern matters. The antenna will certainly work, but will fare worse than many others.
If we go back to the 40 meter linear-loaded vertical I recently looked at you will find that is truly single band. The following SWR scan covering 40 through 10 meters makes that fact clear.The antenna is mounted 2 meters above average ground.
A matching network at the feed point adds complexity, cost and (most emphatically) loss, but it can be done. However it isn't always exceptionally difficult. The present situation is an interesting case study in that regard. The reason is that the antenna is 13 meters tall, which is ~43 feet. Many hams have heard that a vertical of this height is easy to match on multiple bands with a simple network. It is also a subject of controversy for reasons that only some hams can see.
This linear-loaded antenna isn't quite a simple 43-footer since it has already been loaded to resonate on 40 meters. Even so it does show evidence of other resonances, resonances which are not especially evident from the above SWR chart. The actual R and X values do indicate some promise, though you'd have to plot them on a Smith chart to see that.
I "eyeballed" the values and decided to, metaphorically speaking, roll the dice. In EZNEC I connected a λ/2 length (calculated at 7.1 MHz) of open-wire transmission line. This is equivalent to making a full circle around the Smith chart, bringing the impedance right back to what it is at the feed point, though only on 40 meters (but is exact only for zero-loss line). At every other frequency there will be a net reactance that will transform the impedance.
After only a few minutes of fiddling with values in EZNEC I settled on 300Ω open-wire line, and I fed the antenna near the bottom rather than at the centre. The shift downward changes the values of E and I (and phase) at the feed point, which shifts the impedance (Z = E / I). Here is the resulting SWR chart from 7 to 30 MHz.
Some of the SWR dips are useful, such as on 30, 17 and 12 meters, but not all. That isn't too bad for a roll of the dice, and it shows the matching potential of the 43-foot vertical. However, please keep in mind this is only a thought experiment not a serious antenna design.
- The feed line introduces loss, including on 40 meters. The SWR is high on that section of open-wire line. In most cases the loss is modest, but first make sure of it.
- Even without shifting the feed point the antenna is not electrically symmetrical on bands above 40 meters. That means you will have a challenge, perhaps an impossible one, keeping the open-wire line from becoming a part of the antenna (i.e. radiating).
- A λ/2 of open-wire line at 40 meters is close to 20 meters long. In most stations that is a problem since the entire run isn't that long. It also needs to be mounted carefully to avoid a variety of problems.
The pattern on a multi-band antenna is always a challenge. The pattern of any antenna with an electrical length much longer than λ/2 is never simple, and can get quirky, with lobes and nulls in various directions, as you go ever higher in frequency.
For an especially bad example, let's look at how this antenna performs on 17 meters. With our "magical" feed line addition there is an attractive resonance near 18.1 MHz that draws our interest.
Ground losses are high at -9 db, which is considerably worse than on 40 meters; the pattern is asymmetric; and, there is also substantial radiation at higher angles, which does not suit my focus on DX. These figures become clear when we have EZNEC show us the currents on this antenna. Notice the position of the current maxima and minima: the bulk of the antenna's radiation comes from the bottom loading wire. This does not help us get a decent pattern.
So, again, don't be deceived by low SWR. The pattern matters. The antenna will certainly work, but will fare worse than many others.
Sunday, April 7, 2013
"VE3?"
DXing with QRP to a tuner-fed aluminum eaves trough isn't easy. My country count plateaued at 90 countries when the HF conditions turned poor over the past little while. Mind you I am counting from late December when I reestablished a station, not all time with this call, which is well over 300 countries.
I am interested in seeing whether I can work 100 countries with this pathetic station before I finally am able to erect some real antennas. It's certainly possible, but it's been getting difficult as my total rises. In the last 24 hours I had a run of luck working 3 new ones, including two in central Asia. Over-the-pole conditions on 20 meters have been good of late. So now I'm at 93 worked, with many new band-countries.
When I say it isn't easy, I mean it is really difficult. Sometimes I get lucky and some weak and rare (to me) DX station comes back on the first call. Most often I get silence in response: I am just not heard.
In many of the in-between cases I am heard but not at all well. The most common response I get, when I get one at all, is "VE3?". Occasionally this converts into a good QSO though more frequently the other station gives up in frustration. I understand; I, too, get frustrated.
It reminds me of those long graveyard shifts trying to make contest contacts at a multi-single or multi-multi operation on 40 meters SSB. With a beam and a kilowatt there were many that heard me though far fewer were copyable on my end. Hour after long hour I'd repeatedly struggle to pull another call out of the noise, sometimes successfully and sometimes not. The log would fill slowly, too slowly, while waiting for sunrise and the high bands to open. Others would get that thrill as I trundled off for a few hours of sleep. In 30 minutes that lucky operator would surpass my hard-won contact totals.
I could claim this is "payback" time where I get to be the one with the weak signal trying to work the bigger stations, but the reality is that it is no fun at either end of the QSO.
Some operators give up quite easily and will log the first or second guess at my call. I have gone into DX logs as VE3UN, VE7VN, VE3STN (try this on CW and you'll understand), XE3VN, VA3VN, among others. Of course these are all inferior outcomes to being VE3?, since it at least means the DX operator is still trying to get it right.
Alternatively I should perhaps explore getting a special suffix. After all, special prefixes have become routine. Just listen to what was on the bands during the recent WPX contest. How much of a stretch is it to try for a special suffix. I'll bet you've guessed what I'm thinking: my preferred suffix is "?".
Then, when the DX comes back with "VE3?" I can shout back "QSL, QSL!!". This could greatly accelerate my ascension to the DXCC Honor Roll. Or perhaps nothing more than a lot of very confused hams.
I am interested in seeing whether I can work 100 countries with this pathetic station before I finally am able to erect some real antennas. It's certainly possible, but it's been getting difficult as my total rises. In the last 24 hours I had a run of luck working 3 new ones, including two in central Asia. Over-the-pole conditions on 20 meters have been good of late. So now I'm at 93 worked, with many new band-countries.
When I say it isn't easy, I mean it is really difficult. Sometimes I get lucky and some weak and rare (to me) DX station comes back on the first call. Most often I get silence in response: I am just not heard.
