The past week has been cycling between very good and very poor propagation conditions on the HF bands. The high solar flux (~150) is a plus as is the season (just past the equinox). Except when we also get a mix of X-ray flares and geomagnetic disturbances. Here in the mid-northern latitudes we often lose good DX propagation since even minor geomagnetic disturbances cause high attenuation on paths in the quadrant between the northeast and northwest, which covers pretty much all of Asia.
Although propagation and solar prediction are inexact sciences we at least have a surfeit of data in comparison to past decades. That is, we may not know tomorrow's (or this evening's) conditions but we can explain current conditions in exquisite detail. This is the equivalent of discounting weather forecasts to having a look out the window.
Like looking out the window to determine the weather the best way to determine conditions is to turn on the radio and listen. I try to do this often even when I have no intention of making any contacts. It only takes a few minutes. Not only do I get a better feeling of what's going on I sometimes discover unexpectedly good conditions. This occurred on Monday evening this week. The temptation to get active was irresistible.
With the yagi pointing northeast I was looking for early morning activity from stations in Russia, the middle east and east Asia, with one ear out for a rarer south Asian midday opening. Good conditions may not be enough since few stations are likely to be active on a weekday morning where most hams are instead heading off to work.
Perfect for QRP
Although I now typically transmit with 100 watts I can recognize conditions that are ideal for QRP. After all, with only 5 watts the difference has to be made up with exceptional propagation, up to 10 or 20 db enhancement, or more. That evening on 20 meters I was surprised by strong signals from the other side of the world.
There were S9+ signals from a host of juicy DX tidbits: 4X, 9K, EK, A6, HZ, SV5, 3B9, among many others. These are always good catches though none are especially rare. The thing is this was the kind of night when, with QRP, I would have been in heaven. With 100 watts I easily worked every station I called.
While this one path on 20 meters was excellent the same was not true elsewhere. Signals on 30 and 40 were unexceptional and almost no DX was heard on 17 meters other than TX5P on Clipperton Island. Higher bands were closed. The polar path on 20 was heavily attenuated allowing only the strongest signals through. These included JT and a variety of Siberians. All this data told me that the auroral zone was active and that the MUF to the northeast was most likely only slightly above 14 MHz. European signals were also quite good, though there were few to be heard since for them it was the middle of the night.
You can never have it all. Appreciate what you get and work it when the propagation gods smile upon you. Great conditions rarely last. This is particularly worth noting if you run QRP or have poor antennas.
Making propagation
The next evening (Tuesday) the conditions were worse. A series of X-ray flares had ruined most paths on the higher bands. I went lower and found mediocre propagation on 30 on 40 and little in the way of exciting DX. On a whim I went down to 80 and tuned the CW segment. With my noise and poor antenna I didn't expect much, and that's just what I got: nothing.
On my final spin of the dial a strong and steady S9 signal calling CQ caught my attention. I expected it to from the US. Instead it was HA8RM, a call often heard during contests. I listened to his unanswered CQs for a few minutes while I scanned my email on the shack computer. It was more interesting to me to see who would go back to him than to call him myself; I've worked him many times, including with QRP on 80.
In the end I decided to answer his CQ just so he wouldn't think propagation was dead. I mentioned to him (Peter) that conditions seemed to be good. He told me he was running a kilowatt and a 2-element yagi. Yes, those are good conditions! When the ionosphere is less than cooperative an exceptional station often can make up the difference. If I was running QRP I am sure I would have worked him just as easily. His station is that good.
Which goes to show that with a sufficiency of time and money you can go a long way towards making your own good conditions. The rest of us have to make do with what bones nature tosses our way.
Wednesday, April 22, 2015
Wednesday, April 15, 2015
Spring Checkup
It took a while for spring to arrive in Ottawa. Yet it is now here and the snow is mostly gone. This gives me an opportunity to climb and inspect the towers and the antennas they support. It all looks quite good. In the days or weeks following erection of a tower or antenna it is
normal to be slightly anxious no matter how much care went into the task
or even if over-engineered for the local conditions. After a few big
winds and coatings of ice when nothing happens you gradually relax. I am
now very relaxed.
I'll take this as a sign that I've perhaps done something right. Rather than repeat myself you can refer to the following articles (and their links) for the major structural components of my miniature suburban antenna farm. The only mid-winter work I did was to secure with tie wraps the foam bumpers on the coax up the mast to the inverted vee. The tape loosened in the cold air so the bumpers slid out of position and the coax resumed noisily slapping against the steel mast.
As I hinted in my year-end recap I have some thinking to do in 2015. I can either make the best of my present QTH or I can move somewhere more amenable to bigger and better antennas. However that is a discussion for the future once I've made some personal choices. For this article I want to cover the successes and failures over the past winter season, both in the antennas themselves and in operating.
