Nate, The advantages of circular when both antennas are circular is not the 3 dB gain, but the ability to except the rotation of the antenna without signal change and cancellation of the first reflection (reduction of multipath fading) which produces steady signals. Yagi types are subject to receiving reflections from the side and are only circular polarization on the on axis lobe. Off axis reverse circular rejection is limited to the forward gain and side lobe rejection of a Yagi antenna. Helix and QFH antennas are circular over the entire envelope. The use of dish reflectors on frequencies of 1.2 GHz and above is recommended. There are several designs, one in an old AMSAT Journal of a patch feed and polarity switch that works well as a RH/LH circular feed for the popular TVRO satellite dishes that are abundant.
Art, KC6UQH
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Nate Duehr Sent: Saturday, September 20, 2008 11:29 PM To: AMSAT BB Subject: [amsat-bb] Re: Polarity questions
On Sep 20, 2008, at 2:07 PM, Edward Cole wrote:
At 01:53 AM 9/20/2008, Luc Leblanc wrote:
On 20 Sep 2008 at 0:56, Nate Duehr wrote: ======snip
That was a weird snip... made it look like my comments were left in that question and statement from Luc.
I do have a question for the polarity gurus, however...
When *transmitting* circular vs. linear polarity, are the apparent losses the reverse of the numbers being used for receiving? I am inclined to think that has to be true.
Or to put it as a question: With all else being equal, including antenna gain numbers...
How much more power has to be put into a circularly-polarized antenna versus a linearly polarized one of the same gain, to have a distant station receive the same signal if they're linearly polarized?
I'm purposely coming at this from the "other" direction... the transmit side, instead of what is seen at the receiver.
This is mostly a mental exercise to see if I "get it". So if it's a goofy way to analyze it, ignore that for a moment...
-- Nate Duehr, WY0X nate@natetech.com
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