Effect of degraded axial ratio 2

[pwg505]

I always see a 3dB axial ratio cited as metric for CP antenna performance. What negative effects will I see receiving with an ideal RHCP antenna a RHCP signal, say 6dB axial ratio, beyond a slight loss (1 or 2dB) in receive power do to the mismatch? Thanks!

 

[VEBill]

For most applications, loss is the only effect. Loss would very likely (VHF and up) decrease the SNR.

There are some unique applications (remote sensing radar) where the desired polarization return could be 'contaminated' with the undesired polarization return.

Same thing with DBS satellite TV systems where a poor axial ratio might increase the noise floor due to cross polarization bleed through.

There are probably other examples.

Note - if the poor axial ratio is due strictly to loss, then the cross polarization bleed wouldn't be an issue; but this is very unlikely.


3dB isn't a very challenging spec. within 1dB is a more reasonable target for any sort of precision designa and build.


PS: "due" not "do. !!

 

[Higgler]

The 3 dB axial ratio spec can be a challenge over wide angles and wide frequency.

Worse axial ratio can also mean worse multipath single bounce reflection, like GPS ground bounce or bounce off a building.

Dual channel method of DirecTV uses say V&H polarizations to send down twice the number of channels. This needs the V&H to be separated by fairly good polarization purity. I believe RHCP and LHCP might be used for channel separation on some Satellite Systems.

 

[VEBill]

DBS systems like DirecTV, Dish Network and Bell Expressvu are the ones that use RH/LH CP. FSS systems such as Shaw (a.k.a. Star Choice) uses V/H linear.

 

[pwg505]

Thanks for the input. I am trying to design a wideband (1-2 octaves) conformal spiral array for a project. The axial ratio so far was less than ideal (avg. about 4 dB with some lossy backing and 6-7dB just over a ground plane).

 

[Higgler]

You'll need multi-layer lossy backing, single layer won't absorb enough. Might need an FSS LPF to help the high end.
One octave vs. two octaves is a big difference.

Conformal always has "the H plane problem". H plane antenna pattern is always narrow. You might want to measure/calculate V and H polarization patterns separately and watch the H plane narrow as the reason for your axial ratio problem. When flushmounted, there's an automatic rolloff in H plane that narrows the antenna pattern. If you're alllowed to have it protrude slightly or curve, that'll help.

good luck