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If a 2 meter J-pole construction is imperfect enough that
the short and long vertical portions are not exactly parallel it might require an insulated piece of something to force them to be parallel. For what it's worth - the "American Legion" aluminum-rod J-poles which are quite popular in my area have just this sort of separator near the top of the matching section. It seems likely to me that the junctions between the rods, and the aluminum crosspiece which forms the bottom of the J-pole are not rigid enough to keep the rods parallel over the lifetime of the antenna, and that bracing is necessary. These J-poles use a somewhat uncommon feed technique - the SO-259 is attached near the center of the crosspiece and grounded there, and the short side of the matching section is fed through a bent-wire transmission line and gamma match. http://www.pcarc.net/jpole.pdf shows the construction, including the crossbrace. I'm not sure just what they use for the cross-brace - the document indicates acrylic. This doesn't seem to stand up to sunlight all that well in the long term - if I see an American Legion J-pole which has failed, it's usually due to the cross-piece breaking. I'd suggest ABS, black Delrin, or some other plastic which is more UV-stable (or, just coat clear or white acrylic with paint to shield it from UV). -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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