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Summer 2004 Issue |
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 Photo A. A 915-MHz circularly polarized “Cheap Patch” antenna. |
ANTENNAS By Kent Britain
WA5VJBConnecting the |
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What a week! As I start this column, power and
telephones have been out for four days. The computer did not like getting
power from my generator, so I’m typing on a Sharp palmtop and some
lashed-together gel cells. Consequently, a lot of the planned construction
projects have not had a chance to be tested, but we came up with something
to talk about this time. There are quite a few ways to generate circular polarization with a patch antenna. One way is to split the signal into two paths, delay one path 90 degrees of one-fourth wavelength, and then feed adjacent sides of a square patch. Use of a quadrature hybrid is highly recommended, but it isn’t essential. The second way is a slot through the patch at a 45-degree angle, which is certainly easy enough to do with a hobby knife and aluminum-coated styrofoam. However, my metal tabs for soldering coax kind of mess this up. I first saw this third method used on a multi-band AMSAT dish feed at the Weinheim, Germany Hamfest. A circular patch was driven much like a square patch, but 45 degrees to one side of the feed. A variable capacitor to ground was used to generate a phase shift to change the patch from linear to circular polarization. This technique is very good, but to tune it up, you really need a network analyzer.
Okay, I go for cheap and easy. Just cutting
the opposite tips off the square will create enough unbalance to generate
circular polarization. Trim off the upper right corner (and lower left)
and you’ll get RHCP (right-hand circular polarization). Trim off the upper
left corner (and lower right) and you’ll get LHCP (left-hand circular
polarization). There are many jokes floating around the antenna community
that the opposite of right-hand circular polarization is really wrong-hand
circular polarization, but I think I’ll save that one for an April 1st
article! Click here to return to Summer 2004 highlights Click here to subscribe to VHF _________________ © Copyright 2004, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.
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