To: | n4gi@tampabay.rr.com, towertalk@contesting.com |
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Subject: | Re: [TowerTalk] 1/8 wave spaced 80m verticals |
From: | Jim Lux <jimlux@earthlink.net> |
Date: | Wed, 10 Sep 2003 13:50:12 -0700 |
List-post: | <mailto:towertalk@contesting.com> |
At 03:50 PM 9/10/2003 -0400, n4gi@tampabay.rr.com wrote:> At 01:07 PM 9/10/2003 -0400, Roger D Johnson wrote: > I toyed with the idea of feeding > >the elements in phase for transmit and with the delay for receive. > >It made the system a bit more complicated but resulted in better > >transmit signal while still having the F/B on receive. Well... if you're feeding them in phase, it's pretty simple... no calculations needed because the mutual terms don't result in any current flow: think of the two antennas as a pi network, with the mutual impedance being the series "bridge" and the self impedances being the shunt legs of the pi. If the voltage at the two ports of the network is identical, no current will flow in the series element, regardless of what it actually is. If you feed them out of phase, you're basically building a W8JK type two element beam, where the gain increases as you bring the elements closer together, but so does the relative losses. The actual current gets quite high too (think of that series element in the pi net... closer together means lower impedance, so more current into the feedpoint) Even if it isn't too complex to optimize 1/8WL spaced verticals for TX, some have noted only a minimal TX gain (~3db) when phasing verticals.... Is this the norm? Short answer: it depends The pattern you get will be sort of figure-8 shaped, with the narrowness of the lobes determined by the spacing of the antennas. In the limit, with the two antennas side by side, the pattern is identical to the pattern of one element. At 1/2 lambda apart, you get a null (ideally to zero) for signals coming from along the line connecting the two elements. The main lobe is 60 degrees wide (3dB down points) Start spacing wider, and you get multiple nulls and lobes (you're basically building an interferometer) at 3/4 wavelength, the main lobe is 40 degrees wide. Sounds like I might be better off taking one my verticals down and using the metal for ground rods on the remaining one :-) _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |
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