In many of the in-between cases I am heard but not at all well. The most common response I get, when I get one at all, is "VE3?". Occasionally this converts into a good QSO though more frequently the other station gives up in frustration. I understand; I, too, get frustrated.
It reminds me of those long graveyard shifts trying to make contest contacts at a multi-single or multi-multi operation on 40 meters SSB. With a beam and a kilowatt there were many that heard me though far fewer were copyable on my end. Hour after long hour I'd repeatedly struggle to pull another call out of the noise, sometimes successfully and sometimes not. The log would fill slowly, too slowly, while waiting for sunrise and the high bands to open. Others would get that thrill as I trundled off for a few hours of sleep. In 30 minutes that lucky operator would surpass my hard-won contact totals.
I could claim this is "payback" time where I get to be the one with the weak signal trying to work the bigger stations, but the reality is that it is no fun at either end of the QSO.
Some operators give up quite easily and will log the first or second guess at my call. I have gone into DX logs as VE3UN, VE7VN, VE3STN (try this on CW and you'll understand), XE3VN, VA3VN, among others. Of course these are all inferior outcomes to being VE3?, since it at least means the DX operator is still trying to get it right.
Alternatively I should perhaps explore getting a special suffix. After all, special prefixes have become routine. Just listen to what was on the bands during the recent WPX contest. How much of a stretch is it to try for a special suffix. I'll bet you've guessed what I'm thinking: my preferred suffix is "?".
Then, when the DX comes back with "VE3?" I can shout back "QSL, QSL!!". This could greatly accelerate my ascension to the DXCC Honor Roll. Or perhaps nothing more than a lot of very confused hams.
Monday, April 1, 2013
Comparing Those Short 40 Meter Verticals
After all of that modelling it is now time to see what it can tell us about antenna choices for 40 meter DXing. First, to recap, here are the antennas I modelled along with links to the posts where each was discussed:
Although we have made the model simple with respect to the environment the reality is never like that. It is nevertheless critical and a common enough problem in many vertical installations. I therefore will use as one figure of merit the height of the average antenna current: higher is better.
Just to finish off these (important) preliminaries I want to summarize what the antennas do have in common:
Now then, after all of that lead up over two weeks you may be expecting to hear me say something profound about which antenna is best, worst, etc. This is difficult, and did not turn out as I expected. This was a learning experience for me, and perhaps for you as well.
If you were following along you might have noticed one glaring fact: the patterns of all these antennas are almost identical! Sure, we got good matches to 50Ω coax, but that was easy and not especially important. What I want is to find the best possible DX performer for 40 meters. Yet in every case the low-angle gain ends up somewhere in the vicinity of 0.25 dbi at 15° elevation.
The performance difference, such as it is, can only be found in modest differences in the height of the average current. Since all antennas are mounted at the same low height this mostly comes about by how high the antenna reaches. For the two "true" verticals the average is at the centre point, which are 9.5 and 8 meters for the linear-loading vertical and Lazy-H, respectively, since each is mounted 3 meters up. Due to its squat construction the quad loop has its current centre up only 6.3 meters. The more complex delta loop has its average somewhere around 10.4 meters up, which is not too different from the verticals.
On the basis of this and its complex construction I won't hesitate to eliminate the quad from consideration. Of course it can be placed higher above ground without exceeding the top height of the other antennas but it then becomes more unwieldy and fragile.
All this leaves us to ask why these antennas perform so similarly? It turns out the reason is simple enough: the low angle performance is determined by ground, not the antenna! If you want more gain it is necessary to add more elements to create a vertical array. That is not what I want to do, though many hams have followed this route to great success.
What if we raise the vertical higher off the ground? As already mentioned the match will change, but here we will ignore that and focus on the pattern. It turns out you need to add quite a lot of height to get a significant change in the pattern. For illustrative purposes here is the pattern of the Lazy-H raised to 20 meters above ground; its top will be at 33 meters.
The pattern plot is educational. Total losses are cut to -3.6 db, or about 2 db better than when mounted at 3 meters. Unfortunately this helps little since that additional gain goes into higher-angle radiation. That's great is you want to operate a contest like the ARRL Sweepstakes, but helps little with DX. If that's your objective, put up a dipole.
At our target elevation of 15° the gain is actually slightly less than it was before. The lesson here is that you cannot outsmart Mother Earth; if you want better low-angle performance you'll have to move somewhere with better ground. All you've really gained is a clear shot to the horizon, which is helpful but unlikely to justify the construction of a massive tower. If you do have such a tower it will be far better to choose a dipole or yagi.
The antenna would also have to be lengthened since the change in height shifts the resonance upward to 7.175 MHz. We would also have to move the feed point to near the bottom of the vertical element to regain a 50Ω impedance. However these are merely details.
The big factor is ground loss. This is worth attention and so I took the trouble to model the Lazy-H (at its original 3 meters height) over various grounds. While not modelled, the other verticals in this comparison should be similar.
If you use EZNEC I made the results easily reproducible by using the standard set of ground options that come with the application. In the following table the first 2 columns are the ground conductance and dielectric constants, ranging from extremely poor ground at the top to salt water at the bottom. The final 2 columns are the net gain at 15° and total ground loss. The choice of 15° is done to ease comparisons at a typical DX take-off angle, just keep in mind the elevation of maximum gain ranges from about 20° down to 5°.
The differences in gain and loss are substantial across the full range of ground types. However unless you are one of the rare hams sited on a seashore that final row is pretty useless. In my case the second-last row (fresh water) isn't far wrong for a range of useful directions since I am very close to Lac Déschènes, which is a widening of the Ottawa River. I took the following photograph from the peak of my roof with the camera about 9 meters above ground and looking towards Europe (north-east). As you can see the fresh water is a little bit "hard" this time of year.
Apart from the water scenarios the range of gains and losses are quite modest at under 1 S-unit. I would therefore not be inclined to worry too much about ground quality. I do believe it is important to get above obstructions, many examples of which can be seen in the above photograph. Current averages much below the height of the photograph can result in attenuation, perhaps a lot, although I have no easy way to prove it.
