The Eagle Has Landed

Steve Katz WB2WIK/6

Introduction.

Most hams who are active on HF are familiar with GAP Antennas, of Fellsmere, FL: http://www.gapantenna.com/

They have been selling HF vertical antennas for close to twenty years now and have four fairly popular models. I built and installed my first GAP antenna, a model Challenger DX-VI, in 1993 and it worked pretty well for a while, although it did deteriorate with age as I'll explain later. I've also installed a few of the other GAP models over the past decade or more, for other people.

In any case, the two popular “rooftop” GAP models are their Titan DX and Eagle DX. This story is specifically about the Eagle DX model installed at the home of Dr. Kate Hutton, KD6HTN. Kate's a new ham licensed this year, but a full-time scientist and has a scientist's attention to detail so it was fun to work with her and Allen Wolff, KC7O, who I've known for many years but hadn't seen in quite some time. The purpose of our get together was to try to de-bug and troubleshoot the Eagle DX antenna installation performed in early July. The antenna was assembled per the factory instructions and installed properly, but just didn't work, so I was called in to see if I could shed any new light on the problem.

First visit.

First thing we did is check the antenna both at the bottom of 50' of RG-213/U coax and also right at the bottom of the antenna - one more time - using an MFJ-259B antenna analyzer. And, as had previously been confirmed, the antenna didn't really resonate in any of the ham bands, and its “best case” VSWR was poor in most cases, even where “dips” were found.

We removed the antenna from its mounting over Kate's roof and laid it down. We measured every single piece of tubing (numerous) to assure ourselves we didn't have any rogue components in the mix, and all measurements confirmed the factory supplied tubing dimensions (received additional dimensions from Chris). We double-checked all connections both visually and using a portable Ohmmeter.

[I must say that the GAP assembly instructions are not very good, and weren't very good 16 years ago when I built my first Challenger DX (in fact, they likely haven't changed). The antenna is fairly simple but the instructions make it seem very complex, and include too many words and too few pictorials, and not a single actual photograph of any of subassembly - which would make checking the field assembly so much easier. As someone who has spent years in technical writing, I'm astonished the assembly instructions aren't much better than they are, as they could be drastically improved by any technical writer in a couple of hours. (I like the “IKEA” instructions: They use no words but many clear pictorials, making them universally useful anywhere in the world.)]

In any case, Kate, Allen and I all poured through the instructions once again to confirm the antenna had been assembled 100% correctly, and it was. I remembered the other GAP antennas I had assembled years ago all had a capacitor assembly installed at the top of the upper radiator tubing section, and quizzed Allen about that. “Yes, it's there, inside somewhere.” I figured we better fish it out and look at it, since it's the only variable we had not yet explored.

Once the antenna is assembled, the only way to access the capacitor is by taking the top sections of the antenna apart again, so we did that. The capacitor is actually in series with the uppermost antenna connection, so one wire from the capacitor, via an automotive-type “Fast-On” connector, attaches directly to the top of that element. The other connection to the capacitor comes from a length of embedded coaxial cable that feeds up through the tubing.

I removed the capacitor and inspected it visually. The cap is a potted assembly, but the potting is nearly clear so you can see the dipped mica capacitors inside quite clearly. They are a pair of 1200pF units in parallel, which obviously should yield 2400pF total capacitance. The caps looked okay, but we thought it would be a good idea to measure them.

Allen happened to have a handy portable battery-operated capacitance bridge at home, so he took a short ride home, potted GAP capacitor assembly in hand, to make the measurement. When he returned, I asked, “Did you find anything wrong?” and he answered, “I think so.” I was hoping he would, as there was really nothing else left to check.

He had brought the portable instrument back to Kate's house with him. We clipped it to the capacitor terminals protruding from the potting compound. Clipped “gently,” the unit measured 2200 pF, pretty close to what it seemed it should be. But if we pushed a little bit on one terminal, the reading dropped to 30pF. We repeated this test a dozen times, with the same results. There was an internal intermittent connection!

We inspected the wires in the “Fast On” terminals, and could see only one wire coming through each crimp connection (not two, as we figured we should see, since there are two capacitors in there, each having two leads). The leads are crimped and not soldered. There was an obvious air gap in the “intermittent” terminal, allowing that terminal to either make contact with the capacitor lead, or not, depending upon the pressure applied.

