W5WVO 6m Beam Project

W5WVO clone construction so far.

I wrote earlier on my purchase at the Dalton, GA Hamfest of the 6m Mystery Beam. It clearly formed some kind of antenna, given the lengths of the elements. But I had no clue how those elements were intended to be positioned on the boom -- and even if I did, I had no idea what kind of performance to expect.

Course of Action

Unsure of what to do, I asked the folks on he SEDXC mail reflector. Joe Subich, W4TV suggested that I use the components to implement the W5WVO modification of the A50-5S, or perhaps re-create one of YU7EF's five element designs for a 4.5m boom or 4.15m boom. 

Choosing between these options was difficult. What I had wasn't a A50-5S, so the W5WVO medication wasn't straightforward. And the YU7EF designed were even further afield from my starting point.

I decided to adapt my tubing collection to W5WVO's design. 

My elements were too short, they'd need to be extended. But, it isn't as simple as just matching the length W5WVO specified -- the taper schedule is different. 

The A50-5S and the W5WVO designs use 48" of 3/4" tubing in the center extended with 5/8" tubing to the element length. My tubing is 3/4" the entire way. I'd need 5/8" extensions, but how long?

Answering that question required modeling.

Modeling a Solution

As a Mac user, I use CocoaNEC with the NEC 2 engine. It's pretty sophisticated, actually, but getting good results requires using the NC modeling language, which can be a bit tedious. 

My first model was W5WVO's design using the normal taper schedule - inner 24" of each half element are 3/4" with the rest being 5/8". Results were very similar to, but not exactly the same as W5WVO's article. (Part of the reason is W5WVO used NEC 4 engine) But what I had was close enough.

Second model used the 3/4" element lengths I had, spaced according to the W5WVO design. The results were akin to the W5WVO, but with significantly worse F/B.

Third model used the same 3/4" element lengths, with 5/8" extensions on the tips of each element. Because of the different taper schedule, I experimented using a different percentage of the W5WVO dimensions. Lo and behold, at 80% extension length, I modeled something very, very close to the W5WVO design. 

Reflector with
5/8" extension.

The extensions needed on each end are short:

• Reflector - 2.5"
• Driven Element - 0.75"
• Director 1 - 1.75"
• Director 2 - 1.375"
• Director 3 - 0.25"

I added about 3/4" for overlap inside the 3/4" tubing. I secured the extensions using 1/8" Cherry pulled rivets. These aren't ordinary "pop" rivets. Ordinary pop rivets are just a hollow aluminum tube. These leave a steel mandrel filling the tube -- a solid, structural connection.

Building

Extensions on each element.

First step involved cutting the extensions and riveting to each element. I used two rivets on opposite sides. On the D3 element, with the very smallest of extensions, I ended up with one rivet because I broke my #30 drill bit. 

Second step would be to hang the elements on a boom. Oh, wait, I need a boom!

The parts I bought at the Hamfest had three segments of 1" Aluminum pipe which was reinforced by a 13 foot piece 3/4" pipe. None of this fit well together. And the diameter was somewhat small for a 20 foot boom.

I had a 7 foot piece of 1-1/2" tubing I replaced on a Cushcraft A3S. I also had a 12 foot piece of 1-1/2" tubing. Together, they would be 19 feet. The last 10" of the 7 foot tubing had a crack, so I cut that part off, and used a  1 foot 1-5/8" tubing section to join the two together. My only hesitation was that the 12 foot piece was only 0.035" wall (whereas the others are 0.058"). I was worried it might not be strong enough. I figured it was worth a try, perhaps aided by a supporting truss.

I also had to figure out a boom-to-mast plate. I was fortunate to have one in the junk box, along with U-bolts that would work.

Mapping the elements onto the boom was a little tricky. The U-bolts just barely fit over the 1-1/2" boom, but they could not go over the 1-5/8" joiner. I had to move the reflector 8" away from the end of the boom so that Director 2 did not fall on the joiner. 

I managed to get all the elements positioned on the boom. Definitely looks like an antenna now.

Next step will be to figure out how to feed this beast with a gamma match.

Hamfest Special - Mystery 6m Beam

Back in July 2021, I asked members of the SEDXC reflector how best to work Europeans on 6m, one important bit of advice was to use an antenna with more gain than my Cushcraft A50-3S. Three elements just won't cut it on marginal paths. The suggestion was to use a beam with five or more elements. 