Comparing all these short verticals to a full size, vertically-polarized delta loop at the same 3 meters height shows a modest advantage. At 15° there is about 1 db advantage favouring the delta loop, with a comparable average current height of ~10 meters. The gain is -3 db off the ends when compared to the broadside directions.
The final comparison I want to make is to an inverted-vee. This is the only horizontal antenna I will use in this analysis since it has comparable construction requirements. Like the other antennas it is a single, half-wave wire element, and it will be mounted using a similarly sized mast. The legs will be set 90° apart and the apex at 13 meters.
I won't reproduce the patterns of the inverted-vee here since I've shown almost the very same plots previously. In the broadside direction it shows a gain of -0.7 dbi at an elevation of 15°. Despite its low ground loss almost all the radiation goes out at high angles due to its low height, which is not only useless for DX it will increase QRM from domestic stations. I'd rather hear the DX, not the pile-up.
Where does this leave us? The two short verticals options look interesting but not too much. For myself I still favour the full-sized delta loop. This exercise has set my mind at ease about previously-unexplored alternatives.
- Linear-loaded vertical
- Lazy-H vertical
- Linear-loaded quad and delta loops
Although we have made the model simple with respect to the environment the reality is never like that. It is nevertheless critical and a common enough problem in many vertical installations. I therefore will use as one figure of merit the height of the average antenna current: higher is better.
Just to finish off these (important) preliminaries I want to summarize what the antennas do have in common:
- Omnidirectional - Despite asymmetries in all the antennas, they are all effectively omnidirectional. The worst case is a maximum-to-minimum gain differential of 2 to 3 db for the vertically-polarized loops.
- Match sensitivity - Matching and resonant frequency are very sensitive to even small adjustments to antenna height. Environment is also likely to be a significant factor. In all cases it is therefore desirable to make the antennas easy to adjust in place. Some suggestions are in the articles. The loops are hardest to adjust.
Now then, after all of that lead up over two weeks you may be expecting to hear me say something profound about which antenna is best, worst, etc. This is difficult, and did not turn out as I expected. This was a learning experience for me, and perhaps for you as well.
If you were following along you might have noticed one glaring fact: the patterns of all these antennas are almost identical! Sure, we got good matches to 50Ω coax, but that was easy and not especially important. What I want is to find the best possible DX performer for 40 meters. Yet in every case the low-angle gain ends up somewhere in the vicinity of 0.25 dbi at 15° elevation.
The performance difference, such as it is, can only be found in modest differences in the height of the average current. Since all antennas are mounted at the same low height this mostly comes about by how high the antenna reaches. For the two "true" verticals the average is at the centre point, which are 9.5 and 8 meters for the linear-loading vertical and Lazy-H, respectively, since each is mounted 3 meters up. Due to its squat construction the quad loop has its current centre up only 6.3 meters. The more complex delta loop has its average somewhere around 10.4 meters up, which is not too different from the verticals.
On the basis of this and its complex construction I won't hesitate to eliminate the quad from consideration. Of course it can be placed higher above ground without exceeding the top height of the other antennas but it then becomes more unwieldy and fragile.
All this leaves us to ask why these antennas perform so similarly? It turns out the reason is simple enough: the low angle performance is determined by ground, not the antenna! If you want more gain it is necessary to add more elements to create a vertical array. That is not what I want to do, though many hams have followed this route to great success.What if we raise the vertical higher off the ground? As already mentioned the match will change, but here we will ignore that and focus on the pattern. It turns out you need to add quite a lot of height to get a significant change in the pattern. For illustrative purposes here is the pattern of the Lazy-H raised to 20 meters above ground; its top will be at 33 meters.
The pattern plot is educational. Total losses are cut to -3.6 db, or about 2 db better than when mounted at 3 meters. Unfortunately this helps little since that additional gain goes into higher-angle radiation. That's great is you want to operate a contest like the ARRL Sweepstakes, but helps little with DX. If that's your objective, put up a dipole.
At our target elevation of 15° the gain is actually slightly less than it was before. The lesson here is that you cannot outsmart Mother Earth; if you want better low-angle performance you'll have to move somewhere with better ground. All you've really gained is a clear shot to the horizon, which is helpful but unlikely to justify the construction of a massive tower. If you do have such a tower it will be far better to choose a dipole or yagi.
The antenna would also have to be lengthened since the change in height shifts the resonance upward to 7.175 MHz. We would also have to move the feed point to near the bottom of the vertical element to regain a 50Ω impedance. However these are merely details.
The big factor is ground loss. This is worth attention and so I took the trouble to model the Lazy-H (at its original 3 meters height) over various grounds. While not modelled, the other verticals in this comparison should be similar.
If you use EZNEC I made the results easily reproducible by using the standard set of ground options that come with the application. In the following table the first 2 columns are the ground conductance and dielectric constants, ranging from extremely poor ground at the top to salt water at the bottom. The final 2 columns are the net gain at 15° and total ground loss. The choice of 15° is done to ease comparisons at a typical DX take-off angle, just keep in mind the elevation of maximum gain ranges from about 20° down to 5°.
| Ground Conductance | Dielectric Constant | Gain at 15° | Total Loss |
|---|---|---|---|
| 0.001 | 3 | -2.9 dbi | -7.4 db |
| 0.001 | 5 | -2.1 dbi | -6.7 db |
| 0.002 | 10 | -1.1 dbi | -6.0 db |
| 0.002 | 13 | -0.6 dbi | -5.6 db |
| 0.005 | 13 | -0.6 dbi | -5.9 db |
| 0.006 | 13 | -0.6 dbi | -5.8 db |
| 0.0075 | 12 | -0.5 dbi | -5.8 db |
| 0.01 | 14 | 0 dbi | -5.4 db |
| 0.03 | 20 | 1.7 dbi | -4.0 db |
| 0.001 | 80 | 2.3 dbi | -3.3 db |
| 5 | 81 | 5.0 dbi | -0.7 db |
The differences in gain and loss are substantial across the full range of ground types. However unless you are one of the rare hams sited on a seashore that final row is pretty useless. In my case the second-last row (fresh water) isn't far wrong for a range of useful directions since I am very close to Lac Déschènes, which is a widening of the Ottawa River. I took the following photograph from the peak of my roof with the camera about 9 meters above ground and looking towards Europe (north-east). As you can see the fresh water is a little bit "hard" this time of year.