Rotator
My now ancient Ham-M rotator performed well though not without some difficulties. Internally it appeared fine when I opened it up for inspection before discovering my cabling faults. That argued for leaving well enough alone, and putting it up to see how it fared. Although it showed no serious problems it did have some difficulty dealing with the brutally-cold temperatures we suffered. February, for example, was the coldest on record.
Starting around -20° C rotation was sluggish. Below -25° C it turned only very slowly, and only started turning after 5 seconds of power to the motor. Although the grease should be good enough for these temperatures I either need to completely clean the bearings and races or use something even better. Some care is needed since low-temperature grease is not always the best at the height of summer when the temperature inside the bell housing can climb far above the air temperature.
Another factor that is almost certainly contributing to slow rotation is the motor capacitor. The replacement I selected is not what is specified for this motor. This is a simple change in the shack when I purchase the appropriate part.
Of more concern was behaviour of the wedge brake at the lowest temperatures. I had a couple of instances below -25° C when the brake would not engage. That is, it would retract to allow rotation but not fully extend when power was removed. When this happened and it was windy the rotator freely turned. Even though it cannot turn past the end stops it was disconcerting and an inconvenience. After some minutes it would engage properly. This is likely a grease problem.
Rain, snow, wind and ice
There wasn't much rain during this cold winter since thaws were absent for a long time mid-winter. Build up of snow and ice did affect resonance and performance of all antennas. Ice also added mechanical load though never enough to be a concern. These are routine matters that did not really test my design of the masts and towers. Of course I'd rather not test them!
Bolts can loosen in extreme cold and metal becomes more brittle. A survivable wind in summer can prove disastrous in extreme cold. Luckily the strongest winds don't occur here in winter. It is critical to do regular inspections -- preferably twice annually, but at least once -- to ensure nothing is out of spec. Nothing untoward has been discovered during my spring inspection.
DXCC
The numbers are gradually mounting at VE3VN with very little effort at all. With a yagi and 100 watts it is almost too easy. There are enough DXpeditions and decent propagation to give everybody a chance. I worked over 200 countries with QRP (5 or 10 watts) and at least 100 countries on each of 40, 30, 20, 17, 15 and 10. Now that total stands at 243 countries. Included in that is 200 countries on 20 meters, just shy of 50 on 80 meters and 198 confirmed on LoTW alone. This is all CW, though I am well past 100 on SSB. I am only counting what I've worked since returning to the air in early 2013; my actual total is well over 300 mixed, though I can't be bothered to count them.
Perhaps it is my imagination that it seems so much easier to climb the DXCC ladder now than before my 20 year absence from the hobby. I suspect that it is partly due to demographics. Aging baby boomers have accumulated wealth that is spent on their stations and DXpeditions. The quantity of booming signals on 80 and contest super-stations amazes me in comparison to the 1980s.
This may be as good as it gets since younger generations are less involved in amateur radio so this spurt of frenetic activity will wane with the decline of my generation and those older. At least in the USA and Canada, and to a lesser extent in Europe.
When the next solar maximum comes around don't expect quite the same level of DXCC country-hunting opportunities. I think that's a good thing since it ought to be a challenge. If it's easy it quickly becomes boring. My preference is work hams that are native to those currently-rare countries, which is also necessary to the longer term global health of amateur radio.
Contests
My time as a QRP contester can be best described as: big fish in a small pond. I do well because those with bigger antennas and ambitions also run more power. I didn't go into QRP contesting with the intention of winning anything yet I do well regardless. For example, I achieved #1 world in SOAB QRP in last year's CW WW SSB contest.
Going up to 100 watts and the results are less impressive. Here the competition is already more fierce. My antennas are not equal to my ability so my results are relatively poor. This is true on both high bands and low bands. Still it is fun to work more stations, including frequently good runs, regardless of how I place. Jumping up to a kilowatt will hurt rather than help since in that category there are many impressive antenna farms. And my otherwise friendly neighbours would form a mob and lynch me.
I will continue to return to QRP with my KX3 in some of the larger contests, since with the high participation I can work many stations and place well. If nothing else it stokes my ego. For bigger scores I will do better to join a multi-op effort at a station with more substantial antennas.
Low bands
My results on 40 are good for a simple wire antenna though less good on 80. Even with no antenna for 160 I have had some surprising success. Unfortunately it won't get much better at this QTH. With a larger tower I could have a small yagi on 40 (wire or rotatable), and by finessing the placement of radials I could do better on 80 and even on 160.
My neighbours are reasonably accepting of my present towers and antennas. Putting up a taller tower (over 15 meters) requires crossing the regulatory threshold to consult with my neighbours and deal with push back from the city. If I were prepared to go that distance there is another less-correctable problem: noise.
Noise
The number of residential noise sources continues to grow. In tightly-packed suburbia it can get quite bad. In recent weeks I have been plagued by even worse noise than before. Starting a few days back after some strong winds S9+ power line noise appeared on all bands from 80 through 10. As I write this it has become intermittent and I can only hope it disappears before the Ontario QSO Party this weekend. Another recent noise source peaks to the north, often wiping out most signals on the path to Asia.
