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.

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.

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.
  • Lazy-H vertical dipole
  • Linear-loaded vertical dipole
  • Linear-loaded, full-wave loop
Short antennas designs are often controversial though there is no reason for it nowadays since it is quite easy to model them and compare. We no longer need to erect them and pray, and then (of course) tell everyone how great that one antenna is while avoiding substantive comparisons which are in any case difficult to arrange and accomplish (convenient that).

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.
SWR is not on this list nor should it be. Any antenna can be matched by suitable feed system or, less desirable, with a tuner in the shack. The SWR does not indicate anything about an antenna's performance other than the risk of matching losses if you are not careful.

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.

Thursday, March 14, 2013

Verticals - Pro and Con

It has often been said that a vertical is an antenna that radiates equally poorly in all directions. This is unfair though not entirely. The problem is less one of polarization than that of configuration and deployment in too many cases. After all, vertical polarization should, in theory, be a plus for working DX because of the low radiation angle. Theory and practice can markedly differ, hence the problem.

I can summarize my problem with typical vertical antennas as follows:
  1. Ground losses
  2. Environment losses
  3. Multiband inefficiencies
A vertical antenna mounted near or on the ground, whether the radials are on the ground, in the ground or above the ground, still couples strongly to the ground. This is also true of verticals that require no radials. The primary reason is that the antenna's current maximum is low to the ground, but also because a vertical E-field gets "eaten up" by lossy (real) ground. Consider the following EZNEC current plots and characteristics of some 40 meter antennas: ground plane, dipole, and vertical dipole. All are modeled over real, average ground and with ideal (loss-less) elements.

First notice that not all parts of an antenna are equal. The contribution to radiation is in proportion to the current. For a ground plane antenna with the current next to ground (½ on the monopole and the rest equally distributed on the radials) the coupling to ground and an obstructed view of the horizon (buildings, trees, terrain variation) can be disastrous.

Even with a vertical dipole the loss is substantial, and only gets slightly lower when somewhat higher (-5.1 db when mounted at 10 meters). Remember that when you peruse those ads for all-band, no-radial verticals.

In the majority of cases a low dipole will outperform a ground-mounted vertical, and not just for DX. In the above example, although when up only λ/4 the radiation mostly goes straight up, at 26° elevation (where the ground plane peaks) it actually outperforms the ground plane by ~3 db. It also sees fewer low-angle obstructions.

The addition of (many!) more radials can reduce near-field ground losses but cannot address the environmental and far-field losses, which should not be underestimated. I have used verticals that work, but only when mounted at a height that is at least roof level, which at least addresses some of the environmental losses if not other problems.

Other verticals types of antenna that lift the current to higher points do perform better. These antenna include the quarter-wave (or half) sloper, and side-fed full-wave loops. Actually the last is not really a vertical, but is vertically polarized.

Making a vertical multiband is not easy. Unlike with dipoles you cannot make a "fan" vertical since only one or a few of the monopoles can be at or near vertical, so for the other bands the pattern is skewed. It also gets expensive if the structure is anchored to the ground since it will require construction from solid tubing; although element suspension from an overhead tree limb can be an alternative for some locations.

Commercial multiband verticals use a combination of one or more of traps, linear loading and matching networks to achieve their aim. There are unavoidable compromises that must be made. Traps add losses, as do matching networks. They also increase the antenna Q such that they are both difficult to adequately tune on all bands and have a small bandwidth. Both require additional tuning at the transmitter end, which adds its own losses. If horizontal space is at a premium and effectiveness if not a primary aim, these antennas can fit the bill. For my purposes they are unacceptable.

So, why do so many hams swear by verticals? In my own experience this would appear to come down to human nature. Whether it is a house, a car, a smart phone or an antenna, once we've invested time and money into it, and others see what we've chosen, there is a tendency to retroactively justify the choice, to convince not only others but also ourselves. The antenna might even seem to hear well, although this is usually due to attenuation of both signal and noise so that the SNR for all but the lowest-angle DX can be comparable to other antennas; however, they are not comparable at the other end of the path. For many, a common solution is an amplifier. I suppose it's a good thing that so many commercial vertical are rated for a kilowatt or more. Do that, and if you have yet to meet your neighbours you soon will.