Apart from the water scenarios the range of gains and losses are quite modest at under 1 S-unit. I would therefore not be inclined to worry too much about ground quality. I do believe it is important to get above obstructions, many examples of which can be seen in the above photograph. Current averages much below the height of the photograph can result in attenuation, perhaps a lot, although I have no easy way to prove it.Comparing all these short verticals to a full size, vertically-polarized delta loop at the same 3 meters height shows a modest advantage. At 15° there is about 1 db advantage favouring the delta loop, with a comparable average current height of ~10 meters. The gain is -3 db off the ends when compared to the broadside directions.
The final comparison I want to make is to an inverted-vee. This is the only horizontal antenna I will use in this analysis since it has comparable construction requirements. Like the other antennas it is a single, half-wave wire element, and it will be mounted using a similarly sized mast. The legs will be set 90° apart and the apex at 13 meters.
I won't reproduce the patterns of the inverted-vee here since I've shown almost the very same plots previously. In the broadside direction it shows a gain of -0.7 dbi at an elevation of 15°. Despite its low ground loss almost all the radiation goes out at high angles due to its low height, which is not only useless for DX it will increase QRM from domestic stations. I'd rather hear the DX, not the pile-up.
Where does this leave us? The two short verticals options look interesting but not too much. For myself I still favour the full-sized delta loop. This exercise has set my mind at ease about previously-unexplored alternatives.
Friday, March 29, 2013
Two Small 40 Meter Loops
This is the 3rd in my series of post on short (and low) 40 meter vertical antennas I am exploring for my new station. Even though I might not build any of them, still opting for a full-sized delta loop, the exercise is useful in understanding what is gained or lost when attempting to squeeze a low-band antenna into a smaller space. I previously modeled and explored a linear-loaded vertical dipole and a lazy-H design. Now I am moving on to vertically-polarized shortened "full-wave" loops.
Loops, like dipoles and their variants, can be shortened in a similar
fashion. Since there are no "ends" to a loop the positioning of the
loading elements is a little more interesting. As you will see in the two designs I look at in this post they can be structurally complex, more so than the complexity inherent in any full-wave loop antenna.
As always, for me the antenna must be an acceptable DX performer, which mostly means low loss and low angle radiation. This duo of attributes is difficult to attain in a low-height, low-band antenna since horizontal polarization requires height and vertical polarization undergoes sometimes brutal attenuation at low radiation angles.
The two loop configuration under consideration are the delta loop and the square quad loop. To be a primarily-vertical radiator the delta is fed λ/4 from the apex and square is fed halfway along one side. The EZNEC models show the sources (feed point) as circles and the current distribution on these shortened antennas.
The feed point symmetry is retained despite the size in order to maintain the pattern. The loading elements placement ensures that the two current maxima are in approximately the same relative positions on the scaled-down loops. One unavoidable difference is that the distance between the maxima is reduced. That will affect the pattern.
The model for the quad loop is based on one I found in ON4UN, John Devoldere's book "Low-Band DXing". My 1987 version is quite old so I can't say if it's in newer editions. My delta loop model is similar with regard in connecting the linear loading elements 1/4-circumference either side of the feed point.
As modeled, the circumferences of the quad loop and delta loop are 26.4 meters (0.63λ) and 30.16 meters (0.71λ), respectively, with about 2% of the reduction due to the wire insulation (#12 THHN). Loading of the delta loop is lessened to avoid additional construction complexity. Getting significantly shorter than these figures requires coils, which adds losses I want to avoid. Getting much shorter will also reduce the impedance, probably even below 50Ω.
Both antennas are modeled at a height of 3 meters (at the bottom horizontal wire) to keep it away from prying hands for safety and security. The average current height is about half way up on both, working out to about 6.6 meters and 10.4 meters on the quad and delta, respectively. Their respective heights are 9.6 and 11.7 meters above ground.
Both use insulated copper wire (#12 THHN), except that the top wire of the quad is 25 mm (1 inch) aluminum tubing. This latter specification is required to make the loop rigid, assuming the quad is centred on the mast. Thin nylon rope can be used to tension the interior loading wires so as to keep them in their specified positions.
The model assumes that the support mast is non-conductive, such as a (rope) guyed fibreglass mast. A conductive mast significantly changes these antennas because it is vertical and crosses the loops, especially so because the crossing is at their high-impedance points (low current, high voltage). You can use metal but the antennas would be very different, absolutely requiring precise size and placement of the mast within the model.
When tuned low in the 40 meter band (CW) the match bandwidth is still good enough to cover the entire band with either antenna, but the quad will require the services of the rig's ATU. The delta loop retains a somewhat high impedance which is easy to bring closer to 50Ω with a λ/4 transformer made of 70Ω coax. This is incorporated into the model for the delta loop. For both antennas it is desirable to run the transmission line orthogonal to the antenna plane, and mandatory to use a current (common mode) choke close to the feed point. It is counterproductive to carefully craft an antenna for the desired pattern and match and then throw it all away due to transmission line coupling and radiation.
With two current maxima in these antennas the azimuth pattern cannot be as perfectly circular as for the single element verticals we've already seen. Even so the pattern is omnidirectional, with minima of less than 1 S-unit. I can live with that. Both antennas show almost the exact same azimuth and elevation patterns so I am not bothering to show both.
Both antennas are lossy -- about -5.7 db -- similar to vertical polarization ground losses in the previously-modelled verticals. Poorer soil increases losses, such as that found in urban landscapes. The modelled loss changes little with height. Additional height does add some forward gain; for example, raising the quad loop from 3 to 5 meters adds 0.3 db, which is negligible.
Both loops are sensitive to height above ground. Even a change of as little as 1 meter will noticably shift the resonant frequency and ground losses.
A big down side with these short loops is that an ~30% reduction in circumference is not enough to justify the complexity and likely fragility. At least that is my opinion for my circumstances. Others may see it differently.