No matter the time of day or day of week there seems to be noise on at least some bands. Often the only noise free periods are in the middle of the night when everyone's lights are out and asleep. Since I too am asleep then it does little good. Even with 100 watts I prefer to stick with CW where I can narrow the filter bandwidth.
The only true solution is to get out of the city. Even then it is no longer a sure thing to escape the noise from our modern electronic appliances. Investing time and money into a big tower in the suburbs is often not worthwhile.
Going forward
With the warmer weather my operating activity will, as usual, decline. This is a good idea even for the greatest enthusiast since it replenishes the energy when we do return to the shack. I do have a few antenna experiments in mind for this year which I will undertake as time permits. My schedule includes numerous non-ham activities. However, as I said above, there is little reason for significant changes at the station this year. I am not terribly happy about that.
I will continue with antenna modelling and other planning activities with an eye firmly set on the future. Blog activity may temporarily decline since much of what I'll be doing will not be interesting to others. Reading, studying and planning are boring to watch though worthwhile for the one doing them.
Whenever something that may be of interest to others comes up I will write about it. As I've pointed out before my web site statistics clearly tell me that those of you who visit this blog want to hear about antennas more than anything else.
I'll take this as a sign that I've perhaps done something right. Rather than repeat myself you can refer to the following articles (and their links) for the major structural components of my miniature suburban antenna farm. The only mid-winter work I did was to secure with tie wraps the foam bumpers on the coax up the mast to the inverted vee. The tape loosened in the cold air so the bumpers slid out of position and the coax resumed noisily slapping against the steel mast.As I hinted in my year-end recap I have some thinking to do in 2015. I can either make the best of my present QTH or I can move somewhere more amenable to bigger and better antennas. However that is a discussion for the future once I've made some personal choices. For this article I want to cover the successes and failures over the past winter season, both in the antennas themselves and in operating.
Rotator
My now ancient Ham-M rotator performed well though not without some difficulties. Internally it appeared fine when I opened it up for inspection before discovering my cabling faults. That argued for leaving well enough alone, and putting it up to see how it fared. Although it showed no serious problems it did have some difficulty dealing with the brutally-cold temperatures we suffered. February, for example, was the coldest on record.
Starting around -20° C rotation was sluggish. Below -25° C it turned only very slowly, and only started turning after 5 seconds of power to the motor. Although the grease should be good enough for these temperatures I either need to completely clean the bearings and races or use something even better. Some care is needed since low-temperature grease is not always the best at the height of summer when the temperature inside the bell housing can climb far above the air temperature.
Another factor that is almost certainly contributing to slow rotation is the motor capacitor. The replacement I selected is not what is specified for this motor. This is a simple change in the shack when I purchase the appropriate part.
Of more concern was behaviour of the wedge brake at the lowest temperatures. I had a couple of instances below -25° C when the brake would not engage. That is, it would retract to allow rotation but not fully extend when power was removed. When this happened and it was windy the rotator freely turned. Even though it cannot turn past the end stops it was disconcerting and an inconvenience. After some minutes it would engage properly. This is likely a grease problem.
Rain, snow, wind and ice
There wasn't much rain during this cold winter since thaws were absent for a long time mid-winter. Build up of snow and ice did affect resonance and performance of all antennas. Ice also added mechanical load though never enough to be a concern. These are routine matters that did not really test my design of the masts and towers. Of course I'd rather not test them!
Bolts can loosen in extreme cold and metal becomes more brittle. A survivable wind in summer can prove disastrous in extreme cold. Luckily the strongest winds don't occur here in winter. It is critical to do regular inspections -- preferably twice annually, but at least once -- to ensure nothing is out of spec. Nothing untoward has been discovered during my spring inspection.
DXCC
The numbers are gradually mounting at VE3VN with very little effort at all. With a yagi and 100 watts it is almost too easy. There are enough DXpeditions and decent propagation to give everybody a chance. I worked over 200 countries with QRP (5 or 10 watts) and at least 100 countries on each of 40, 30, 20, 17, 15 and 10. Now that total stands at 243 countries. Included in that is 200 countries on 20 meters, just shy of 50 on 80 meters and 198 confirmed on LoTW alone. This is all CW, though I am well past 100 on SSB. I am only counting what I've worked since returning to the air in early 2013; my actual total is well over 300 mixed, though I can't be bothered to count them.
Perhaps it is my imagination that it seems so much easier to climb the DXCC ladder now than before my 20 year absence from the hobby. I suspect that it is partly due to demographics. Aging baby boomers have accumulated wealth that is spent on their stations and DXpeditions. The quantity of booming signals on 80 and contest super-stations amazes me in comparison to the 1980s.
