I'm trying to throw together a helix antenna, similar to one shown he
http://www.antenna-theory.com/antenn...ling/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?

"SteveO232" wrote in message
...

I'm trying to throw together a helix antenna, similar to one shown
he
http://www.antenna-theory.com/antenn...ling/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?




--
SteveO232

Hi Steve

I dont have the answer I think you are asking for. I have wondered how
to estimate the amount of signal loss to expect from my antenna when it is
elliptically polarized and receiving a circularly polarized signal.
I decided the math was too difficult for me and thought of it this way --
If my antenna is linearly polarized, it will be 3 dB down in sensitivity
from ideal.
If I am willing to assume that by making my Axial Ratio smaller, the
received signal will increase LINEARLY with the added orthogonal component,
an AR of .5 will be about 1.5 dB down from ideal.

I hope you get a better explanation of the desensititization related to
having an AR less than unity. I'd like to learn too.

Jerry KD6JDJ

Hi Steve

Having absolutely nothing to do with your question but thought I'd
mention it...

Be aware that in a reflective/multipath environment you get a
polarization reversal. This makes for loss not unlike linear cross
polarization issues but only for the 1st, 3rd, 5th etc reflection.
Although I have never looked into it this would seem a useful feature
for data comms where out of sync/delayed signals might be a problem.

My first though to your actual question though would be to use a linear
antenna and rotate it whilst recording the signal level. Since however
you want to use a circular RX antenna you may be able to use the
reflection behaviour above to get some information. It appears however
that you want the maths, which I am also not really up to.

Cheers Bob VK2YQA

SteveO232 wrote:
I'm trying to throw together a helix antenna, similar to one shown
he
http://www.antenna-theory.com/antenn...ling/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?

SteveO232 wrote:
I'm trying to throw together a helix antenna, similar to one shown
he
http://www.antenna-theory.com/antenn...ling/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?



I use those daily. There are better forms of helical that are much
cleaner off-axis.

Bob Bob wrote:
Be aware that in a reflective/multipath environment you get a
polarization reversal.

Interesting thought. That means that with two antennas,
one clockwise and one ccw, you could adaptively mix the
two signals to cancel the 1st reflections.

Clifford Heath.

Fun stuff!

I actually found this behaviour in experimentation (or rather I noted
what was already in the theory books). I was looking at mobile FM ops on
70cm and had found that horiz polarization yielded a much lesser
peak/null phase addition problem ("picket fencing") than on vertical
(the antenna aperture being wider in the horiz plane). I went to
circular in an attempt to test if there was a large enough component of
major polarization shift from multpile refectections, and promptly lost
most of the first reflection. On the whole it was about a 3dB loss
(9-12dB having been the horiz vs vert improvement) but that was very
terrain amd obstruction dependent.

It was a pity because the mobile antenna looked real trendy! (A skew
planar loop)

Cheers Bob VK2YQA

Clifford Heath wrote:
Bob Bob wrote:
Be aware that in a reflective/multipath environment you get a
polarization reversal.

Interesting thought. That means that with two antennas,
one clockwise and one ccw, you could adaptively mix the
two signals to cancel the 1st reflections.

Clifford Heath.

On Mar 10, 5:26*pm, Clifford Heath wrote:
Bob Bob wrote:
Be aware that in a reflective/multipath environment you get a
polarization reversal.

Interesting thought. That means that with two antennas,
one clockwise and one ccw, you could adaptively mix the
two signals to cancel the 1st reflections.

A c-pol receive antenna having the same rotation sense as a direct-
path, c-pol radiated wave will reject reflections/re-reflections of it
from any odd number of surfaces. The first-surface source usually is
the dominant source of reflections.

However if the rotation sense of a c-pol receive antenna is opposite
that of the transmit antenna, then it will reject the direct path
signals and receive only those reflections of it.

RF

SteveO232 wrote:
I'm trying to throw together a helix antenna, similar to one shown
he
http://www.antenna-theory.com/antenn...ling/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