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Conducting the Autopsy on a Dead Antenna
By Don Keith N4KC
Copyright 2018 by Don Keith
Maybe I have seen too much CSI on TV. But as I examined my recently expired homebrew hexagonal wire beam, I felt for all the world as if I was conducting an autopsy. An autopsy on an old friend who had stood by me faithfully for better than two years. But I also realized that such a post mortem can be helpful, to me (and hopefully to you) in future projects of a similar nature.
Let me back up a few steps. Several years ago, I began research on various possibilities for an antenna that would offer me some gain, some zapping of signals off the back side, and some side rejection, yet would otherwise fit a long list of other requirements I had imposed on myself. You know, the usual stuff like size, weight, cost, availability of components, and complexity of construction.
There were several possibilities, but I settled on the hexagonal wire beam, that odd-looking critter that most resembles an upside-down umbrella, but without the fabric on a real umbrella that keeps the rain off our bald spots. One reason for its choice was because truly wonderful notes and directions for its construction existed on the Internet, courtesy of Leo Shoemaker K4KIO and Steve Hunt G3TXQ. And I kept talking to guys on the air with big signals who sang their praises, usually at several dB over S-9.
So I gathered materials in my usual plodding way and built it, veering off in a few ill-advised (in retrospect) directions on my own. I was (and still am) so proud of my little upside-down umbrella that I wrote an article about it on eHam.net. It really did perform beyond my expectations, and especially for what amounts to a two-element beam with its elements folded into shapes a contortionist would avoid.
At least it performed well until a few weeks before Thanksgiving. That is when I noticed the SWR running higher than normal. I had long since stopped looking up at the beam every time I was outside and had started just assuming everything would be fine. But a quick glance upward the next time I was outside told me that the 20-meter wire element had come loose and was draped all over the other elements. And the 17-meter element hung down below the spreaders and base plate on one side. Somehow, the top two elements had let go their grip.
Aarrgh! Well, that was not exactly what I said, but you get the idea. The neighbor kids have sensitive ears.
So, I asked myself, do you pull it down and fix it or do you spring for a commercially made model that you can trust up there?
I wrestled with that quandary for several weeks, continuing to use the antenna at 100 watts and, believe it or not, actually working a couple of new ones.
See, I confess I have been concerned about a couple of things in my construction. One was that I had put more tension on the 20-meter wires than Leo’s instructions recommended, mainly because it looked perfectly fine to me the way I wanted to do it. And I also worried about where the driven elements tied into the bolts at the beam’s center post. I had used short coax jumpers made from stiff RG-8U between each set of elements. The beam uses one feedline for all bands and the jumpers to connect from the top down the length of the center post between sets of bolts that served as terminals for each set of driven-element wires. I had worried that those would either work their way loose or start taking on water, regardless the amount of goop I had used to protect them.
There was one determining factor in my ultimate decision about repairing or starting over: I no longer climb towers, and I do not wish to keep someone who does on retainer, so whatever went back up there would have to last.
See, the truth is, I know the idiot who built my hexbeam. I did not totally trust his fix any more than I did his original construction job. I bought a commercially made beam after doing some more research. Lots of research, because I am a miserly so-and-so.
There are several very, very good suppliers of these beams, but I chose the one sold by K4KIO for several reasons of my own. I have not regretted the purchase. K4KIO, DX Engineering, and Mike Traffie all offer wonderful products and all seem to have stellar customer service. I chose the one I did primarily because I liked the way the elements attached to the center post, which is sealed, and the fact that the antenna is not a kit and appeared to be very easy to put together, even for a guy with minor nerve damage from a previous immune-system disorder and who lacks most normal mechanical skills.
In short, this version offered no short, stressed, thirsty coax jumpers. Good ropes out to all the spreaders to help hold the shape. And only a couple of hours of work to put it together. Even the price was not outrageous when you consider the fact that it is a six-band beam—I opted for the six-meter element, too, which would never have worked in my previous homebrew configuration.