This may be as good as it gets since younger generations are less involved in amateur radio so this spurt of frenetic activity will wane with the decline of my generation and those older. At least in the USA and Canada, and to a lesser extent in Europe.
When the next solar maximum comes around don't expect quite the same level of DXCC country-hunting opportunities. I think that's a good thing since it ought to be a challenge. If it's easy it quickly becomes boring. My preference is work hams that are native to those currently-rare countries, which is also necessary to the longer term global health of amateur radio.
Contests
My time as a QRP contester can be best described as: big fish in a small pond. I do well because those with bigger antennas and ambitions also run more power. I didn't go into QRP contesting with the intention of winning anything yet I do well regardless. For example, I achieved #1 world in SOAB QRP in last year's CW WW SSB contest.
Going up to 100 watts and the results are less impressive. Here the competition is already more fierce. My antennas are not equal to my ability so my results are relatively poor. This is true on both high bands and low bands. Still it is fun to work more stations, including frequently good runs, regardless of how I place. Jumping up to a kilowatt will hurt rather than help since in that category there are many impressive antenna farms. And my otherwise friendly neighbours would form a mob and lynch me.
I will continue to return to QRP with my KX3 in some of the larger contests, since with the high participation I can work many stations and place well. If nothing else it stokes my ego. For bigger scores I will do better to join a multi-op effort at a station with more substantial antennas.
Low bands
My results on 40 are good for a simple wire antenna though less good on 80. Even with no antenna for 160 I have had some surprising success. Unfortunately it won't get much better at this QTH. With a larger tower I could have a small yagi on 40 (wire or rotatable), and by finessing the placement of radials I could do better on 80 and even on 160.
My neighbours are reasonably accepting of my present towers and antennas. Putting up a taller tower (over 15 meters) requires crossing the regulatory threshold to consult with my neighbours and deal with push back from the city. If I were prepared to go that distance there is another less-correctable problem: noise.
Noise
The number of residential noise sources continues to grow. In tightly-packed suburbia it can get quite bad. In recent weeks I have been plagued by even worse noise than before. Starting a few days back after some strong winds S9+ power line noise appeared on all bands from 80 through 10. As I write this it has become intermittent and I can only hope it disappears before the Ontario QSO Party this weekend. Another recent noise source peaks to the north, often wiping out most signals on the path to Asia.
No matter the time of day or day of week there seems to be noise on at least some bands. Often the only noise free periods are in the middle of the night when everyone's lights are out and asleep. Since I too am asleep then it does little good. Even with 100 watts I prefer to stick with CW where I can narrow the filter bandwidth.
The only true solution is to get out of the city. Even then it is no longer a sure thing to escape the noise from our modern electronic appliances. Investing time and money into a big tower in the suburbs is often not worthwhile.
Going forward
With the warmer weather my operating activity will, as usual, decline. This is a good idea even for the greatest enthusiast since it replenishes the energy when we do return to the shack. I do have a few antenna experiments in mind for this year which I will undertake as time permits. My schedule includes numerous non-ham activities. However, as I said above, there is little reason for significant changes at the station this year. I am not terribly happy about that.
I will continue with antenna modelling and other planning activities with an eye firmly set on the future. Blog activity may temporarily decline since much of what I'll be doing will not be interesting to others. Reading, studying and planning are boring to watch though worthwhile for the one doing them.
Whenever something that may be of interest to others comes up I will write about it. As I've pointed out before my web site statistics clearly tell me that those of you who visit this blog want to hear about antennas more than anything else.
Sunday, April 5, 2015
Improving the 2-element Connected-radial 80M Parasitic Vertical
In my earlier article on a 2-element parasitic vertical array for 80 meters I made no attempt to optimize the design. That was put aside in order to focus on radial topologies. With that out of the way I now want to take the best of those radial topologies -- connected radials -- and get more out of the 2-element antenna.
Since I will not be repeating myself you should use the link above to refer back to that article. For this one I have some objectives in mind when I use the word optimize or, if you prefer, improve the design:
With that introduction out of the way let's proceed to improving (or optimizing) that 2-element parasitic vertical antenna.
Step 1: Optimize the element spacing
The 0.25λ element spacing I used in the earlier article was not selected for its superiority. Rather I chose it as a reasonable value that would allow radial coupling for overlapping radial systems, good though not necessarily optimal coupling between the monopoles, and lastly as an initial basis of comparison with 4-squares (typically arranged in a square with 0.25λ sides). Don't confuse this parasitic antenna with a dual-fed 2-element end-fire array which has a different optimal element spacing.
I ran the EZNEC model by adjusting element spacing in steps of 0.05λ while changing nothing else about the antenna. As before each standalone element is resonant at 3.6 MHz. I successively changed spacing in the increasing and decreasing directions until it was clear that the performance possibilities were exhausted. The model (which has close to 500 segments) requires some work at each step in order to properly maintain radial topology, so it is not a trivial process.