If the low bands are your thing -- 80 and 160 meters -- there may be no alternative to a ground-mounted vertical. Get them out in an open field and put down lots of radials and/or ground screens. They can also be made multi-element for directivity and gain. Since I cannot achieve any of this in my current situation I plan to initially focus on 7 MHz and up for my new station's antennas. The lower bands will have to wait.

Monday, March 11, 2013

Tricks of the Trade

In an earlier post I posited:
This makes me wonder what it truly takes for a DXer in 2013 to win an advantage over the crowds...what can I do, today, to gain an advantage?
When I do get some proper antenna installed it will help but not solve the challenge. I will still be QRP. Before I get back to talking about antennas I think it is worth a few moments to discuss tactics for working DX, especially the rare ones.

Perhaps the seminal work in this area is "The Complete DX'er" by Bob Locher, W9KNI. It is now quite an old book yet still full of insights about the tricks of the trade in working DX. If you enjoy pursuing DX and you haven't read this book, make the effort to find a copy and read it. Any edition will do. Times have changed, and so have many of the tactics, but the way of thinking about the topic has not.

With my current pipsqueak station tactics are about all I have. I will quickly run down some of the ones I have been using lately. Perhaps a few others will find them of use. The list is in no particular order, and the non-appearance of others is not intended as a critique. I simply do not intend to cover everything. It is also worth mentioning that some of these points only apply to CW.

Change it up -- All too often I call a DX station and get no response despite that station having no other callers. With my puny signal this isn't surprising. However we mustn't give up so easily! Let's look at this from the DX operator's perspective. Your signal is riding the noise, there may be QRM or QRN, or they simply can't be bothered to struggle with weak callers. Be a little persistent (sometimes that works).

Sometimes it's simply because you are not transmitting in their "sweet spot". Everyone tends to tune in a CW signal that sounds good to them. That tone is rarely at their transmitter's zero-beat frequency. Further, some receivers (like the KX3) default to receiving CW on the lower sideband rather than the upper sideband. The solution is to vary your frequency on each of your calling attempts. Shift your transmitter up or down 200 or 300 Hz and call again. Many times this works to get their attention. Or maybe is just your persistence!

Come back later -- Persistence need not imply a continuous attempt to get through. If you aren't having success just stuff the frequency into a memory and spin the dial and look for other DX. Every few minutes flip back to the original frequency and toss in your call. You can catch a lull in the pile-up or perhaps the (for you, unheard) QRM they were fighting is gone. This technique often works very well, even for non-QRP,

Move fast -- Let's face it: most people are lazy. That includes hams. Many callers of the rare DX are not listening carefully, just tossing their calls into the pile-up hoping for the best. Listen to whom they are responding and shift your transmit frequency to a spot just a few hundred Hz away (usually up is best). Do this on every contact, not just once. Yes, it is more work than just pressing the memory button that sends your call, which is why it works: many don't bother.

Here's another example where speed works wonders. A couple of months back I made a few calls to a station on South Shetlands on 30 meters, not really expecting to get through the small pile-up. He was weak and I had my usual puny signal. Then in response to the growing pile-up he sent: now up 1. In under 3 seconds I had activated the XIT and spun it up to about the right spot and called. I got him. Even as I worked him it seemed that many had not (yet) noticed he was operating split and kept calling on his transmit frequency.

Second tier DX -- During the recent spate of DXpeditions (TX5K, XT2TT, etc.) there were other less rare DX stations calling lonely CQ's. Little stations don't get through the rare DX pile-ups very often, so take second best while the taking is good.

Avoid spots -- If a DX station is even moderately rare, once it's spotted on a workwide DX cluster the little stations have little chance. By that point there is at least a small pile-up and with a station like mine even one other caller can be as bad as a deep pile-up. The trick is to catch the DX before it's spotted. Plan and listen, then listen some more. Listening doesn't stop you from occasionally taking your cue from spots but you ought to focus on finding the DX yourself. Don't be lazy like everyone else. Know the propagation, the time/mode/frequency habits of the desired DX and then carefully tune the dial and listen.

Call the weak ones -- There is some justification for a small station to only call the stronger signals. The majority of stations run at least 100 watts and probably have a real antenna, so it they're weak you would expect them to not hear your far weaker signal. This is often not the case. Many times the other station has a particularly quiet spot on the band or is also QRP. Further, since most others are lazy (yes, that again) they will tend to pass right over the weak stations and only pay attention to those they can comfortably copy. So call them. To relate a recent example, a few days ago I came across a barely-copyable SU on 20 meters. After 2 or 3 calls I worked him.