Such antennas are several hundred dollars new. The A50-3S was used from a local club for $80. Yes, I'm cheap, but it has served me well. Since then, I've been looking for a reasonable, used antenna. I'm even willing to do some minor repairs.

As I was leaving the Dalton, GA hamfest at the end of February, I stopped by a tailgate area where a guy had a trailer load of stuff. I could see a Cushcraft tribander, a Hy-Gain tribander, house brackets, guy brackets, feed lines, a gin pole and other stuff. I wondered if he might have something for 6m. So I asked.

The owner wasn't present, so his kid called him on a digital walkie-talkie. He said he had a 5-element Cushcraft 6m beam. By the time he made it back to the trailer, we pulled it out, and he changed his tune, he said it was a 6-element Hy-Gain beam. You could see the gamma feed on the driven element. 

Sounded great to me. I negotiated him down to 63% of his asking price, and walked away with the antenna bundle for $125. Sweet.

Getting home, before  I took the antenna off the truck, I went looking for Hy-Gain six-element 6m antennas. I found manuals for models 66B and VB-66DX. They are very similar. The VB-66DX appears to be a hardware-update of the 66B design. These antennas are also fed with a beta-match, not a gamma-match. What I bought is not a Hy-Gain antenna.

Taking the antenna off the truck, cutting it apart and laying the pieces out on the deck.What I found was surprising:

• 

  The components I purchased
  

  REF - 3/4" Al - 9' 9" - 117"
• DE - 3/4" Al - 9' 2" - 110" (Gamma match)
• D1 - 3/4" Al - 8' 9" - 105"
• D2 - 3/4" Al - 8' 8" - 104"
• D3 - 3/4" Al - 8' 7" - 103"
• Misc - 1/2" Al - 50" - Swaged to 5/8" last 6" (2) - Hy-Gain bracket adds 1 1/2" - 101 1/2" total
• Boom - 1 1/4" Al totalling 24 feet in three sections with 1" thicker wall inner tubing

First five elements mount with a single 1 3/4" U-bolt and saddle in the center. The Misc segments could mount in a single Hy-Gain bracket, giving a total element length of 101 1/2" -- which might be a forth director.

Gamma match is a total of 16" 1/2", most of which is a 1/4" Aluminum rod. The shorting bar is at 14 1/2". The first 1 1/2" is a 1/2" Al tube flattened at one end for a screw. The open end hid a disc ceramic capacitor that sadly I broke in transit. Looks like a 3-6 kV capacitor, value unknown.

The boom is a piece of work. There are three 1" Al pipe sections: 75 3/4", 144", 68". The 68" section has a 156" piece of 3/4"Al pipe with a ticker wall as reinforcement. It is mounted asymmetrically, so more of the end extends into the 144" piece than the 75 3/4" piece. There is no boom to mast bracket.

I'll note that the boom is aluminum pipe. Not tubing. It's designed to carry liquids, not be structural.

Clearly, this is not the parts to a Hy-Gain nor a Cushcraft 6m beam. First off, no commercial 6m beam ships with single-tubing size elements. They all use a taper schedule. There are two good reasons for this. 1) It makes the antenna adjustable. 2) they can ship sections shorter than 7 feet, which allows the package to go UPS, 

These parts are a collection different ideas. The U-bolt mounting is Cushcraft-style, but the boom size is too small for a Cushcraft. The boom is 24 feet long, but it is clearly not a Hy-Gain. The boom is way too small, since Hy-Gain used a 2" boom. Plus, it apparently had a truss (now broken), probably because the    for the 24 foot length.

What I appear to have is a collection of parts used to cobble together a poor imitation of something like the Hy-Gain 66B / VB-66DX. Not at all what the guy at the hamfest told me.

There's plenty here to put together a solid five element beam on a 12 to 18 foot boom. The elements are already cut. The hard question is how far should they be spaced? Once I know what the right spacing is, I would then know how much boom I need. 

The broken gamma match is annoying, but fixable. Once I know where to place the elements....

This project is going to take some work.

Demise of the 80/40m Dipole

Sad to say, I recently lost my 80/40m trap dipole. Which is too bad, because it was a good antenna. I used this antenna from four different locations in Georgia: Gwinnett, Floyd, Walton and Fulton counties. I made thousands of contacts on this antenna. It had been the first antenna I put up at Floyd and Walton counties, and the only antenna at Fulton county.