My tentative conclusion on these two antennas is that if I had to choose between them I would choose the short delta loop. Considering the near identical performance, I like the simpler construction and likely lower cost. The 70Ω coax for the matching transformer is no obstacle since it's readily available and adds almost nothing to the total cost.
In my next post I'll compare all of these vertical 40 meter antennas and try to come to some conclusion of whether to stick with a full-sized delta loop or opt for one of these short antennas. They will also be compared to an inverted-vee antenna that would employ a mast of similar height.
Loops, like dipoles and their variants, can be shortened in a similar
fashion. Since there are no "ends" to a loop the positioning of the
loading elements is a little more interesting. As you will see in the two designs I look at in this post they can be structurally complex, more so than the complexity inherent in any full-wave loop antenna.As always, for me the antenna must be an acceptable DX performer, which mostly means low loss and low angle radiation. This duo of attributes is difficult to attain in a low-height, low-band antenna since horizontal polarization requires height and vertical polarization undergoes sometimes brutal attenuation at low radiation angles.
The two loop configuration under consideration are the delta loop and the square quad loop. To be a primarily-vertical radiator the delta is fed λ/4 from the apex and square is fed halfway along one side. The EZNEC models show the sources (feed point) as circles and the current distribution on these shortened antennas.The feed point symmetry is retained despite the size in order to maintain the pattern. The loading elements placement ensures that the two current maxima are in approximately the same relative positions on the scaled-down loops. One unavoidable difference is that the distance between the maxima is reduced. That will affect the pattern.
The model for the quad loop is based on one I found in ON4UN, John Devoldere's book "Low-Band DXing". My 1987 version is quite old so I can't say if it's in newer editions. My delta loop model is similar with regard in connecting the linear loading elements 1/4-circumference either side of the feed point.
As modeled, the circumferences of the quad loop and delta loop are 26.4 meters (0.63λ) and 30.16 meters (0.71λ), respectively, with about 2% of the reduction due to the wire insulation (#12 THHN). Loading of the delta loop is lessened to avoid additional construction complexity. Getting significantly shorter than these figures requires coils, which adds losses I want to avoid. Getting much shorter will also reduce the impedance, probably even below 50Ω.
Both antennas are modeled at a height of 3 meters (at the bottom horizontal wire) to keep it away from prying hands for safety and security. The average current height is about half way up on both, working out to about 6.6 meters and 10.4 meters on the quad and delta, respectively. Their respective heights are 9.6 and 11.7 meters above ground.
Both use insulated copper wire (#12 THHN), except that the top wire of the quad is 25 mm (1 inch) aluminum tubing. This latter specification is required to make the loop rigid, assuming the quad is centred on the mast. Thin nylon rope can be used to tension the interior loading wires so as to keep them in their specified positions.
The model assumes that the support mast is non-conductive, such as a (rope) guyed fibreglass mast. A conductive mast significantly changes these antennas because it is vertical and crosses the loops, especially so because the crossing is at their high-impedance points (low current, high voltage). You can use metal but the antennas would be very different, absolutely requiring precise size and placement of the mast within the model.
When tuned low in the 40 meter band (CW) the match bandwidth is still good enough to cover the entire band with either antenna, but the quad will require the services of the rig's ATU. The delta loop retains a somewhat high impedance which is easy to bring closer to 50Ω with a λ/4 transformer made of 70Ω coax. This is incorporated into the model for the delta loop. For both antennas it is desirable to run the transmission line orthogonal to the antenna plane, and mandatory to use a current (common mode) choke close to the feed point. It is counterproductive to carefully craft an antenna for the desired pattern and match and then throw it all away due to transmission line coupling and radiation.
With two current maxima in these antennas the azimuth pattern cannot be as perfectly circular as for the single element verticals we've already seen. Even so the pattern is omnidirectional, with minima of less than 1 S-unit. I can live with that. Both antennas show almost the exact same azimuth and elevation patterns so I am not bothering to show both.Both antennas are lossy -- about -5.7 db -- similar to vertical polarization ground losses in the previously-modelled verticals. Poorer soil increases losses, such as that found in urban landscapes. The modelled loss changes little with height. Additional height does add some forward gain; for example, raising the quad loop from 3 to 5 meters adds 0.3 db, which is negligible.
Both loops are sensitive to height above ground. Even a change of as little as 1 meter will noticably shift the resonant frequency and ground losses.A big down side with these short loops is that an ~30% reduction in circumference is not enough to justify the complexity and likely fragility. At least that is my opinion for my circumstances. Others may see it differently.
My tentative conclusion on these two antennas is that if I had to choose between them I would choose the short delta loop. Considering the near identical performance, I like the simpler construction and likely lower cost. The 70Ω coax for the matching transformer is no obstacle since it's readily available and adds almost nothing to the total cost.
In my next post I'll compare all of these vertical 40 meter antennas and try to come to some conclusion of whether to stick with a full-sized delta loop or opt for one of these short antennas. They will also be compared to an inverted-vee antenna that would employ a mast of similar height.
Saturday, March 23, 2013
Lazy-H Short Vertical Dipole for 40
I should first mention that it's my own idea to call this antenna "Lazy H"; it may already have an appellation of which I'm unaware. That's how it looks to me: an "H" that's fallen over on its side. It is basically a stunted vertical dipole loaded at its ends (top and bottom) with a capacitance hat, configured so that the whole thing fits in a vertical plane. It takes up more space that the linear-loaded dipole, which is not a problem in my backyard.
This is the 2nd in my series of 3 candidate short, vertically-polarized antennas I am contemplating for 40 meters. The first was the linear-loaded vertical dipole and up next will be a linear-loaded loop. I am comparing these to each other and to other, full size candidates: in particular the delta loop and inverted-vee. My aim is good DX performance without making too much a statement in the neighbourhood. I do not have a tower in my plans.
At the side is a picture of the EZNEC model and the current distribution. (It's a bit askew so you can see the current profiles.) This is a plumber's delight style antenna since it pretty well has to be constructed from aluminum tubing. It stands 10 meters tall and the loading hats are 7.8 meters tip-to-tip. The horizontal elements can be shortened by making the antenna taller.
