Date: Tue, Oct 4, 1994 12:23 AM EDT From: becker@shell.portal.com Subj: Summary: Invisable Antennas To: KA9FOX Looks like we do have some ideas for the stealth antenna out there. Here is the promised summary of constructive comments. From: Dave G4BUO /G=LAWLEYD/S=LAWLEY/PRMD=IBMMAIL/ADMD=IBMX400/C=GB/@mhs-relay.ac.uk I regularly use a half square antenna on 40m (gardens and supports are smaller over here) and I call it a half bobtail which I think is more descriptive. It is an excellent performer and unobtrusive. If you model the antenna over 'real ground' in Elnec the pattern is not good, but over perfect ground the pattern is excellent. This shows that although the antenna does not need radials, it is strongly dependent on good local ground conditions in order to work well. The original posting suggests that if you feed it at the bottom corner you need radials. This is a high impedance point and you can either use a zepp feed or an LC network. In the latter case, a ground rod or short radial is all that is needed. A set of radials in the conventional sense is not needed. As with any vertical, you could improve the performance by extending radials out to several wavelengths unless the antenna is already placed on salt water. I saw the QST article and am a bit uneasy that coax feed at a top corner may lead to current imbalance, depending on the position of the coax. Feeding at a bottom corner is nice and easy. Remember the bottoms of the vertical wire carry LOTS of voltage so should be protected from accidental touching by children, pets etc. From: Roy Hradilek, AD5Q, 73374.2465@compuserve.com I have been modelling half squares with ELNEC and AO and find them very interesting. There is no perceptible difference between the half square and the full Bobtail using ELNEC models. Looks like the simplest half square is corner fed at the top: easy 50 Ohm match. I have tried this and it works. From: Ed Gilbert, WA2SRQ My suggestion for your antenna improvements: Forget the indoor antenna. Get some #24, 26, or 28 wire which will be nearly invisible outside. Run something center-fed with open wire, as high and as long as you can. With open wire you will be able to use it on all bands with a tuner. From: Ken Silverman, WM2C, ken.silverman@atlas.ccmail.AirTouch.COM Wires in the trees is a good bet. You can run a nice vertical/inverted-L up the tree. Just shoot a line over the tree, and don't care if the inverted-L part is curved, bent etc. Will work much the same. Use camouflaged wire! Try brown, black, or my favorite in our neck of the woods (California), a green and yellow spiral stripe. In my area, most of the year, the foliage is a combination of evergreen trees, and golden dried out grasses. When hanging in the sky, the wire blends in perfectly. Even appeased a cranky neighbor. From: Palle Preben-Hansen, OZ1RH (OZ9EDR contest team) Do you know the trick with antennas made of very thin wire << 1 mm? phosphorous bronze quite strong and available as wire. I have used it once, though the material is more often used for springs in relays ect. Try a full size 2x20M dipole for 80M in the top of your trees. Isolators can be small pieces of plastic. Replace the wire if it breakes/is stolen/taken down by authorities. The wire may be errected using bow and arrow. If a flagpole is allowed try that. Better still: move to the country side! Contests -> QRO -> TVI/BCI! One other possibility: telescoping fishing rods in fiber glass are available in lengths over 30 ft, and may be put up in the dark and support some wire. Carbon reinforced fishing rods may be conductive, this could be a problem if you wind the wire around it to create a coil. From: Pete Smith, N4ZR, N4ZR@netcom.com For a number of years, I used coax to a set of dipoles fed from the same center insulator. Had 10,20,40 (also used on 15) and 80. Radiators were fanned out both in altitude and azimuth to minimize interaction. Should be little problem with your tuner anyhow, if you can bury the coax and get it under your wall and out to the trees. If you can string wire from your home to any sort of existing support, such as a tree, some people have reported good results with wire as fine as #30. Of course, you'll be restringing it a lot, but an end-fed long wire might be preferable to what you have now. You could also use such fine wire for a half-square, if you were of that mind. Depending on how much metal there is in the structure of your house, you might be better off with wire strung right under the eaves, rather than in the attic. From: David McCarty, K5GN, dkmc@chevron.com There was a post on here a few days ago about feeding a half square at a 50 ohm point (the corner). I bet you could build one for 20m and get it up where it would help you. I built a full bobtail curtain for 15m and hung it over my house roof (one story). It was about 2:1 at the resonant point, feeding it with 50 ohm coax at the top of the center vertical element. It was not *real loud*, but better than a dipole up in the trees, which qualified as an improvement in my book. I expect it would be far better than an indoor dipole. One caveat about such an antenna: the high voltage at the ends of the wire, which is usually in reach of unsuspecting neighborhood kids. From: Dan, (no call given) PEMS_ST_DK@noeca.ohio.gov I have used the half square and would highly recommend it. I modeled many versions of it and have found the following: 1. It cares not about a ground system. The ground effects are determined by the general condustivity in your area - FORGET RADIALS. 2. It doesn't have to be straight - it can be bent and run along the ground to make up missing height (ex. 20 feet vertical and 10 horizontal.) 3. Spacing is not real critical. You still get some low angle radiation if the top part is not 1/2 wave. 4. CAUTION: The bottoms of the vertical ends are HOT so should be kept away from human contact !!!! 5. Feed with a parallel tuned network at the bottom of one vertical elemant. You can load it on more than one band using a switched network. I used a 40 meter job on 160 with good results (this use DOES call for radials) 6. In short, if you can get one up, even bent and twisted, do it. I used one from an apartment and it worked fb. From: Jim, KD0AV, JimKD0AV@aol.com (we used to do FD together ed.) I too am condemned by restrictive CCR's to stealth antenna: I use a center fed zepp @ 40 feet in large maple trees, a biodegradeable set up that can come down occasionally due to proximity of the Windy City. Hard to get through on SSB with low power. Also, I have been looking at half-squares and other goodies that show up in ham mags. Editorial comment and semi-humorous further challenge So, fellow hyper-texted, hyper-competitive, hyper-testosteroned, type-A, aerial-aerially-challenged condo-commando-contesters ... draw breath :) It seems clear that the conventional wisdom (or could it be just a passing fad) calls for AE0M and his spin-casting rod to be over the garden wall and up in the trees with a half-squared length of two-toned litz wire before the NCCC bored drums him out for sub-standard SS scores again. BUT! How to feed the beast? Digging a trench deep enough throuth 40 feet of the adobe hard clay (are these just rocks are did someone put re-bar in here?) we make due with for soil in the west Santa Clara Valley is more than this swivel-chair cowboy is willing to attempt with this level of evidence. I want PROOF! Well, I'll settle for anything I get. Just how does one go about this fed- at-the-corner method. Can I just let the coax drop down along one vertical side of the half-square, or will the RF induced on the outside of the coax be enough to curl my mustache? How about if I use one of those "current baluns" at the feed point? Remember, we are the best of the best here, so any further comments? E-mail me, and I'll summarize for the reflector. AE0M - Tony Becker - becker@shell.portal.com - Silicon Valley, U.S.A.