It was last up at the parsonage in Fulton county. Apparently a group of teens was doing some volunteer work near the parsonage. They saw the rope tied off to the parsonage fence and thought it was something other than it was, so they untied it.

After this, half the antenna fell down to the ground and was left there. When the landscapers came by to mow the grass, they ran over it....

So, about 80 feet of rope, insulator and one of the traps was completely destroyed. The other half of the dipole is still intact, up in the trees. But most of one element is gone -- clearly the landscapers threw it away.

Plan is to design an 80/40/20m trap dipole, using traps made with coils and capacitors, since they have much higher Q than the coax cable traps. I'll also place the traps on frequencies well off the operating frequency.

What's Next for DXCC?

At the end of summer every year, I start thinking about DXCC. For several years, it was getting more confirmations for 80m DXCC, so I could complete 5BDXCC. Last year, I accomplished that, including the 30m endorsement.

At the time of this writing, I have 109 confirmations for 17m. That band will be next - I'll submit that this year. This will also push me over the 1000 confirmations threshold for DXCC Challenge - I'll do that one at the same time.

The downside of earning DXCC awards on each band is there are fewer bands left.  On 12m, I need just 17 more confirmation. Given the low level of sunspot activity, that might take some doing. However, I did manage to add 10 confirmations since last year, so it is possible.

160m would be the next one. With 46/45 confirmations, it's further off. I'm not even half way to the necessary 100. I will definitely put some work into 160m this winter. But, honestly, DXCC on 160m may take a few years.

Speaking of years, on 6m I have a whopping seven confirmations. In the spring, I figured out how to use FT8 on this band, and while I heard a ton of DX, I only managed to work a handful. Definitely looking forward to the winter sporadic-E season. I think the most excitement I'll get for a while is finishing off 10-band confirmations. I have USA, Canada, Mexico, and Suriname on all 10 bands -- but there's several countries where I just need a 6m contact.

I've inched closer to Honor Roll, with 279/276 confirmed Mixed. have worked about nine of the remaining 64, but I need confirmations. Perhaps it is time to learn more about QSLing effectively.

And, if nothing else, it's fun building endorsements for the bands I already have. That's the fun of DXing -- it's long-term fun.

5BDXCC - YES!

For the last four low-band seasons, I've been hoping I'd capture enough confirmations for 5BDXCC. I had already confirmed 40, 20, 15 and 10m DXCC, I just needed to get to the magic 100 entities on 80m. As I wrote in the fall, I just needed a few more to put me over the top.

At that time, I had 91/90 confirmed on LoTW, plus a couple of cards.

Today, I received a confirmation on 80m from CE2VMF, bringing my totals to 98/97 on LoTW. Plus, I have THREE cards with additional confirmations, which brings the overall total to 101/100.

I remember talking to Jim Streible K4DLI (who now, unfortunately, is a silent key) about his 5BDXCC plaque. Jim said he had spent 35 years earning that award. And Jim earned it back in the day before electronic confirmations, too.

So, when I apply for DXCC awards later this year, I'll be petitioning for 6BDXCC (got 30m, too!).

I'll have to seriously consider ordering the plaque.

Pursuit of 5BDXCC IV - 80m

Shunt-feed wires rise up the side of the tower.

For the last three years, near the end of summer, I have written about converging on 5BDXCC. I wasn't close when I only had thirty countries to confirm on 80m. The next year, I had inched a little closer, but taking the proposition a bit more seriously. And last year,  I made some serious progress, and was hopeful I was within striking distance.

Let's review the numbers, shall we? Last year I stood at 89/88, right? Well, turns out, this was slightly off. I discovered that a confirmation of CP1FF on 15m Phone was credited to 80m. This has since been corrected, so I was really at 88/87.

Today, I hold 91/90. Yes! At least down to the last ten confirmations. Not a lot of progress from last year, but at least progress.

I do have paper QSL confirmations for a couple more entities on 80m. According to ClubLog, I've worked 103 countries on 80m. Looking at the data, perhaps a couple of those have busted calls on one end or the other.

This leaves me with two methods of achieving this goal: I can pursue getting paper confirmations from contacts I've already made, or I can strive to make more contacts and get LoTW confirmations from them.

Let's look at the latter.