There is at least one commercial version of this antenna, the Sigma-40 by Force 12. It is substantially shorter (~7.3 meters) by virtue of the loading coils (doing a search I found that W0SJS took a picture of the feed and coils). I dislike loading coils since, of necessity, they must be located where the current is high, thus having a noticable impact on loss. Force 12 claims no more than 10% (~0.5 db), which is quite good if true.
Another concern I have is that by being short the average height of the Sigma-40 current distribution is several meters lower than in my model. This will increase ground losses and shadowing by obstacles (houses, wiring, local terrain, etc.). From reports it does work well for many people, but that is not enough to stop me from trying to do better. That antenna is also not an inexpensive choice for what it is and does.
My Lazy-H model places the antenna bottom 3 meters off the ground to keep the antenna out of hand reach. If you run more than QRP the tips of the horizontal elements are a serious shock hazard. At this height the top of the antenna stands 13 meters above grade. My choice would be to guy it, though it is possible to make it free-standing with a concrete base. The same is true of the vertical dipole.
Like the Sigma-40 I have chosen to feed my version at the center. This gives a good match to 50Ω but makes construction and tuning challenging since the vertical element will have to be split or fed with an out-of-easy-reach matching network. In either case it is vital to use a high-resistance current (common-mode) choke on the transmission line and to run it for a distance at a right angle to the plane of the antenna. Since this point is nearly 8 meters off the ground, I would have to run it across the roof of my two-story house, then down toward the basement shack. That might be a minor challenge.
Let's first take a look at what EZNEC has to tell us about the SWR across the 40 meter band. It has an even better SWR curve than the vertical dipole. As always I am focussed on CW, yet even so it does fine above 7.2 MHz. At the resonant frequency of 7.08 MHz the impedance is 53Ω.
The pattern is also quite good. It peaks a little higher than the vertical dipole (20°), but still outperforms it at 15° (see note at bottom). The additional gain is almost entirely due to lower ground losses, which are modeled at -3.9 db for this antenna, or about 1.5 db better than the linear-loaded vertical dipole.
The azimuth pattern is almost perfectly omnidirectional, which surprised me since the loading elements are not axially symmetric. I haven't bothered to look more deeply at this particular result although it does intrigue me. Perhaps those horizontal currents are cancelling in the far field.
After playing with various parameters I can say that this antenna is very sensitive to element diameters and lengths and small changes in height. For example, using wire for the loading elements their length must be increased by 5%. In the model the mast is 50 mm (2 inches) and the loading elements are 25 mm (1 inch). Actual construction will undoubtedly use different diameters plus tapered tubing. Because of the mentioned sensitivity I recommend redoing the model with actual tubing choices.
Tuning can be accomplished by adjusting the length of the mast (best choice) or the lower loading element. These can be adjusted without lowering the antenna if tapered tubing is used. The feed point will require breaking the mast with a dielectric or using a matching system such as the gamma match. One idea I have is to have the upper mast half slide into the lower one and insulated with a plastic sleeve. If you do this you may want to model the capacitance between the overlapping sections to avoid surprises. You will probably need to guy with antenna, so choose rope or kevlar, not metal.
Next up will be the linear-loaded loop. I plan to cover both square and delta loops. I'll get to this in a few days as time permits.
NOTE: I did not mention in the previous post that I am using "good" suburban ground in my model. If your urban environment is worse the performance of any vertical will also be worse. Keep that in mind when comparing to horizontally-polarized antennas since they are less sensitive to poor ground. I'll say more about these comparisons after I wrap up the series on short verticals for 40 meters.
This is the 2nd in my series of 3 candidate short, vertically-polarized antennas I am contemplating for 40 meters. The first was the linear-loaded vertical dipole and up next will be a linear-loaded loop. I am comparing these to each other and to other, full size candidates: in particular the delta loop and inverted-vee. My aim is good DX performance without making too much a statement in the neighbourhood. I do not have a tower in my plans.
At the side is a picture of the EZNEC model and the current distribution. (It's a bit askew so you can see the current profiles.) This is a plumber's delight style antenna since it pretty well has to be constructed from aluminum tubing. It stands 10 meters tall and the loading hats are 7.8 meters tip-to-tip. The horizontal elements can be shortened by making the antenna taller.There is at least one commercial version of this antenna, the Sigma-40 by Force 12. It is substantially shorter (~7.3 meters) by virtue of the loading coils (doing a search I found that W0SJS took a picture of the feed and coils). I dislike loading coils since, of necessity, they must be located where the current is high, thus having a noticable impact on loss. Force 12 claims no more than 10% (~0.5 db), which is quite good if true.
Another concern I have is that by being short the average height of the Sigma-40 current distribution is several meters lower than in my model. This will increase ground losses and shadowing by obstacles (houses, wiring, local terrain, etc.). From reports it does work well for many people, but that is not enough to stop me from trying to do better. That antenna is also not an inexpensive choice for what it is and does.
My Lazy-H model places the antenna bottom 3 meters off the ground to keep the antenna out of hand reach. If you run more than QRP the tips of the horizontal elements are a serious shock hazard. At this height the top of the antenna stands 13 meters above grade. My choice would be to guy it, though it is possible to make it free-standing with a concrete base. The same is true of the vertical dipole.
Like the Sigma-40 I have chosen to feed my version at the center. This gives a good match to 50Ω but makes construction and tuning challenging since the vertical element will have to be split or fed with an out-of-easy-reach matching network. In either case it is vital to use a high-resistance current (common-mode) choke on the transmission line and to run it for a distance at a right angle to the plane of the antenna. Since this point is nearly 8 meters off the ground, I would have to run it across the roof of my two-story house, then down toward the basement shack. That might be a minor challenge.
Let's first take a look at what EZNEC has to tell us about the SWR across the 40 meter band. It has an even better SWR curve than the vertical dipole. As always I am focussed on CW, yet even so it does fine above 7.2 MHz. At the resonant frequency of 7.08 MHz the impedance is 53Ω.