Contests -- What can I say? Wall to wall DX, all operating simplex and desperate for you to call them. Pick any DX contest and run the bands from one end to the other. You are surely guaranteed to pick up new ones, maybe lots of them. You don't have to enter the contest, just make sure you know what info to exchange.

599 -- Purists may not like this one. When the going is tough (and for me that's almost all the time with my station) you have to concentrate on getting copied correctly. The critical piece of information to ensure that the DX stations copies is your call. The signal report is just a formality. They are already having difficulty copying you so don't make it any harder. Their ears are tuned to hear you send "5NN", so send it. Even if they send your report as 229 (I've gotten a few of those recently), you send 599. Make it easy for them and there's less chance that they'll get your call wrong. The longer the QSO continues the less patient they become and there's more opportunity to decide that your call is VE3UN rather than VE3VN (this has happened to me a few times).

When I first read Locher's book all those many years ago it immediately struck me that pretty much everything in there were tactics I already used. What I gained was an awareness of what I was doing, which allowed me to plan rather than react. As each new country showed up on the bands I would mentally review my plan of attack to get it in the log. It helped. It still does.

I promise to get back to antennas in my next post.

Thursday, March 7, 2013

Bending the Vee

In my previous post I showed a multi-band inverted-vee antenna that would fit on my roof and cover the bands from 30 to 10 meters. It is not a great DX antenna but it does work if that is all one can put up.

This antenna, as I covered in earlier posts, has a few well-know advantages that makes them a common choice:
  • Single mast for support
  • Good match to 50Ω coax
  • Easy to make multi-band
  • Inexpensive
There are performance concerns as well that should not be dismissed.
  • Potential to strongly couple to the (metal) support and feed line
  • Pattern and match distortion and ground loss due to low end points
The thing is, just how bad can it get? Once again EZNEC comes to the rescue. It is a simple matter to model an inverted-vee and see just what happens when you bend those dipole legs downward. That's what I did.

The diagram depicts the modelling experiment I undertook. I varied the angle between each leg from the horizontal (standard dipole) position and measured the parameters that were of interest. The results are shown in the following table. For this exercise I set up the following situation, with the goal of making the results applicable in real world but without too much distraction of environmental factors.
  • Start with a horizontal dipole that resonates in approximately the middle of the 20 meter band
  • Antenna apex at 10 meters (λ/2)
  • Real ground of medium conductivity and dielectric properties, similar to a typical suburban setting
  • No obstructions of any sort, including no conducting mast and a feed line that is entirely decoupled from induced or conducted antenna currents
In this table, the angle α is for each leg; the angle between the legs is therefore 180 - 2α. Resonance is where the feed point reactance X=0 (per R+jX), not where the SWR is minimum. The R value at resonance is also shown.

α (angle)ResonanceResistanceMaximum gainLosses
14.13574Ω8.0 dbi @29°-1.3 db
10°14.15074Ω6.9 dbi @30°-1.3 db
20°14.17071Ω6.6 dbi @32°-1.4 db
30°14.24064Ω6.3 dbi @33°-1.4 db
40°14.34054Ω6.0 dbi @33°-1.5 db
50°14.48041Ω5.6 dbi @34°-1.7 db
60°14.69028Ω5.3 dbi @34°-1.9 db
70°14.98015Ω5.0 dbi @33°-2.2 db

Perhaps the most noticable item is how rapidly the resonant frequency rises and the feed point resistance drops as the legs are bent. The lesson is to beware standard dipole equations for the inverted-vee! If you are cutting wire for an inverted-vee it would be a good idea to make it longer than required so that you have enough to play out to lower the resonance to where you want it.

The pattern changes but at a lesser rate for large angles. In fact, the elevation of maximum (broadside) radiation changes very little. Some of that is accounted for in the "Losses" column.

Losses need to be looked at. Some of the loss is in the wire (modeled as 12-gauge uninsulated copper wire), which climbs as the feed point resistance plunges. Most of the loss would be ground losses. Those losses disappear when modelled over a perfect ground, however none of us has access to perfect ground.

I did model this antenna over perfect ground to compare the results. To summarize, the maximum gain drops less but the radiation angle rises to nearly 40° for α=70°; resonance rises a little less; and, feed point resistance differs little from the real ground values shown above.

That should do for inverted-vee antennas for the present. Except, I do want to leave you with a couple of unusual situations that are, in my opinion, enlightening. Inverted-vees can do DX wonders at times!

