Observations from Scotland            28/05/2008        GM1SXX

Why CP aerials on the ground are pretty much a waste of money.   

In an earlier Observation, I pointed out the various losses incurred by not using CP aerials in amateur satellite groundstations.  In short, if you use linear rather than CP aerials, pretty much the worst case scenario is that you lose 3Db of signal.  Thanks to reciprocity, it works both ways (TX and RX).  Why night one ask, is there so much stuff in print about using CP in your groundstation?  From where I'm at, it doesn't make much sense.

Why put CP on the satellite?

Ah, that's a different matter.  Placing CP on a satellite confers certain advantages with regard to the groundstation options. The ONLY type of aerial on the ground that doesn't work well with a satellite carrying CP aerials is one with CP of the opposite sense.  In other words, if the satellite is carrying LHCP, then RHCP on the ground will not be good!

The most important thing to grasp about circular polarisation is that the polarisation 'seen' by the receive aerial is "ONLY CP DOWN THE BORESIGHT".    In short, if the bird is not 'looking' DIRECTLY AT YOU, then your aerial (any sort!) will NOT see a CP signal.

First things first.  LEO satellites

It therefore logically follows that a satellite in low earth orbit can NEVER provide a CP signal to two users who are separated geographically!  (He means if they are in different places!... LA2QAA)

Just a little additional though will lead to the conclusion that a CP signal on a satellite can rarely ever produce a CP signal on the ground. 

Satellites in a Molniya  orbit are somewhat different.

A Molniya bird at apogee (Earth pointed)  looks at the Equator (or should do). This means that for users North or South of the Equator, the signal is likely to be elliptically polarised rather than circular.  In such a situation, it can be useful to have a pair of yagis in different orientations (typically V & H) with a means of switching between them.  This has worked well for me in the past.

At apogee, these birds are are typically/ideally  at 36000Kms range.  For operational reasons, these satellites are frequently off-pointed.  The main reason is usually sunlight, or the lack of.  This in turn means that users on the ground see a variety of polarisations but rarely ever true CP.  The Polarphasor is worthy of mention here. I don't know if these devices are still made but they consisted of a printed circuit delay line connected to a rotary switch. This allowed the user to vary the polarisation at the groundstation by altering the phasing of the aerials. I used to own one of these and I can assure the reader that even with slow moving Molniyas at or near apogee, the polarisation DOES vary in real time.

So to recap.

1) The signal polariation of LEO's varies throughout the pass.

2) The signal polarisation on Molniyas also varies, but at a lesser rate thanks partly to their greater range.

To be continued (Wanna write a bit for a Joint OBS?  SXX)