The One About the Hidden Antenna
By Barry Boothe, W9UCW © 2009
One Solution to The Hidden Antenna Dilemma
Having been a longtime student of antennas and one that’s not bashful about spouting off on what I think I’ve learned, I get lots of questions. Because of the widespread application of “Covenants, Conditions, and Restrictions” by homeowners associations, plus the zoning regulations in place today, many of those questions concern stealthy antennas. Recently I received another request for ideas on that subject. It was typical of many others, in that the person had a home with a low attic, no big trees, had small open ground spaces, planned to limit power to 100 watts, and wanted to use the station on several bands to make a reasonable number of successful DX contacts with unspecified stations.
Things that come to mind are antennas made of very fine wire, disguised verticals, raingutter/downspout arrangements, etc. Availability of stable supports, complexities of multi-band switching, ground radial installation, and home construction variables immediately come to mind. My advice would be to ignore those ads for breadbox sized “Wonder” antennas in the ham mags. There’s no free lunch… but there are lots of possibilities that can get you on the air. Will you be the next “DX King?” I doubt it. If that was a part of your plan, I think you should be living somewhere else.
Well, one of the fastest, simplest, and most versatile solutions to your problem would be a horizontal wire loop in the attic. It should enclose the largest area possible, so you'll have to crawl around to get it out to the edges of the attic space. Maybe you should hire a skinny kid to do that job. You can supervise. The shape of the loop is not a big factor. If it were perfect, it would be a circle… and I’ve yet to see one like that. There are no calculations involved. It does not need to be sized for resonance on any particular frequency. "The bigger the better" as they say.
Use any wire you can get, like the smallest gauge single conductor house wiring. Look for #16 or even #18, MTW, TNNN, or TWW. Even 'bell' wire will work if you can get a big enough spool. Of course, you can always splice it. You can staple the wire to the rafters & joists. Although the loop is very forgiving as far as proximity to house wiring and such, try to keep any space you can between it and parallel runs of cables or metal ducts, etc. Oh, and by the way, if you have a metal roof stop reading this and try something else.
The break in the loop, where the feedline wires are connected, should be as close as possible to the ceiling right above the rig. Use good quality 300 ohm TV twinlead for the feedline. That's the old-fashioned flat line with a solid dielectric. Connect each of the two wires in the feedline to one end of the 'broken' loop. Drill a half-inch hole thru the ceiling as close to the wall behind the rig as you can and run the TV twinlead down to the station. The hole can be easily filled and blended in if that ever becomes necessary.
BTW, in answer to that nagging question you have that goes something like "Why can't I just feed it with a piece of coax?" First of all, it’s a balanced antenna. There’s a long, technical explanation as to why you can't feed it with an unbalanced line, but simply put, you're trying to make a silk purse out of a sow's ear and the only way to do that successfully is to follow the rules and don't cut corners. It's the only way it will do what you want to be done.... trust me on this one.
Don't coil up any excess feedline! That’s an absolute No-No with a balanced feedline. Cut it off at the point where you have just enough to reach your equipment conveniently. You want the feedline to be as direct a run as is reasonable between the loop and the station. It should not run parallel to other wires or metal conductors without being spaced at least an inch away in the case of TV twinlead.
As a general rule, any balanced line, ladder line, or open wire line must be spaced away from parallel conductors at least about two times the distance between the two wires. On the other hand, it can pass through perpendicular to a conductor like a metal plate with much less spacing. "How much," you ask. Well, good question! Give it a little breathing room…. perhaps one half the spacing between the two conductors.
You must use an antenna tuner with a balanced output to feed the loop. This will allow you to use the antenna on any frequency on several bands with a perfect 1:1 SWR. The best choice would be a truly balanced output tuner, made for a balanced feed line. But, if you have a single-ended one with a good ferrite balun at the output that offers a connection for a balanced line, try it. Some tuners of this type call for a jumper wire on the back of the box to connect the balun into the circuit. Check your tuner manual. There are some very well-designed auto tuners on the market but if it doesn’t have an internal balun, you will need one outside the box. The same thing is true of rigs with an internal autotuner.
Your feed line will be relatively short and so a single-ended tuner with an output balun may work flawlessly. At any rate, it will "work," but how flawlessly remains to be seen. After all, "Beggars, as well as those with daunting CCRs, cannot be choosers," to update an old adage my grandad used to say.