The first story is about a 2-element inverted-vee wire 40 meter yagi I built for my old tower. The apex was at 17 meters, so not too high. The boom length was 6 meters (~0.15λ) and the ends were only about 1 meter apart, at a height of about 3 meters. I modeled the antenna with the original ELNEC (MiniNEC) program by W7EL. It was electrically switchable between NNE and WSW directions to suit my operating interests.

This antenna worked very well. The gain was not much better than the delta loop it replaced but in contests it played very well because of the F/B ratio. The QRM in contests can be fierce. For a relatively low, horizontally polarized antenna it did very well for me.

The second story regards an 80 meter inverted-vee used in a CW contest (one of the CQ DX contests, if my memory is correct). A guest operator put up this temporary antenna with the aim of being able to work more US stations. My main 80 meter antenna, a quarter-wave sloper, was a superb DX antenna but not so good for continental use. The apex of this inverted-vee was around 18 meters, which is under λ/4 -- very low.

The antenna performed to expectations. Where it exceeded expectations was on certain DX paths. For example, he found that JA stations were perhaps 2 S-units stronger on the inverted-vee than the sloper during the morning twilight/grayline opening. This allowed him to fill the log with a surprising number of Asian contacts. This was no a unique situation since many DXers with multiple low-band antenna have often observed similar effects. Count one more success for the inverted-vee as a supplemental, not primary, DX antenna.

Wednesday, March 6, 2013

Contemplating a Fan Dipole

As I pondered the construction of simple wire antennas for my new station I naturally looked at a fan dipole. This is nothing more than a multiplicity of dipoles sharing a common feed point. It is simple, cheap and extendible to multiple bands.

Any dipole's DX performance depends on height: the more the better. You could, in theory, tilt the fan dipole on its side to make it a primarily vertical radiator, but this is not a serious contender since it suffers from obvious construction challenges.

When I first started playing with these designs I chose to model a fan dipole from bands from 30 meters to 10 meters that could fit on my house roof. I did try adding in 6 meters but gave up because the interactions made it a mess. My roof is not really suited to a 40 meter dipole unless it is shortened by loading, and in any case would do poorly due to the low height.

Let's briefly look at just such a fan dipole that I designed for my roof. The EZNEC antenna view can be seen alongside the text. It's a bit difficult to show well since, other than looking straight down on it, the wires look like a tangled heap from most viewing angles.

First, note that these are in fact inverted-vee antennas. This gets the apex higher (14 meters), thus maximizing the distance between the apex, where the bulk of the far-field pattern originates (current maximum), from the snow load on the roof and metal wiring and conduit within the house. The ends of the element tilt towards the roof's edge, where they would be attached by rope fasteners. (You'll have to mentally picture the presence of the roof.)

I modeled a metal mast (as a 50 mm diameter tube or pipe) to check for interactions. The mast does not touch any of the dipole elements. The coax feed line itself must be isolated from the feed with a common-mode choke, and possibly even with another further along its length (across the shingles then down the side of the roof) since it will strongly couple to the antenna on one or all bands depending on its length and positioning. However this is a topic I won't address in this post.

Please take note that NEC2 has some limitations when it comes to 2 or more wires that are joined at acute angles. The EZNEC documentation discusses how to deal with those difficulties. I addressed most of those in my model so the results should be reasonably correct.

An interesting aspect of the fan dipole is the relative degree of non-interaction among the elements. Where the bands are not harmonically related there is very little current flowing on other than the two dipole halves that are cut for that band. One important benefit of the low interaction is that for the most part each element can be designed as if not part of a dipole array, requiring little tweaking in the design or, ultimately, once erected. The mast is 4 meters long and shows low induced current on all the listed bands, and therefore is not a concern in the design.

For a dipole of this type, the one harmonic of real consequence is the 3rd, where a dipole is 3λ/2 in length. With dipoles for 30, 20, 17, 15 and 10, there is only one such case: the 30 meter dipole on 10 meters. This does in fact occur and explains why I decided to make the 10 meter dipole a true horizontal antenna, constructed with aluminum tubing: to get a decent match.

Below is a graph of the antenna's SWR when fed with 50Ω coax.

It turns out that, despite being inverted-vees, the antenna matches better when fed with 72Ω coax. That is, except for 10 meters where the match gets worse. With an automated antenna tuner the antenna should match fine as is, suffering little in the way of additional transmission line attenuation due to the mismatch.