Last year, I ran into a few obstacles trying to work DX on 80m. Since moving from Floyd County to Walton County, I had to take down the 80/40m dipole and also the very effective 160/80/40m Inverted-L. The 80/40m dipole is up at the Walton County QTH, but the low height and local noise limits its usefulness. With the Gwinnett County QTH just minutes away, I've been more tempted to operate from there, when I can. The 160/80/40m Inverted-L and all the 30 lbs of radials are still sitting in a box. I don't really have a location to put it up right now.

That leaves me with the shunt-feed tower for 80m. I ran into a number of issues last fall with the shunt-feed matching network. The 80m cap had to be replaced. I also found that the shunt isn't as effective as it might have been. Certain times of the early evening, the 80m dipole was much more effective.

Plus, I'm seeing some RFI-related issues when using the shunt-fed tower. These didn't affect the Elecraft K2/100 or the Kenwood TS-430S, but they sure make a mess of the Elecraft K3/100.

I also wanted to put up the K9AY loops with a new loop controller. The new loop controller didn't work properly, and even using the old mechanical switch, I discovered I had other RFI feedback from the K9AY loop feed line.

So, It's clear to me I need to take care of a number of things before fall begins:

• Fix the Shunt-feed network. To improve matching on 80m, I'll add a relay and an extra variable capacitor, which would give me settings for 160m, 80m and 75m. I'll also bypass the control lines to prevent RFI ingress. Decoupling on the feed line couldn't hurt, either. 

• Add Radials. The 29 or so radials for the shunt-fed tower may have deteriorated in the last 15 years, and a few of them have been damaged. I have about 2,000 feet of surplus cloth-covered copper wire, and my plan is to put it ALL down as radials. 

• 80m Dipole. I'm working on a design for a trap 80/40/20m dipole, which may double as a second-radio antenna. That should go back up where the old 80/40m was.

• Amplifier. One benefit of the 80m dipole is I can use the amplifier. The shunt-feed network can't handle much more than 100 watts. 

• K9AY Loops. Fix the controller. Re-route the feed line to avoid RFI pick-up. 

Once all that is done, there's the small matter of being there. I guess I should start operating more from Walton County. Perhaps plan to work a few DX contests on 80m. Perhaps the ARRL DX contest would be good.

How about the former? I've already used ClubLog and OQRS to request paper QSLs for a couple of the contacts for which I have matching information. Those should come eventually.

Plus, I need to go through my batch of QSLs and see if I have anything matching on 80m. I think I did this exercise last year, but I've gotten some new QSLs from the bureau that I haven't processed yet.

The final step is to use ClubLog to identify some of those contacts I've worked and not confirmed and request paper QSLs. Those may take a while to arrive, but at least I don't have to fix antennas or wait for propagation to get that done.

Mark V Shunt Feed Matching Network (80m)

Inside of the Mark V matching network. Notice the larger
capacitor in the back upper right, and the new inductor,
both for 80m.

I wrote earlier that I was having trouble with my 80m shunt-feed matching network. I managed to identify the problem using an enormous variable capacitor, but that was only temporary. As it was, the Bread-slicer obtained several spots of rust for the week I left it outside. I needed a permanent solution.

Having nothing in my junk box that was suitable, I found a 80 pF variable cap with reasonable (4 kV) plate spacing on eBay. It would fit into my NEMA box, but just barely.

In my earlier hunt for the 80m issue, I had already re-wound the 80m inductor on a T200-2 core with insulated wire.

While I was doing this, I realized I had no more T200-2 cores. I was giving idle thought to going to 2 T200-2 cores, as I had for the 160m inductor. Double cores would allow me to use fewer turns, so I could use a larger gauge wire, increasing the inductor Q.

T200A-2 core in center, cover in
fiberglass tape on left, and a
finished 80m coil on a T200-2 core
for comparison on right.

While I was shopping on-line, I came across a new product from Amidon. It is variously labelled T200A-2 or T200-2B, but it is essentially a T200-2 core that is 1 inch thick -- the same as stacking two cores together. Needless to say, I bought two.

New 80m inductor.

Using the larger core, I went from 38 turns on the T200-2 to only 28 turns on the T200A-2 core. The most difficult part of this exercise was finding suitable wire in the junk box. I ended up using a bright orange 16 gauge wire that had much thicker insulating than I needed -- but it is what I had. The resulting coil was a bit difficult to wind, and I used a couple of wire ties to keep the ends from unravelling.

New cap and inductor for 80m.

Shoehorning the capacitor and the larger inductor into the NEMA box took some doing. I had to move the relay just to get the capacitor in the box. And the wiring had to be re-done.