From studying these charts I believe that 0.25λ is the optimum spacing. You can get more gain at 0.20λ but the gain bandwidth is narrower. Better F/B can be had at smaller spacing, but again with narrower bandwidth and with higher SWR. SWR increases at smaller spacing due to decreasing feed point resistance (~25 Ω at 0.15λ spacing) and higher Q (more rapidly changing resistance and reactance with frequency).
In all cases the gain and F/B curves can be moved up or down the band by adjusting monopole length (standalone element resonance). SWR curves can be shifted with a matching network as simple as a series capacitor. Impedance, and thus SWR, will change with different ground, wire type and gauge, and radial count, though probably not by a lot.
Gain relative to a single vertical of the same construction is greater by 0.7 db than shown in the charts since its gain is -0.7 dbi. All gain and F/B values are for an elevation angle of 15°, a good median value for typical DX paths on 80 meters. Over medium ground and the specified radial system the gain peaks at an elevation angle of about 25°.
Step 2: Double the gain
Now that the 2-element array is optimized we can stack them. That is, put an identical array beside it and try for 3 db of additive gain. The idea is to exceed the gain of a 4-square with the same quantity of verticals, and to accomplish it with the same simplicity as each 2-element antenna.
Use of term stack is deliberate and accurate. It is the same as we saw in my article on the basics of stacking yagis. The differences are that the two arrays are ground-mounted, 2-element vertical parasitic antennas and that they are stacked horizontally rather than vertically. In other respects the stacking arrangement is the same.
For this to work we must space the two antennas far enough apart that mutual coupling does not grossly interfere with the additive nature of the array. This is complicated by the radials which would touch, and therefore need to be connected for predictable behaviour, at a spacing of 40 meters or less. Thus the minimum spacing is 42 meters, or 0.50λ. There is no theoretical maximum spacing, though land use and transmission line loss are constraints. We ideally want the minimum effective spacing. So let's try 0.50λ and see what we get.
Gain peaks at 6.33 dbi at 3.65 MHz, and again standardizing on 15° elevation. The gain remains in a narrow range over the band segment of interest. In comparison to the single 2-element vertical array the gain increase is proportional to frequency, from 3 db at 3.5 MHz to 4.5 db at 3.8 MHz. The mutual coupling between the close-spaced antennas is having an effect, and that effect is in our favour in that the gain is for the most part higher than the 3 db expected with zero mutual coupling.
Mutual coupling is also having an effect on F/B, and again it is mostly beneficial. We are now seeing some reasonably good F/B, although only higher in the band (SSB segment). SWR has actually improved, staying below 2 from 3.5 to 3.8 MHz.
The forward lobe in the azimuth pattern has become somewhat narrow, spanning just less than 60° at the -3 db points. This is what happens in stacks: the gain comes from narrowing of the forward lobe in the plane of the stacking direction. Vertically stacked yagis narrow the elevation beam width and horizontally stacked antennas narrow the azimuth beam width.
The feed system is included in the model, composed of several length of coax. Performance will not be accurate if two in-phase sources are instead used in the model. Power splitting is accomplished by tying together (T connector) two λ/4 70 Ω transformers, and placing the single source there. I specified 14 meters of solid dielectric coax (0.66 VF), for which RG-11 is an example. The far ends of the transformer sections must be equal lengths of 50 Ω coax. The transformer lengths alone will not reach from the power splitter to the antennas.
I made the transmission lines loss-less in the model. Real (lossy) coax of the builder's choice can be substituted when selected. In any case the loss in the cabling is not high and would not substantially affect the modelled performance.
As already mentioned, 0.50λ is the minimum spacing. If it is desirable to increase spacing there is a constraint we need to discuss. Unlike a multi-element yagi the beam width of each 2-element antenna in the stacking plane is so wide that there is substantial radiation off the sides (see pattern in the previous article). One advantage of the chosen minimum spacing is that off the sides the radiation from each antenna is 180° out of phase. This is perfect for improving F/S of the array. Until you increase separation to 1.5λ spacing the cancellation can be poor.
The pattern at right is for a spacing of 0.75λ (63 meters). This is not a pattern you are likely to favour. In an array of this type the spacing between antennas is an important parameter.
Step 3: More directions
While 0.75λ is not a particularly effective way to get more than two directions from this array it is possible to steer the forward lobe of the optimum 0.50λ-spaced array. We do this by switching in (or out) a length of 50 Ω coax between the 70 Ω transformer and just one antenna's feed point. However, exact phasing of feed points is not possible since the unequal lines alter the mutual coupling resulting in what might be unexpected results.
In the skewed pattern shown I have "switched" out 10 meters of 50 Ω coax going to the left antenna. While heavily distorted you can see how the main lobe has turned about 30° to the left and partially filled the side nulls. The same can be done in the right direction by the same change to the right leg of the feed system.