Eureka!

Here's a photo of the original GAP capacitor assembly, with terminals potted, crimped and unsoldered:

Information exchange.

This started a series of e-mail exchanges with “Chris” at GAP. Kate began that with a description of what we had found, and notifying the company she was Fed Ex'ing the faulty part back to them with expectation of a good replacement unit. She did receive a new replacement and 2 additional caps with the leads twisted together and 2 additional connectors only about five days later, which was reasonable service.

In the interim (during the five days) we had a number of e-mail exchanges with the company. Allen and I strongly recommended the capacitor leads be soldered, since that's the only way to assure a real connection when crimping a solid wire into a connector which is actually intended for use with stranded wire. The connectors used are also actually intended for minimum 16AWG wire, and the little capacitor lead is about 20AWG, so it's undersized for the terminal. Chris replied they only crimp the leads, and never solder them, and they never have any problems with this. I continued to recommend solder, and he replied they wouldn't solder these terminals as the soldering heat could melt the connector insulation.

I did some research on the terminals used to find they are absolutely rated for soldering heat, with a 15 second dwell at 250°C (such as using a solder pot). When Kate received the replacement unit, Allen did solder the terminals to the wire, using heat applied to the sides of the terminals and flowing solder down inside them to reflow. He did a great job, and the insulation still looks brand new, no signs of melting or scorching. So the company's refusal to solder based on that theory doesn't have much merit.

The second visit, and the fix.

Here's a photo looking “into” the new (replacement) potted capacitor assembly terminals, after Allen flow soldered them: Now, the leads are very well soldered to the terminals, with no damage to the terminal insulation.

We re-assembled the antenna with the new, soldered capacitor module and re-installed the Eagle DX the same way it was originally installed, and now found it “dipped” in each of the amateur bands, in some cases down to <1.5:1 VSWR, except for the 15m band, which experiences two distinct dips, one each above and below the ham band. Kate was primarily interested in 20m and 40m right now, since these are the “most open” bands at this point in the sunspot cycle, and the antenna loaded up well on both. (Note the best readings were achieved only after experimenting with the transmission line earthing point as discussed at the end of this article.)

During our e-mail conversations with Chris at GAP, he continued to claim this problem really doesn't happen so our experience was an anomaly. However, in reviewing the Eagle DX “product reviews” on the popular website eHam.net, I found this antenna has a user review rating of 3.7 (out of a possible 5.0) and 13 out of 51 reviews stated the antenna worked very poorly, didn't work at all, wouldn't load up or resonate, or some similar problem. That's 25.5% of the reviews, which can be found here: http://www.eham.net/reviews/detail/452

I brought that to Chris's attention and he replied these are users who did not assemble the antennas correctly, or did not install them properly, or some similar problem. I agreed people sometimes make mistakes, but I doubted 25% did. In researching some of the reviewers who gave the antennas bad marks, I found half of them were very experienced amateurs, some had engineering degrees, and most reported they were perfectly satisfied with some other vertical antenna they had chosen to replace the GAP, installed at the same location.

As such, I speculated that these users, or at least some of them, very likely had exactly the same problem we experienced with Kate's antenna. Chris pushed back on this and didn't accept that hams do things properly, and I replied that in our case, we did: And while he was dealing with a neophyte ham in Kate, she's a Ph.D. scientist, and was assisted by two graduate engineers in Allen and myself, so I didn't think we were all idiots. I didn't receive any more correspondence after that, but Kate did get a new replacement part, and it did work.

Finally, it works!

Photo of the finished, installed Eagle DX atop Kate's roof (the guy lines are just lightweight ropes, recommended by GAP to prevent wind damage):

Hopefully this piece will help resolve some other hams' issues with GAP antennas, which are all of similar design. If the capacitor is open or intermittent, that does create many problems - and you can't really tell if the cap assembly is good or not just by looking at it: It must be measured.