I used two 100 pF 6 kV disc ceramic caps in parallel with the 80 pF variable to get enough capacitance to match.

All assembled, it tunes up nicely around 3800 kHz with a 1.1:1 SWR. And the SWR doesn't change at all when going to 100 watts. Perhaps with the new cap and inductor, I could put a few hundred watts through it with no problems. I'll have to wait to fix the AL-80A before I can test that.

Next step will be to get the 160m network to tune better, it's a little off lately, I think I need more capacitance.

Pursuit of 5BDXCC III - 80m

I wrote of this dream first two years ago. Back then, I thought it would be an easy thing, perhaps to work (and confirm) nearly 30 countries on 80m in a single winter. Boy, was I naive. I only added six confirmations on 80m the entire year.

Can this shunt-fed tower put me over the top?

The next  year, I at least had the sense to understand the difficulty of the proposition. I enumerated things I might do that year. In the end, I only did a couple of them. I did not add more radials to the Inverted-L, I did not bring the amplifier up to Floyd county, I never did finish the new K9AY controller. 

However, I was there a lot. I spent a lot of time on 80m. As a result, I now stand at 89/88 confirmations - I added a dozen over the year. That's twice as productive as the previous year. 

If I can pull it off again this year with a dozen confirmations, I should be able to meet all the requirements of 5BDXCC.

It's going to be interesting. At the moment, the Inverted L, with over 30 lbs of radials, is sitting in a box. I'm still trying to figure out if I can put it somewhere where I can lay down those radials safely. My only 80m DX antenna is the shunt-fed tower in Gwinnett county. 

Wish me luck.

Things I've Learned about Antennas - Horizontal Antennas

I've been fooling around with radio for more than 40 years. Finished my first receiver January 1971, so I guess it's closer to 44 years. In those early years, I didn't know anything about antennas. My initial antennas were nothing more than magnet wire strung up around my attic bedroom. They worked - Badly.

Over the years, I've learned a few bits of wisdom about antennas. This article is about:

• Horizontal Antennas

Be they dipoles, center-fed Zepps, yagis, quads, Vee beams or rhombics -- horizontal antennas share one key characteristic -- their most important dimension is height above ground in wavelengths.

The height determines the radiation pattern, impedance and much of the loss. I remember a few years ago on the QRP mailing lists there was a hot debate about one of W4RNL's designs -- the "88 foot" dipole. When LB modeled this antenna -- meant to be a secondary or spare antenna when your beam failed in the middle of a contest -- he did so at 100 feet and also at 70 feet. 

This design was supposed to give reasonable performance on 80, 40 and 20m. 88 feet worked out to be about right. Long enough not to have too crazy impedance on 80m, and short enough to not have a lot of deep nulls on 20m. At 100 feet, I bet it is a pretty good performer. At 70 feet, it wasn't a slouch, either. The odd-ball impedance would make for some loss in the feed line, but for a spare antenna, that wasn't a huge concern.

From the discussions, you'd like that 88 feet was somehow a magic number that made everything work better. Heck, if you have the room make it a full-size 80m dipole then add a couple of traps, for pete's sake. And do you think those QRP stations put up that 88 feet of wire at 100 or even 70 feet? Heck no, they were down at more practical heights of 20-35 feet. 35 feet might be passible for 20m, since it is 1/2 wavelength up. But it is only 1/4 wavelength for 40m, and 1/8 wavelength for 80m. 

Here's the deal: the pattern of a dipole is hugely affected by the height above ground. About 1/2 wavelength, it just starts to have a bidirectional pattern, and only that at pretty high angles. Lower than 1/2 wavelength, it's basically got an ice-cream-cone shaped pattern going straight up. This pattern is rarely desirable.

How high is enough? At some point above about 2 wavelengths, the dipole pattern looks a lot more like free space. For beams, at these heights, you can start to get nulls in your pattern at useful angles, so you have to be careful. Somewhere between 1/2 and 2 wavelengths is generaly the "sweet" spot for horizontal antennas. For specific applications, your best bet is to model the antenna at the desired height and watch for undesirable nulls.

Given that most hams don't have supports for antennas above 50-70 feet, it's likely that any antenna below 20m is too low. Get those antennas as high as you can.

