Re: Polarization Loss?

SteveO232 wrote:
I'm trying to throw together a helix antenna, similar to one shown
here:
http://www.antenna-theory.com/antennas/travelling/helix.php

I'm trying to get it to have circular polarization, but it deviates
away from that pretty quickly away from the main beam. If my receive
antenna is also circularly polarized, is there a way to estimate the
polarization mismatch loss based on how skewed the polarization is? Like if its elliptically polarized, how can you figure out how much
power a circularly polarized antenna would receive from this?

The mismatch won't exceed 3 dB as long as the sense (right or left hand) of the elliptically polarized wave is the same as the receiving antenna. Here's why:

An elliptically polarized wave can be mathematically split into left hand and right hand circular components just like a linearly polarized wave can be split into orthogonal (e.g., vertical and horizontal) components. A right hand circularly polarized antenna will respond only to the right hand component of the elliptically polarized wave, and a left hand circularly polarized antenna will respond only to the left hand component. So the general answer to your question is that the received signal will be proportional to the circularly polarized component of the wave which has the same sense as the receive antenna.

As you make a right hand (for example) circularly polarized wave more elliptical, the right hand component decreases and the left hand component increases. This continues until the wave becomes linearly polarized, in which case the constituent right and left hand circular components are equal and 3 dB less than the total, linearly polarized component. If the wave becomes elliptical and increasingly circular in the other sense (left handed for the example), the right hand component continues to decrease and the left hand component continues to increase. When the wave becomes completely left handed circular, the right hand component is zero, so there will be infinite attenuation when received with a right hand circular antenna.

In general, both the transmit and receive antenna will be elliptical, so both components (right and left handed) have to be evaluated separately and summed.

Me, I'd just model the system with EZNEC+ and let it figure it out for me. A number of other programs are able to do it also.

Roy Lewallen, W7EL
.

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