While far from the directional performance of a 4-square it is one way to compensate for the fairly narrow main lobe. Yet it is no substitute for getting good performance in the missing 2 quadrants. SWR changes little for the range of cable lengths differences I experimented with in the model. The 10 meters of coax I cut to skew the pattern was arrived at by trial and error; other lengths, I tried gave poorer results. Keeping the 50 Ω feed sections as short as possible (before switching a section out of one leg) gives better patterns.
Step 4: Direction switching and cable management
Direction switching and cable management in the elevated 40 ground plane array requires some care to avoid the coax and relay lines from degrading performance. The same is true here though with some welcome reduction in complexity. In fact the methods used have much in common with typical construction of a 4-square.
For an individual 2-element ground-mounted antenna the presence of the coax (transmission line to shack and between elements) can be put to use rather than cut out of the picture using common-mode chokes. Relays for direction switching are also simpler. This can be done since these conductors are in the same plane as the radial systems, and several of the radials are connected.
First, let's look at the coax and switching line between elements. Unlike the ground plane array we can permanently connect the ends of the coax outer conductor to the radial origins at both elements. It is only the centre conductor that needs to be switched. We are in effect turning these cables into radials. If the cables are buried deep (for weather and rodent protection) they can be complemented with a separate surface radial above the cables and running directly between the elements per the previous article. No chokes are needed. An SPDT relay at the far element switches the monopole between the radial system (reflector element) and the coax centre conductor (driven element).
The element where the transmission line connects still requires two relays, though these can now be SPDT rather than DPDT. One switches the monopole between the second relay (driven element) and the radial system (reflector element). The other switches the transmission line centre conductor between the first relay (driven element) and coax to the far element (reflector element). The transmission line outer conductor connects to the radial system and to the outer conductor of the coax going to the far element.
The transmission line can also become a radial if run where a radial would normally run and it is (optionally) terminated out 20 meters with a common mode chock. In the 2-antenna array these runs would terminate at some convenient point between the 2-element antennas where power splitting takes place. It is likely that the coax-radial will include a transition from 50 Ω coax to the 70 Ω transformer so there may be a need for superior sealing of that joint. If DC relay lines are run the same way (central DC/RF splitter rather than one for each antenna) similar common-mode precautions are needed for the DC lines.
All transmission lines in each half of the array must be identical for the required symmetry. This includes the 50 Ω lines from the 70 Ω transformers to the feed points and the 50 Ω line between elements.
As with stacking yagis there is the alternative of running 50 Ω coax from the power splitter to the individual antennas by using a 2:1 transformer rather than λ/4 transformers. This has a few benefits worth considering:
As I stated up front I am willing to sacrifice ultimate performance in two of the directions provided by a 4-square if I can get better performance in the two directions of greatest importance to my operating. This array can accomplish that and do so with electrical simplicity and a little more land to accommodate the greater separation required between the 2-element antennas.
The cost is one of time and effort to roll-your-own rather than buying a commercial 4-square splitter/phasing box. However the tuning effort in my opinion is simpler and quicker with this array than with a 4-square. You also won't find one day that the "dump" resistor of a 4-square warns of failure or burns up hundreds of precious watts due to weather or other adverse event that will occasionally upset the sensitive tuning of the array.
To me this is good food for thought. Since I am not yet in a position to build either antenna there is time to consider my options. Your situation and objectives may be different and so should your choices.
Since I will not be repeating myself you should use the link above to refer back to that article. For this one I have some objectives in mind when I use the word optimize or, if you prefer, improve the design:
- Simplicity: The more common 4-square used on 80 meters in many high-performance stations is in the judgment of many hams a good return on investment of time and money. But at its heart it is not simple, dependent as it is on some intricate phasing and power splitting electronics. My preference is for an antenna with similar performance that is less vulnerable to weather or component failure, and is far easier to tune and keep in tune.
- Gain and F/B: Gain is paramount to me though I'll take all the F/B I can get on 80 meters. With noise so prominent on the lowest bands gain has an outsize effect compared to higher bands since signals are often so near the noise level. Odds of working the rare DX improve and many more QSOs can be logged during contests. The same applies to F/B but only on receive, which can alternatively be addressed with a separate high-directivity receiving antenna. Thus for me gain trumps F/B.
- DXing and contesting from my QTH: For me the most productive directions are Europe and central USA, which happen to be in exactly opposite directions. The 4-quadrant switchable directivity of a 4-square is nice, in general, but when aligned to those two directions the other two are not so useful: namely the north Pacific and south Atlantic Oceans. One of those will at least garner some JA QSOs while the other is mostly useless, except perhaps for instances of skew-path propagation. I am willing to sacrifice performance in those directions to do better towards Europe and central USA. Other, simpler antennas can fill the pattern holes.
With that introduction out of the way let's proceed to improving (or optimizing) that 2-element parasitic vertical antenna.