160m / 80m / 40m Inverted-L

Can you see it? I can't either. The black wire in the middle
ascends into the tree, but the traps are virtually invisible

The 160m Inverted-L went up nearly a year ago. Six months ago, I added a trap for 80m, making it an 160m / 80m Inverted-L. At that time, it seemed like a good idea to add a 40m trap. It's taken me a few months to do this.

I used CocoaNEC 2.0 to model the trapped inverted-L. With multiple traps, there's lots of interaction in the antenna segment lengths. More than you'd think, since the trap resonant frequencies are well outside the operating frequencies of the antenna. This means that the trap impedance, while high, doesn't completely cut off the flow of current in the rest of the antenna. This means the 160m segment length affects 40 and 80m and vice versa.

My model showed the 40m segment would be about 32.5 feet, the segment between traps would be about 3.6 feet and the rest of the antenna would be about 69 feet. The problem with this model is that it uses NEC 2 -- which assumes a perfect ground, so the real antenna different. My approach was to insert the 40m trap at about 34 feet, and then slowly trim to a resonance in the band. Then I'd trim the segment between traps, and finally the 160m segment.

Comparison of 40m (left) and 80m (right) traps. Note the
turns on the 40m trap are a bit loose

40m trap was built for 6.7 MHz. This required about 7 turns of wire on my 3" schedule 20 PVC pipe form and a 100 pF capacitor.  Same technique was used to trim the trap -- the Heathkit gate-dip oscillator.

Trap went in at 34' and after three trims it was a 29' 7". A couple of trips and the 80m segment was 7' 4" long. Everything looked good.

Note that the between traps segment is considerably longer than the model. Could very well be the traps I built are not exactly the same as what I modeled, in addition to the lossy ground effects. I think the modeling work is useful, though, because it has gotten me in the ballpark.

Another view. 40m trap uses 100 pF, 80m trap uses 200 pF.

Last trip was 4 feet off the 160m segment, for a total of 63'. Hmm. 40m is now resonating just above the band. Drat.

I'm not sure if maybe the turns on the trap may have loosened when I raised it last, or if the 160m trim caused the shift. I ran out of daylight to test it. However, the antenna still works pretty well. I've been running this antenna on all bands through the KAT3 antenna tuner, and it is pretty effective on the designed bands, as well as on 30, 17, 12, 10 and 6m. It works ok on 20 and 15m, but I always feel I'm competing with tribanders and other beams there.

Hasn't been a whole lot of activity on 80m this fall -- I think part of it is because everyone is enjoying the conditions on 10 and 12m. Hope to see more activity as the nights lengthen, so I can work some more DXCC entities there. I also want to try this puppy in the ARRL 160m contest.

This antenna really worked well during SS CW. I used it on all bands through a tuner. Although, for domestic contests, the 80/40m dipole works much better.

160m / 80m Inverted L

Just left and below center you can see the trap hung up on
a branch at around 42 feet.

From the minute I put up the 160m Inverted L, I had planned to add a trap for 80m. First, however, came more radials. With the original four 125 foot radials, the antenna seemed rather quiet, and that should have been a sure sign it was too lossy.

Adding four more 125 foot radials made a big difference. Noise level went up, along with the performance. Eight more 62.5 foot radials followed, for a total of sixteen - eight long and eight short ones. For any antenna with ground-mounted radials, sixteen should be considered the minimal number of radials. At least, for any antenna not mounted near salt-water.

I used this antenna to work and confirm two new countries on 80m phone in the WPX Phone contest. 

OK, so radials are easy. Not cheap, since a 500 foot spool goes for $45 these days. The next step was to add an 80m trap. The easy way to do this is to simply create a resonant trap on the operating frequency and stick it into the antenna by trial and error.

80m trap wound and set up for
trimming, Note the temporary solder
connections with the capacitors.

However, that's not the most efficient. W8JI wrote an excellent article about making efficient trap antennas. Two important lessons from this article: traps work best when they are made from very high-Q components, traps should never be resonant at the operating frequency. 

With my previous 80/40m trap antenna, I had followed half of this advice -- but still used coaxial traps. W8JI found that these traps are much more lossy than those made with discrete components.

And since wire is so expensive, trial and error isn't the best way either. The tricky part about using traps that are not resonant at the operating frequency is that some antenna current flows in all parts of the antenna at all frequencies. This means adjusting one part of the antenna affects the resonance at all frequencies. And while a trap at resonance offers an effectively infinite impedance regardless of the actual values of capacitance or inductance -- off-resonance impedance is definitely affected by the choice of capacitance and inductance.

