[amsat-bb] Loss Between Right and Left Circular

[Paul Andrews]

Roger / Greg,

Greg is correct the CubeSats appear to be mostly linear polarized.
Using a RHCP or LHCP polarized ground antenna will limit your
"satellite tumble" loss to 3db.   I also have x-pol yagis and using CP
on CubeSats makes operation a little easier but you give up a minimum
of 3db on RX and TX   However, satellite operation is not really weak
signal and there is usually plenty of link margin.   I agree, using 2m
and 70cm preamps will greatly enhance your satellite experience.

I have used both switchable linear x-pol yagis and RHCP yagis on
CubeSats and FO-29 / AO-7.   They both work - RHCP (circular) will
minimize the deep signal nulls as the satellite tumbles.  Be careful,
I think FO-29 is a CP satellite so you need to pick the correct uplink
sense.  Linear will be ok FO29.

This is the beauty of the handheld Arrow yagi.   The human brain / ear
combination directs this low gain linear polarized yagi to exactly the
correct polarization everytime without the 3db CP penalty.   The only
problem with the Arrow is that you need to be outside in the rain and
snow for every satellite pass.  :)      BTW - I have seen Arrow yagis
mounted on Az/El rotors but now you lose the polarization correction.

It's ham radio - try everything!

73 - Paul - W2HRO











On Fri, Oct 7, 2016 at 2:00 PM, Greg D <ko6th.greg at gmail.com> wrote:
> Hi Roger,
>
> Going from left to right, or right to left, with both sides being
> circular is in that 30db loss realm.  I recall it being more like 20db,
> but once you get in that range, a few db either way kind of doesn't
> matter.  In practice it's not quite that bad, since in practice nothing
> is perfectly circular, but, yeah, not a recipe for success.
>
> Crossing circular (either handed) with linear is 3db, I believe.  Or was
> it 6?  Anyway, not impossibly bad.  Hopefully you also have a preamp at
> the antenna, which will offset a lot of this.  If not, I'd invest in
> one, regardless of the polarization topic.
>
> I have a pair of switchable beam antennas for satellite work.  My
> observation is that most of the newer satellites these days appear to be
> linear, and I've noticed that while there appears to be some difference
> between right vs left on the ground, crossed with their linear signals,
> most of the time it's not fatal.  Either can work for at least part of
> the pass.
>
> Bottom line is that you've got a workable antenna.  Try it and see what
> you get.
>
> Greg  KO6TH
>
>
> Roger Cooper wrote:
>> About 10 years ago got interested in sat radio and bought a Left Circular
>> Quadrifilar Helix Antenna which got good review in QST.  Didnt use it and
>> now retired and trying to get on sat. Evidently most ham sats are right
>> circular and depending on
>> what I read. there is a 30db OR 3db loss going from one circular
>> polarization to another. Which is it?
>>
>> Thanks  Roger N3RC
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