The radiation angle is poor for DXing, except for the higher bands where it is acceptable though still less than I'd like. On 30 meters the radiation peaks at 30° elevation, and is 10° on 10 meters. There are additional high-angle lobes on 10 meters (contributed in part by the 30 meter dipole), and there is ample unwanted radiation at high angles.

But as I said, it's a nice usable simple antenna. In fact I started with just such a dipole for 20 and 15 meters in my very first (1972) station and I worked the world. Yet its height was even lower.

I have a little more to say about inverted-vees which I'll leave to my next post.

Sunday, March 3, 2013

DX and Modern Technology

How things change.

During the 20 years I was QRT the technology in the hands of every DXer has been greatly enhanced. It's a bit of a marvel to tie my KX3 to a  computer, load a logging program and then connect to a DX cluster. Click on a call in the cluster panel and there I am, right on top of the DX station. Then all I need to do is work them.

The change all happened gradually but in my case it came in one fell swoop. Before you roll your eyes, no, this is not going to be a rant about technology versus the old days -- I am after all deep into technology in the hobby and in my academic training and career. I like it.

That is not to say there is no problem. However it is not one of technology but its ubiquity: everyone, and I mean everyone, has access to the identical tools of the trade. In the so-called olden days the way a DXer strove to stand above the crowd was by station design and construction, and then (most importantly) operator skill and networking. I mean networking in the human sense, where one has contacts to share information about DX activity, thus getting an advantage over the majority.

I mentioned in an earlier post that I had little hope of working the XT2TT DXpedition with my 10 watts and eaves trough antenna. One time I came across them as they were just starting up on a frequency and so had just a few callers who, like me, had stumbled onto them. Even so I had no chance. Within 30 seconds they were spotted and the DX hordes descended.

While this is a disadvantage to me it is also a disadvantage to everyone else since all are on the same plane due to their use of the same tools. They have to contend with the very same hordes as I must. Years ago it might take 10 minutes for the pile-up to grow big, whereas today it often takes less than a minute. There is now less advantage to having skill (listening and timing) than to the brute force of cracking the pile-up with power and big antennas.

Having said all that I ought to mention that I did work XT2TT yesterday. What with everyone else having already worked them or distracted by the ARRL DX contest I had surprisingly little trouble working them on 30 meters. This was more a curiosity than an achievement since I have worked XT many times in the past, and in fact I have well over 300 countries in the log. I just like to count from my recent fresh start: now at 82 countries and climbing.

This makes me wonder what it truly takes for a DXer in 2013 to win an advantage over the crowds. Technology was not always so available or universally used. At one time that could be used to advantage. I was thinking about this as I listened to the pile-ups on TX5K (Clipperton I.) and the first time I worked that entity back in the late 1970s when I had a small station consisting of a TA-33jr @15 meters and a barefoot FT-101.

My station was small but I had a temporary and (as it turned out) a strong advantage over others. I was in the process of upgrading from an FT-101B to an FT-101E. For a time I had them both in the shack. We need to go back in time to understand what this means. The best stations at the time were not transceivers, or at least not solely transceivers; the best equipment was separate receivers and transceivers, each optimized to that one task. When from the same manufacturer (Collins, Drake, and even Heathkit) they could be easy locked to a single VFO for transceive operation or with two independent VFOs for split operation.

With two transceivers I could do better, something that only a minority of stations could then achieve. I used one rig to monitor the DXpedition transmit frequency and the other to monitor the split frequency where they were listening. As was typical then as now for the very biggest and rarest DXpeditions they listened over a range of frequencies. Those with transceivers were in difficulty since the rigs of those days rarely had two VFOs. While those with separate receivers and transmitter could split but could not easily determine where the DX was listening. Almost everyone called blind within the announced listening range of frequencies.

Two transceivers made for an elegant solution. With my "transmitting" transceiver I could listen for the current contact and instantly transmit there when the contact was done.

With two quick calls I had Clipperton in the log on 10 meters. Within a few days I had them logged on all bands for which I had antennas. All I could do was sympathize with my friends who were not so fortunate.

Soon that FT-101B was sold (to finance my post-grad education) and my temporary advantage was lost. Now no one has that advantage. Which leaves me at that original question: what can I do, today, to gain an advantage? I will ponder that while I continue to design and build antennas for my new station.