So, how does one figure out all these variables? Antenna modeling! I used CocoaNEC, developed by Kok Chen W7AY. While it is pretty easy to use the spreadsheet model for very simple wire antennas, I ran into some bugs trying to model trap antennas. After e-mailing Chen, I discovered that Chen recommends the NC interface (a C-like programming language) for modeling, rather than the spreadsheet.

Once I figured out the NC interface, I started to get better results. Then the virtual trial and error part began. Lots of programming, running and bug-fixing later, I had a pretty good idea what was needed to build this antenna.

I found some 100 pF 15 kV ceramic disc caps from Mouser, and used two of them for 200 pF. I used a piece of 3 inch schedule 20 PVC for the coil form. I computed that it would take about eleven and a half turns on this form of close-wound 14 gauge THHN wire. THHN is really designed for house wiring inside a conduit, but it is relatively cheap and easily obtained at your local home improvement store. The wire is secured to the form by drilling 1/8 inch holes through the PVC.

Proper technique for measuring trap
resonant frequency. Note wooden work
surface and nothing metallic nearby.

Unless you have a vector impedance analyzer (and who does?), the easiest way to see if you trap is even close to the right frequency is to use a Grid/Gate/Emitter dip oscillator. Mine is a Heathkit HD-1250. I found it at a hamfest years ago for about $30 including all the coils and carry case. It was modified to add a switch to test the battery condition on the meter, and whoever did the mod did a great job. Only two things wrong with this unit: the lettering around the meter has completely rubbed away, and the foam inside the case to hold the coils down completely disintegrated. (I have never seen a HD-1250 carry case where the foam has held up)

Once you have your trap built, careful technique is necessary to measure the frequency. The coil end of the dip oscillator couples to the trap. Couple too closely, and the trap will pull the oscillator, and it will be difficult to find the exactly frequency. Couple too loosely, and you won't find the dip at all.

I set my trap on a wooden workbench and cleared away everything metal for at least 12 inches around. First, couple the coils very closely to be sure you can find a dip in the meter indications somewhere close to what is expected. Then slowly move the meter away for less and less coupling. The best spot is where you get a good meter indication, but it doesn't pull the oscillator too much -- which you can see as you tune across the dip. From the photo, you can see that the best spot was found with the dip oscillator coil just outside the trap form.

Great -- so you've got a good dip on the meter with the right amount of coupling. What frequency are you on? Good question. Unless your dip oscillator has an output for a frequency counter (another useful mod), the easiest is just to spot the oscillator frequency in a nearby receiver. In reality, the exact frequency of the trap doesn't matter so much, so the dip oscillator dial is probably good enough.

Completed trap ready to install.

I modelled my traps at 3450 kHz, but the trap I built was resonant at around 3350 kHz, which I deemed close enough. I drilled a four more 1/8 inch holes for securing the antenna wire to the trap.

With the trap ready, it's time to install the trap into the antenna. My model told me that the trap should go around 47 and 1/3 feet above the feedpoint. I started at 48 feet and used an MFJ-259 antenna analyzer to spot lowest SWR. About three trims later, I have 44 feet to the trap bringing the low SWR just below 3800 kHz.

Completed trap temporary installed for trimming. No solder
used on the connections yet, just twisted together. Note
how the antenna wire (black) is looped up and back down
the trap. This will hold it securely in place.

The modeling work told me that the 80m frequency wasn't affected terribly by changes to length of the upper portion of the antenna. I did a couple of quick calculations on my phone and cut the upper portion to 67 feet. Low SWR came right in at 1830 kHz. Perfect. 

Using the analyzer, the actual antenna measurements don't exactly match the model. For one thing, the NEC 2 model software assumes perfect grounds, and the actual ground is a bit more lossy. The model shows very sharp resonances, but the antenna measures more broadly -- indicating expected ground losses. 

Using NEC 4 would allow more realistic ground models, but I didn't feel like it was worth the $300 to get a license to it. In any case, the modeling did exactly what I expected -- it guided me to produce a workable antenna design in the field.

How does it play? Works pretty well on 160m still, and I used it in the WPX CW to work another new country on 80m with 100 watts. Of course, it's not the right season for low band work right now, but I think this antenna has promise for next fall.

Of course, I could slip in another trap for 40m. Hmm. Let's fire up that antenna modeling software....