A last bit to add: GAP's instructions call for making a coaxial RF choke current balun of the transmission line, and bonding the outer conductor of the transmission line where the choke attaches to the coax supplied with the antenna (short pigtail out the bottom of the lower tubing) by strapping that UHF “barrel” adapter directly to the very bottom of the antenna (prior to weatherproofing that assembly). We did that, then we “un-did” that, to see if it made any difference. It does, but not a lot. However, this isn't the end of the story…

Installing the antenna, Allen did a great job of using a high quality lightning arrestor bonded directly to an 8' ground rod at the base of the antenna support mast, before the coaxial feedline enters Kate's home. That, of course, introduces a second “shield” grounding point, directly to earth. When we first measured the “repaired” antenna, VSWR readings still weren't great, as measured “inside the shack,” at the rig end of the coax, which was 50' long from the lightning ground point to the radio.

Here's a photo of the lightning arrestor ground point outside the house:

We disconnected the lightning arrestor ground by bypassing the arrestor entirely and isolating the coax from earth where it had been previously tied. Bingo! Now, the match across the bands was greatly improved and we enjoyed the results discussed above. Re-grounding the outdoor splice point, with or without the arrestor, brought it back out of resonance again, with higher VSWR readings in most places.

Fun experimenting with the cable lightning ground attachment and lengths of coax:

This obviously indicates the transmission line is not well isolated, and still very much “part of the antenna.” Kate e-mailed this information to Chris at GAP, and he responded that, “Yes, we find it's best to use more than 50 feet of coax…” I don't doubt that, and he's probably correct - but this indicates a design deficiency. Any time your feedline is part of your radiation system, this becomes an uncontrolled element that cannot possibly be optimized across multiple bands. This, too, could explain some of the “bad results” by hams reporting low grades for the antenna on eHam.net: They just didn't use the right length of coax!

I mentioned earlier that my original GAP Challenger DX had “deteriorated” over time. This is anecdotal and may be of help to someone: My Challenger DX, installed in 1993, actually did work fine when first installed. Some time later (I think about two years), I was on 80m CW operating with a kilowatt during a contest and noted my output power had dropped significantly, and SWR rose from normal to very high. I stopped transmitting on that antenna and switched to something else.

Inspecting the antenna from the ground the next morning, I could see at least one, maybe two of the interconnection wires that come from the center “gap” feedpoint to the parallel elements that serve for the higher bands appeared to be broken. I had to take the antenna down and lay it on the ground for better inspection. The wires were not just broken, they were “burned” clean through. Hmm. I started disassembling the antenna and ultimately retrieved and removed the capacitor located at the top of the main upper radiating element. It also looked burned, and had failed.

I ordered a replacement and installed that a couple of weeks later, repaired the burned wires, and the antenna was up and running normally again. Over the ensuing years (I moved from that house in '99 and removed the antenna at that time), the antenna failed again, but this time it was evident that birds had been pecking away at those wires for some reason - maybe they tasted good? In any case, the external wiring used by the GAP antennas is a weak point and if I were to redesign for longer life, I'd replace them with solid aluminum straps that won't fall prey to local wildlife.

Finale.

Kate's antenna now works, but she's just getting started on HF and in ham radio, so it will take some time to tell how well it works. At least it resonates, and with time…and propagation…she should enjoy many contacts. To her benefit, although Kate's a new “no code” ham, she did take it upon herself to learn code and her very first contact with the new Eagle DX was on 40m CW!

ADDITIONAL COMMENTS FROM ALLEN WOLFF KC7O FOLLOW:

The original “bad” cap had two 1200 pf caps and measured 2200 pf when working. The new caps consisted of two 1100 pf caps and measured 2200 pf.

When I did the rework to the “new” cap, I Dremeled away the insulation, (see picture).

That exposed the crimp, which I tinned. With a thin piece of solder slid into the exposed crimp hole at the spade end, I heated the tinned crimp area and the solder flowed into the hole.

Once both lugs were soldered and cool, shrink sleeving was placed over the encapsulated cap, heated and formed a dam. Epoxy was then poured into the dam filling it and covering the solder on the lower end of the space lug.

Current configuration of the antenna coax, arrestor radio is:

Antenna

25 feet of coax (antenna to ungrounded splice to experiment with feedline lengths)

50 feet of coax (experimental addition spliced to first 25 feet from antenna)

Arrestor (grounded)

50 feet of coax (arrestor to shack)

Antenna switch

Radio/tuner

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