Step 1: Optimize the element spacing
The 0.25λ element spacing I used in the earlier article was not selected for its superiority. Rather I chose it as a reasonable value that would allow radial coupling for overlapping radial systems, good though not necessarily optimal coupling between the monopoles, and lastly as an initial basis of comparison with 4-squares (typically arranged in a square with 0.25λ sides). Don't confuse this parasitic antenna with a dual-fed 2-element end-fire array which has a different optimal element spacing.
I ran the EZNEC model by adjusting element spacing in steps of 0.05λ while changing nothing else about the antenna. As before each standalone element is resonant at 3.6 MHz. I successively changed spacing in the increasing and decreasing directions until it was clear that the performance possibilities were exhausted. The model (which has close to 500 segments) requires some work at each step in order to properly maintain radial topology, so it is not a trivial process.
- Move one element to the required position. I nominally set λ = 84 meters (3.57 MHz), and I rounded the spacing to the nearest 0.5 meters to simplify radial calculations without significantly affecting the results.
- With the element moved it is necessary to extend or contract the radials that must be connected. These are radials that would otherwise cross at their nominal length of 20 meters. The included angle between radials must be preserved. I used Cebik's method here, just as I did in the earlier article. All connected radials connect at a line orthogonal to and that bisects the line between element centres.
- Segment counts for the connected radials must be adjusted so that segment length in all radials is as close to equal as possible. This assures best accuracy from NEC2. I normalized segment length as 1.25 meters. I was able to get within a few percent of this for all of the truncated (connected) radials.
From studying these charts I believe that 0.25λ is the optimum spacing. You can get more gain at 0.20λ but the gain bandwidth is narrower. Better F/B can be had at smaller spacing, but again with narrower bandwidth and with higher SWR. SWR increases at smaller spacing due to decreasing feed point resistance (~25 Ω at 0.15λ spacing) and higher Q (more rapidly changing resistance and reactance with frequency).
In all cases the gain and F/B curves can be moved up or down the band by adjusting monopole length (standalone element resonance). SWR curves can be shifted with a matching network as simple as a series capacitor. Impedance, and thus SWR, will change with different ground, wire type and gauge, and radial count, though probably not by a lot.
Gain relative to a single vertical of the same construction is greater by 0.7 db than shown in the charts since its gain is -0.7 dbi. All gain and F/B values are for an elevation angle of 15°, a good median value for typical DX paths on 80 meters. Over medium ground and the specified radial system the gain peaks at an elevation angle of about 25°.
Step 2: Double the gain
Now that the 2-element array is optimized we can stack them. That is, put an identical array beside it and try for 3 db of additive gain. The idea is to exceed the gain of a 4-square with the same quantity of verticals, and to accomplish it with the same simplicity as each 2-element antenna.
Use of term stack is deliberate and accurate. It is the same as we saw in my article on the basics of stacking yagis. The differences are that the two arrays are ground-mounted, 2-element vertical parasitic antennas and that they are stacked horizontally rather than vertically. In other respects the stacking arrangement is the same.
For this to work we must space the two antennas far enough apart that mutual coupling does not grossly interfere with the additive nature of the array. This is complicated by the radials which would touch, and therefore need to be connected for predictable behaviour, at a spacing of 40 meters or less. Thus the minimum spacing is 42 meters, or 0.50λ. There is no theoretical maximum spacing, though land use and transmission line loss are constraints. We ideally want the minimum effective spacing. So let's try 0.50λ and see what we get.
Gain peaks at 6.33 dbi at 3.65 MHz, and again standardizing on 15° elevation. The gain remains in a narrow range over the band segment of interest. In comparison to the single 2-element vertical array the gain increase is proportional to frequency, from 3 db at 3.5 MHz to 4.5 db at 3.8 MHz. The mutual coupling between the close-spaced antennas is having an effect, and that effect is in our favour in that the gain is for the most part higher than the 3 db expected with zero mutual coupling.
Mutual coupling is also having an effect on F/B, and again it is mostly beneficial. We are now seeing some reasonably good F/B, although only higher in the band (SSB segment). SWR has actually improved, staying below 2 from 3.5 to 3.8 MHz.The forward lobe in the azimuth pattern has become somewhat narrow, spanning just less than 60° at the -3 db points. This is what happens in stacks: the gain comes from narrowing of the forward lobe in the plane of the stacking direction. Vertically stacked yagis narrow the elevation beam width and horizontally stacked antennas narrow the azimuth beam width.
The feed system is included in the model, composed of several length of coax. Performance will not be accurate if two in-phase sources are instead used in the model. Power splitting is accomplished by tying together (T connector) two λ/4 70 Ω transformers, and placing the single source there. I specified 14 meters of solid dielectric coax (0.66 VF), for which RG-11 is an example. The far ends of the transformer sections must be equal lengths of 50 Ω coax. The transformer lengths alone will not reach from the power splitter to the antennas.
