I can't see a problem in making a test unit doesn't look like rocket science
http://www.hardhack.org.au/polarisation some copper and solder http://www.astronwireless.com/topic-archives-antennas-polarization.asp To: Repeater-Builder@yahoogroups.com From: k...@catonic.us Date: Thu, 2 Sep 2010 00:31:15 -0500 Subject: Re: [Repeater-Builder] Circular polarization for VHF repeaters? On Thu, 2 Sep 2010, Gary - K7EK wrote: > In the 80's there was a amateur radio repeater book by a fellow, > Pasternak I believe, that took two gamma match style Cuschcraft Four > Pole antennas, combined them, and did some magic with phasing lines to > end up with a four bay circularly polarized repeater antenna. > Unfortunately the description leaves much to be desired, at least for > me, so I never built one. If he would have included specifics on > phasing line lengths, cable types, etc, the job would have been a > whole lot easier. Has anyone actually gone circular with Cushcraft > Four Poles, and if so, could you please share it with me and/or this > group? > > I have done some inquiring to commercial companies about a custom > built two meter four bay circularly polarized array, but that is > entirely out of the question. They want thousands of dollars. There > must be an easier (and cheaper) way. > > Similarly, is anyone in this group running circular polarization on > your amateur repeater(s), and if so, could you please share the > details in a manner that could be duplicated without a lot of guess > work? > > I know that I could easily solve my multipath problem by installing > one or more remote receivers, however I would like to keep that as a > last resort and shoot for a circularly polarized antenna system at the > main repeater site. I do understand that there is approximately 3 db > of loss as a result of this, but that is quite acceptable. The > dividends would greatly outweigh the down side. > > Thanks for any constructive ideas, suggestions, links, etc, that you > might be willing to share concerning this situation. There's also a recent article in QST about a passive Lindenblad using an active dipole as a center section and four passive aluminum wires suspended from a plastic mechanism. I've looked at that and said... that wire/plastic assembly, and one of the old Motorola TAD series dipoles ... would make a DC-grounded circularly polarized antenna that should be good for a few hundred watts, and a minimally preferential (non-circular) pattern. You're in unexplored territory. The best way in is to locate a low-power FM station that upgraded to a higher-power transmitter, buy the antenna from them, and get Jampro to cut it down for your frequency. Of course, that's real dollars... anything else will be improvised. It would be a good idea to look at the terrain you're trying to cover and see what beamwidth fits it best. If you have the money to spec out an antenna with some null-fill and a lot of gain, like a customized DB-228, you'll find that coverage is second to none. Typically speaking, you don't need much antenna gain to cover close in to the tower because the distance losses are less. And you can always address that with a 1/4 wave dipole on the top of the building connected to a voter reciever. Theoretically speaking, circular polarization results when the vertical and horizontal components are 90-degrees seperate from each other. The Cycloid dipole accomplishes this simply, using a vector sum of the two to make circular polarization. It's synthesized -- 1/2 horizontal + 1/2 vertical = so many degrees hypotenuse. It gets complicated to start working up in those regions, almost to the point of designing panel antennas and putting one on each side of the tower to get a decent pattern. It's difficult to phase antennas and preserve some form of a pattern with them. -- Kris Kirby, KE4AHR Disinformation Analyst