The pattern is also quite good. It peaks a little higher than the vertical dipole (20°), but still outperforms it at 15° (see note at bottom). The additional gain is almost entirely due to lower ground losses, which are modeled at -3.9 db for this antenna, or about 1.5 db better than the linear-loaded vertical dipole.
The azimuth pattern is almost perfectly omnidirectional, which surprised me since the loading elements are not axially symmetric. I haven't bothered to look more deeply at this particular result although it does intrigue me. Perhaps those horizontal currents are cancelling in the far field.After playing with various parameters I can say that this antenna is very sensitive to element diameters and lengths and small changes in height. For example, using wire for the loading elements their length must be increased by 5%. In the model the mast is 50 mm (2 inches) and the loading elements are 25 mm (1 inch). Actual construction will undoubtedly use different diameters plus tapered tubing. Because of the mentioned sensitivity I recommend redoing the model with actual tubing choices.
Tuning can be accomplished by adjusting the length of the mast (best choice) or the lower loading element. These can be adjusted without lowering the antenna if tapered tubing is used. The feed point will require breaking the mast with a dielectric or using a matching system such as the gamma match. One idea I have is to have the upper mast half slide into the lower one and insulated with a plastic sleeve. If you do this you may want to model the capacitance between the overlapping sections to avoid surprises. You will probably need to guy with antenna, so choose rope or kevlar, not metal.
Next up will be the linear-loaded loop. I plan to cover both square and delta loops. I'll get to this in a few days as time permits.
NOTE: I did not mention in the previous post that I am using "good" suburban ground in my model. If your urban environment is worse the performance of any vertical will also be worse. Keep that in mind when comparing to horizontally-polarized antennas since they are less sensitive to poor ground. I'll say more about these comparisons after I wrap up the series on short verticals for 40 meters.
Wednesday, March 20, 2013
Linear-loaded 40 Meter Vertical Dipole
There are many ways to shorten antennas. Before choosing how we must first state why we are doing it. Without a clear purpose to the exercise there can be no clear approach to take. My own reasons are, for the low HF bands, to lower the visual impact and reduce the engineering challenge while still achieving DX performance similar to a full-sized antenna.Keep in mind that any length conductor can be an antenna. What we usually mean by shortening is to design an antenna that is short but still resonant on the selected frequency. A suitable matching network can match any conductor, though there are considerations of cost and losses due to attenuation within the matching system and due to high SWR on the transmission line.
The match does not impact the antenna's efficiency or pattern, except if one is careless about keeping antenna currents confined to the radiating elements, not least of which is feed line radiation.
If you are like too many hams of my acquaintance you may want to reread the previous paragraph. Mismatch does not affect the antenna pattern. However it can be very desirable to get the antenna's raw feed point impedance close to 50Ω in order to avoid having to design around those potential losses. That is one of my intentions in this exercise.Enough of that, time to design that 40 meter vertical dipole.
The EZNEC model shown here is an approximately 13.7 meter tall aluminum mast mounted 3 meters above ground. It is symmetrically loaded with two 6 meter long wires (12 AWG, THHN) that are attached to the top and bottom, and parallel to the mast at a separation of 40 cm. I've mentioned it before but I'll do so again, that modeling closely-spaced conductors with NEC2 must be done carefully and I believe I've gotten it right. Even so some difference between the model and reality is expected.
There are other ways to deploy the loading elements to further shorten the antenna or to change its impedance. My choice was a compromise between getting it tall enough to place the current maximum (the broad middle of the antenna as the current plot shows, about 10 meters above ground) as high as possible without making the antenna too visible or requiring excessive engineering. Many commercial designs also employ small loading coils to reduce the length, but this lowers the current height and, perhaps, 1 db of loss.
As it is the antenna has -5.45 db of losses that are almost entirely due to near field and far field vertical-polarization losses, with negligible conductor losses. There are no matching system losses since I have tuned it to near perfect 1:1 SWR at resonance. You can also see from the SWR chart that no additional matching is required over the entire 40 meter band, though if that is your aim you ought to resonate it a little higher than I have. You can see I do have a CW bias.
To get that SWR I moved the feed point (EZNEC source) along the mast until I found the spot I wanted. This is 15% (2 meters) from the bottom where Z=55Ω at 7.05 MHz. Doing this does not affect either the current distribution or the antenna pattern. The choice is a trade-off between feed line match, antenna construction and feed point accessibility.
The rationale at work here is simply one of Z=E/I (Ohm's Law for AC) where the product of E and I is constant, P=EI, for cases, like this one, where conductor losses are negligible. To get a higher Z value, move the source to where the current is lower; do the opposite to get a lower Z. At resonance (Z = R + j0) and no conductor losses Z is simply the radiation resistance.
When fed at the centre of the mast Z=50Ω (resonance @ 7.03 MHz) and at the bottom Z=72Ω (resonance @ 7.11 MHz). Since there are environmental factors at play (ground, houses, etc.) construction should allow for some adjustment of impedance matching and tuning for resonance. Breaking the mast with a dielectric spacer will work but does not allow for adjustment and may reduce the structure's strength. It may be better to use a stub match or just let the rig's antenna tuner (if you have one) make the small adjustment. I am undecided on which way to go should I choose to build this antenna.
Perhaps the easiest way to adjust the resonant frequency is to make the bottom "wire" a solid aluminum rod and slide the attachment point of the vertical loading element. This seems to work well over a range of 100 kHz, at least according to the model.
Getting back to the pattern of this antenna, an elevation view is shown here. You can see the affect of ground losses on the antenna's gain and low-angle performance. While that loss of 1 S-unit may seem excessive, what matters is the comparison to alternatives. For example, a dipole up 10 meters has little in the way of ground losses but at the same elevation of 15° where the vertical peaks the dipole's gain is -0.6 dbi. Most of the dipole's radiation is at high elevation angles.
To beat this vertical at 15° the dipole would have to be up at least 14 meters. That would require two support structures or at least a tower to support an aluminum-tubing dipole. It would also be directional. However as an interted-vee on a 15 meter tall mast it would be almost as good and also more omnidirectional than the dipole.
That's a useful comparison since the vertical dipole could instead be used to support an inverted-vee. I'll come back to this later.