I made the transmission lines loss-less in the model. Real (lossy) coax of the builder's choice can be substituted when selected. In any case the loss in the cabling is not high and would not substantially affect the modelled performance.
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| 0.75λ spacing between the 2-element antennas |
The pattern at right is for a spacing of 0.75λ (63 meters). This is not a pattern you are likely to favour. In an array of this type the spacing between antennas is an important parameter.
Step 3: More directions
While 0.75λ is not a particularly effective way to get more than two directions from this array it is possible to steer the forward lobe of the optimum 0.50λ-spaced array. We do this by switching in (or out) a length of 50 Ω coax between the 70 Ω transformer and just one antenna's feed point. However, exact phasing of feed points is not possible since the unequal lines alter the mutual coupling resulting in what might be unexpected results.In the skewed pattern shown I have "switched" out 10 meters of 50 Ω coax going to the left antenna. While heavily distorted you can see how the main lobe has turned about 30° to the left and partially filled the side nulls. The same can be done in the right direction by the same change to the right leg of the feed system.
While far from the directional performance of a 4-square it is one way to compensate for the fairly narrow main lobe. Yet it is no substitute for getting good performance in the missing 2 quadrants. SWR changes little for the range of cable lengths differences I experimented with in the model. The 10 meters of coax I cut to skew the pattern was arrived at by trial and error; other lengths, I tried gave poorer results. Keeping the 50 Ω feed sections as short as possible (before switching a section out of one leg) gives better patterns.
Step 4: Direction switching and cable management
Direction switching and cable management in the elevated 40 ground plane array requires some care to avoid the coax and relay lines from degrading performance. The same is true here though with some welcome reduction in complexity. In fact the methods used have much in common with typical construction of a 4-square.
For an individual 2-element ground-mounted antenna the presence of the coax (transmission line to shack and between elements) can be put to use rather than cut out of the picture using common-mode chokes. Relays for direction switching are also simpler. This can be done since these conductors are in the same plane as the radial systems, and several of the radials are connected.
First, let's look at the coax and switching line between elements. Unlike the ground plane array we can permanently connect the ends of the coax outer conductor to the radial origins at both elements. It is only the centre conductor that needs to be switched. We are in effect turning these cables into radials. If the cables are buried deep (for weather and rodent protection) they can be complemented with a separate surface radial above the cables and running directly between the elements per the previous article. No chokes are needed. An SPDT relay at the far element switches the monopole between the radial system (reflector element) and the coax centre conductor (driven element).
The element where the transmission line connects still requires two relays, though these can now be SPDT rather than DPDT. One switches the monopole between the second relay (driven element) and the radial system (reflector element). The other switches the transmission line centre conductor between the first relay (driven element) and coax to the far element (reflector element). The transmission line outer conductor connects to the radial system and to the outer conductor of the coax going to the far element.
The transmission line can also become a radial if run where a radial would normally run and it is (optionally) terminated out 20 meters with a common mode chock. In the 2-antenna array these runs would terminate at some convenient point between the 2-element antennas where power splitting takes place. It is likely that the coax-radial will include a transition from 50 Ω coax to the 70 Ω transformer so there may be a need for superior sealing of that joint. If DC relay lines are run the same way (central DC/RF splitter rather than one for each antenna) similar common-mode precautions are needed for the DC lines.
All transmission lines in each half of the array must be identical for the required symmetry. This includes the 50 Ω lines from the 70 Ω transformers to the feed points and the 50 Ω line between elements.
As with stacking yagis there is the alternative of running 50 Ω coax from the power splitter to the individual antennas by using a 2:1 transformer rather than λ/4 transformers. This has a few benefits worth considering:
- There are fewer joints to protect from weather and burial, especially the ones at the far end of the λ/4 transformers.
- One antenna can be easily disconnected from the array. This is a simple way to broaden the forward lobe (135° beam width versus 60°) and thus reduce the need for array steering. Although we give up 3 to 4.5 db gain when doing this it is one way, for example, to cover almost all of the continental USA from my QTH. In contests this can be highly desirable.
- Pattern skewing as described earlier can be done at the power splitter by switching in (or out) a length of 50 Ω coax feeding either of the antennas.
As I stated up front I am willing to sacrifice ultimate performance in two of the directions provided by a 4-square if I can get better performance in the two directions of greatest importance to my operating. This array can accomplish that and do so with electrical simplicity and a little more land to accommodate the greater separation required between the 2-element antennas.
The cost is one of time and effort to roll-your-own rather than buying a commercial 4-square splitter/phasing box. However the tuning effort in my opinion is simpler and quicker with this array than with a 4-square. You also won't find one day that the "dump" resistor of a 4-square warns of failure or burns up hundreds of precious watts due to weather or other adverse event that will occasionally upset the sensitive tuning of the array.
To me this is good food for thought. Since I am not yet in a position to build either antenna there is time to consider my options. Your situation and objectives may be different and so should your choices.
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