Antenna question - "elliptic ratio"?

[Darktrax]

The requirement says..
"The elliptic ratio of the antenna shall be better than 1dB"
For a different lower frequency band, another value given was 3dB.

For me, I know what is Axial Ratio, and that it can be expressed as number ratio, or in dB, and the common values are not anything like 3dB. Usually fractions of dB.

I also know Cross Polar Discrimination (XPD), but this seems unrelated, because values are usually more than 24dB. Google search for "elliptic ratio" did not help much!

A circular polarised wave can be less than perfect, and be elliptical polarized, the extreme form being plane polarized, but that is property of of a wave, not an antenna

So I try to untangle what is really being asked here. More importantly, what does one do to demonstrate or measure it?

(Antennas are dish types 3m to 8m diameter, for S-Band through Ka-Band)

Thanks if you can help

 

[volker@muehlhaus]

I would interpret this as axial ratio spec. Imperfect axial ratio gives elliptic polarization, as you already mentioned.

 

[Darktrax]

I would interpret this as axial ratio spec. Imperfect axial ratio gives elliptic polarization, as you already mentioned.

Thanks for your reply Volker. I do agree. I think it is really asking for Axial Ratio.

It was the magnitudes that gave me concern. An antenna has to be quite bent, or the polariser performance quite poor to give such variation if the incoming wave plane is rotated.

How to measure it is a problem. A 6m dish on a mount is a major structure. It cannot easily be taken to a range, and costs would be huge. Most incoming signals are not nearly perfect circular polarized. Maybe I can point a test feed at it from quite close.

Maybe, even, I need only test the feed with polariser. Everything else is very good circular.

 

[vfone]

In real life there is no perfect circular or linear polarized antennas, and all of the antennas are more or less elliptical polarized.
I found that, especially in satellite dish antennas, the terminology of "elliptic ratio" is used, but as was stated they actually refer to "axial ratio".
I think better is (to make everybody happy) is to use the "polarization loss" characteristic, which depends very much by the axial (elliptic) ratio.
The polarization loss never get the absolute theoretical values (zero or infinite dB's), due to the real life elliptical (more or less) polarization of antennas.

 

[juppydu]

Axial ratio and XPD cn be realte by smple formual

XPD : Cross-Polar Discrimination = fo each dierecton is the delat in dB Copolar vs cross-polar.