When I got the old beam down, I decided to do a careful autopsy, not with scalpel and bone saw but with nut driver and diagonal cutters. If you do likewise, I suggest you do all you can to keep the antenna intact, just as it was, broke stuff and all. Avoid the temptation to tell your tower guy to just drop it. Try not to do any more damage on the way down and over to the corner of the yard for its autopsy…er…inspection.
When you get the chance, give the thing a thorough going-over. What else broke that you did not notice or that did not show up on the SWR meter? What gives the appearance of being on the verge of going bad before the other malady happened first? What can you do in the future to avoid this? On the other hand, what has held up well?
Here is what I found in my case:
· My so-called stainless steel hardware from a big-box store rusted terribly. The bolts in the center post that held the ends of the wire elements were the worst, though they had not yet failed. It would have been only a matter of time, though.
· On the other hand, the stainless steel U-bolts and hardware that I bought along with a commercially made aluminum base plate (from W4RDM) looked as good as they did the day the thing went up. Bright and shiny.
· The fiberglass spreaders—also purchased commercially, from Maxgain Systems—looked fine. I did spray-paint them prior to construction to protect from those pesky UV rays.
· Amazingly, those coax jumpers I most worried about held up just fine. I pulled the insulation back on a couple of them to see if they had taken on water and I saw no signs of it.
· The antenna wire—a non-insulated flex-weave with stainless steel strands woven in—showed some signs of rusting but except for a few strands that had sprung loose, still seemed to be solid, even on the two elements that were drooping. The wires had not broken.
· Ah, the real culprit. I used tie-wraps through hose clamps to hold the wires in place on the spreaders. At high-stress points, I backed those up with small bits of rope. Well, either the antenna wires sawed through several of the tie-wraps—maybe because of the wind moving the spreaders a bit—or the “UV-proof” tie-wraps weren’t. Several of those not also supported by short lengths of rope had broken and that was what allowed the elements to break loose. If they had held, I would not be writing this article. At least until the rusty “stainless steel” bolts gave way.
· I did discover a double-whammy shaky connection in my coax connector that attached my feedline to the pig-tail I had going up to the antenna feedpoint. Whew! Glad I did! Turns out both the PL-259 and a barrel connector there had problems and were intermittent. That gave me and my tower guy a bit of heartburn for a few minutes until we isolated the problem. Neither of us wanted to take the antenna back down and check connections on it again. (And yes, we had done that—thoroughly—before we put it up there! The problems only showed up when we did the weather-proof wrap at the junction point.)
Take advantage of having everything apart to thoroughly inspect feedline, connections, rotor and anything else that stays out there in the weather 24/7 while you are inside in your toasty, warm shack. Sometimes we concentrate on fixing whatever the glitch is and overlook something else that could go goofy on us.
So, if I decide to build a wire beam in the future, I will choose better quality hardware and use some kind of backup to the tie-wraps. I will also be more careful about putting undue stress on the wire elements and their attachment to the center post. I do still have a perfectly good base plate and spreaders and am contemplating doing a single-band, portable version, just for the heck of it.
By the way, I do not want this article to reflect negatively on the hexbeam, bought or homebrewed. It is a very good antenna option if you want something light, durable, and surprisingly effective.
You know, I still have a suspicion that a very large hawk that was frequenting our neighborhood—hopefully feasting on coax-insulation-eating squirrels, but that is another story—may have had something to do with the sudden failure. We did not really have any serious wind or ice, but that phoenix-like critter might have hurried along the inevitable failure of my pride-and-joy beam.
Do hawks forfeit any protected status if they get in the way of my quest for DX and long-winded ragchews?
We’ll see what the guys at CSI have to say about that one!
Don Keith N4KC has been a ham radio operator for more than fifty years. After a long career in broadcasting and advertising, he now writes full time and has published more than thirty books, fiction and non-fiction, on a wide range of subjects including amateur radio. See www.donkeith.com or www.n4kc.com for more info.