You'll need a short coax jumper to round out the setup. Connect the feedline wires to the balanced output terminals on the tuner. Did I say that you cannot use coax for that purpose? I think I did. Connect the short coax jumper to the input of the tuner. The other end of that jumper will be connected to the output of the transceiver in the final layout, but first, you need to make a tuning chart.
When you get things put together, an antenna analyzer, like the MFJ would be the handiest device for making a tuning chart while not clogging the bands with "tune-up" carriers, nor stressing your radio. If you don't have an analyzer, borrow one! Or, get some "brewskies" (807's) and ask a friend to bring his "MFJ" over. While he's soakin' up the suds, you can make your chart. If you have an autotuner you can eliminate this step because it will make it’s own “chart” and will memorize it… meanwhile, you and your friend can share the beers.
Connect the analyzer to the input of the tuner using the coax jumper. The transceiver plays NO role in this process. Start at 10 meters for instance, and set the analyzer to a frequency of interest. Tune the tuner controls to find a place where the SWR is as close to 1:1 as possible. Follow suggestions in the tuner manual for "starting points." Mark down the dial settings for that frequency. Now proceed to other frequencies and bands until you have settings for all the places you might likely frequent. This will allow for fairly quick band changing. You might say that this is not as fast or simple as you want it to be. In that case, I might be moved to remind you that before you put up this loop you weren't able to operate HF at all.
This tuning chart task could be accomplished with a low-power carrier from your radio and an SWR meter, but it is much more tedious for you... and for others, if you do it that way. The "how" of getting a low-power carrier out of your modern rig is dependent on the rig itself. There are a dozen variations on that theme. It may be a TUNE position, an RTTY mode position, or perhaps an AM or FM mode. If you use such a method, crank the power as low as you can go and still get meaningful readings on a sensitive SWR meter. Unless you are trying to build up your list of enemies, make sure no one is using the frequency you will be transmitting on, and, for the sake of your rig, limit transmit times to an absolute minimum needed to get a 1:1 setting on the tuner.
This kind of setup can allow for surprisingly effective results on many bands, but it will not make you "King of the Band." I had a loop like this in an outside shop building for many years. I used it for on-the-air testing of rigs on the workbench. The loop was 14' X 28' and was 8 feet above the surface of the ground. It was made of #20 bare copper with several unsoldered splices, stapled to the rafters and in among shop wiring and all kinds of other conducting junk. I could tune it 1:1 on all bands, 80 through 10 meters. It performed famously. For a long time, I had our 20-meter 3-watt QRP sideband transceiver connected to it and never missed a QSO with a station that was at least 55.
Now, can you end up having a sharp null in a couple of directions on certain bands? It's possible. Might you find some frequency on some band where you can't get it down to 1:1? That could happen. Will it be immune to interference problems with phones, stereos, electronic organs, and such? It's hard to say, but probably not. Even with these troubles, it will be a high-value antenna in a place that is not supposed to have antennas.
Oh, and a couple more points. First, as a reminder, the description of this setup is based on the use of a barefoot transmitter in the 100-watt range. If you plan to use an amplifier, we better do a lot of "beefing up" of components. Secondly, I've been asked a number of times about grounding. In the case of a loop like this, the ground plays no active part in the function of the antenna as a radiator. A ground connection will not affect its efficiency as a transmit or receive antenna.
This is not to say that you shouldn't have a station ground for safety purposes. You should. The ground comes into play only as a reflector/refractor of the RF radiation, but that’s out some distance from the antenna. This phenomenon affects the predominant angle of radiation from the system. If you need a sure cure for insomnia or think you need more info on this factor, look in the books under "The Effects of the Ground" and read about the Pseudo-Brewster angle.
The low horizontal loop will have surprising gain as you go up in frequency. It will probably not be the “absolutely best” antenna you could use for any particular contact, considering distance, direction, frequency, band conditions and so forth. On the other hand, it will very possibly be the absolute best antenna to meet the criteria stated, while limited by those daunting CCR’s.
73, Barry, W9UCW
Questions or comments?
1. Mobile Antennas
2. Low Ban Antenna Survey
3. Actual Measured Performance of Short, Loaded Antennas - Part 1 (PDF from original article published)
4. Actual Measured Performance of Short, Loaded Antennas - Part 2 (PDF from original article published)
5. Creeps and Weirdos (Tongue in Cheek)
6. The All Band Antenna
7. The Ultimate Ultimate Transmatch
8. The Invisible 20-Meter Beam
9. The Hidden Antenna
10. The Loop Antenna