Sunday, March 17, 2013
Introducing Three Short 40 Meter Verticals
My original plan was to delay installing an antenna for 40 meters until later this year. The purpose being to focus on the daylight-loving bands for the near term and then address the low bands in time for fall. Unlike the 1980s I do not spend the majority of my leisure time on amateur radio so I need to set priorities.
With this blog acting somewhat of a diary of my return to the hobby after many years I ought to do things in chronological order. Except that the time I spend designing antennas is not calendar-aligned with the time I will be building and erecting antennas. While still in the grip of winter I am focused on design. It's also a lot less effort than building antennas!
Although I have not yet discussed my plans for the high HF bands, I do have a favoured design in hand. I will come to that in the coming week or two (maybe). It is why I have been recently looking more closely at 40 meters, a favourite of mine. I thought it worth some effort to evaluate short, but not too short, antennas, comparing them to a delta loop.
There is a full-sized delta loop conveniently coiled up in my basement that needs only a mast to be hoisted into the air. I scanned my 40 meter log from 1984 to 1990 to recall how it performed. I can see that it was very competitive with 100 watts, and even better with a kilowatt. The list of rare ones from around the globe filled the pages. It first went up with a mast made of scrap aluminum I scrounged from another's toppled tower, then moved to the 20 meter tower when it went up in 1985.
I am tempted to be lazy and simply reinstall it. Then I decided it wouldn't hurt to evaluate other designs, if only to affirm that my choice was the right one. Now that I've done some modeling I am inclined to reconsider. Another thing at the back of my mind is to avoid the visual impact of a delta loop, and thus hopefully reduce the attention of neighbours.
Over the next week or so I will look at each antenna in turn. The short verticals I eventually selected for evaluation are as follows. Each requires no more than the single support plus guys that the delta loop requires.
There are some design considerations that make the goal of good DX performance achievable in a short low-band vertical antenna:
That is why I called this blog "Pattern and Match": first get the antenna pattern to where you want it, then, and only then, match it. A beautiful 1:1 SWR is worthless if the antenna is not launching those precious watts where you want them. As it is often said, if you want a perfect match buy a dummy load. The entire feed system from antenna feed point to the rig has only two responsibilities: to keep the transmitter happy; and to minimize losses due to feed line attenuation and radiation.
For example, if I find a tweak to my antenna that reduces environment or conductor losses by 3 db I am only too willing to trade 1 db of coax attenuation due to a high SWR. The reverse is also true: if I can get a perfect match at the cost of driving all those watts into the ground (or a neighbour's house) it is a dreadful decision. Never judge an antenna by the sole criterion of SWR.
With all of that out of the way I will as promised discuss each of the listed antennas in turn over the next few blog posts. I'll provide modelling detail and comparisons, and what I like and dislike about each. You can refer back to the criteria I set out for my antennas in this earlier post.
With this blog acting somewhat of a diary of my return to the hobby after many years I ought to do things in chronological order. Except that the time I spend designing antennas is not calendar-aligned with the time I will be building and erecting antennas. While still in the grip of winter I am focused on design. It's also a lot less effort than building antennas!
Although I have not yet discussed my plans for the high HF bands, I do have a favoured design in hand. I will come to that in the coming week or two (maybe). It is why I have been recently looking more closely at 40 meters, a favourite of mine. I thought it worth some effort to evaluate short, but not too short, antennas, comparing them to a delta loop.
There is a full-sized delta loop conveniently coiled up in my basement that needs only a mast to be hoisted into the air. I scanned my 40 meter log from 1984 to 1990 to recall how it performed. I can see that it was very competitive with 100 watts, and even better with a kilowatt. The list of rare ones from around the globe filled the pages. It first went up with a mast made of scrap aluminum I scrounged from another's toppled tower, then moved to the 20 meter tower when it went up in 1985.
I am tempted to be lazy and simply reinstall it. Then I decided it wouldn't hurt to evaluate other designs, if only to affirm that my choice was the right one. Now that I've done some modeling I am inclined to reconsider. Another thing at the back of my mind is to avoid the visual impact of a delta loop, and thus hopefully reduce the attention of neighbours.
Over the next week or so I will look at each antenna in turn. The short verticals I eventually selected for evaluation are as follows. Each requires no more than the single support plus guys that the delta loop requires.
- Lazy-H vertical dipole
- Linear-loaded vertical dipole
- Linear-loaded, full-wave loop
There are some design considerations that make the goal of good DX performance achievable in a short low-band vertical antenna:
- Capacitive loading (linear or hats). These are low-loss loading elements when installed with good dielectric mounts.
- No coils! This includes traps. Coils as antenna shorteners work where the current is high, which is also where they will dissipate the most power.
- Avoid a too-low raw (unmatched) feed point impedance, and aim to get it near to the transmission line impedance. Low antenna impedance and matching networks introduce resistance losses. It is therefore important to model the antenna with real (lossy) conductors, not ideal (loss-less) elements to measure its performance.
- Get the points of maximum antenna current as far as possible from ground and other conductors in the vicinity. Do this to reduce the inevitable vertical-polarization ground losses in both the near and far fields, and to get the radiating parts of the antenna above local obstacles.
That is why I called this blog "Pattern and Match": first get the antenna pattern to where you want it, then, and only then, match it. A beautiful 1:1 SWR is worthless if the antenna is not launching those precious watts where you want them. As it is often said, if you want a perfect match buy a dummy load. The entire feed system from antenna feed point to the rig has only two responsibilities: to keep the transmitter happy; and to minimize losses due to feed line attenuation and radiation.
For example, if I find a tweak to my antenna that reduces environment or conductor losses by 3 db I am only too willing to trade 1 db of coax attenuation due to a high SWR. The reverse is also true: if I can get a perfect match at the cost of driving all those watts into the ground (or a neighbour's house) it is a dreadful decision. Never judge an antenna by the sole criterion of SWR.
With all of that out of the way I will as promised discuss each of the listed antennas in turn over the next few blog posts. I'll provide modelling detail and comparisons, and what I like and dislike about each. You can refer back to the criteria I set out for my antennas in this earlier post.
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