OBSERVATIONS FROM NORWAY & SCOTLAND 14 April 2007
Aerial (Antenna) polarisation revisited.
This joint Observation is deliberately short. Bite-sized chunks of information always work better than meandering explanations.
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From talking to people and reading the many postings about this subject on the various BBS's, one could easily be forgiven for thinking that the ONLY suitable type of aerial for satellite use is one with the 'correct' sense of CIRCULAR polarisation.
The truth, as is so often the case, is somewhat different.
Al, mentioned the effects of using a less than 'perfect'' satellite aerial system in an earlier Observation antenna_polarisation.
Now, in the real world, as opposed to the 'I guess' world, it is not really possible to support two completely contradictory points of view. Only one view can be tenable (except perhaps when talking Quantum Physics)
On one hand, it is blindingly obvious that the polarisation of a radio signal from a satellite (and especially on a LEO) on the bands lower than 23Cms does indeed change as a function of time during a pass. Witness the 'arrow dance', a well documented type of behaviour in the satellite community. Were it the case that those who post all this stuff about needing CP 'antennas' fitted the observed facts, then 'arrow dancing' would be a redundant activity.
In short, somebody is plain wrong!
We'll give you a hint.... its NOT the 'Arrow Dancers'. The polarisation really does change and it's very obviously the case if you actually try the experiment for yourself.
One wonders then, why so many people post this stuff about CP aerials being somehow necessary when they are so obviously wrong. We can only assume that some people read the various AMSAT website material and treat it as 'gospel'. If that's really what's happening, then we'd respectfully suggest that AMSAT review and revise some of their documentation.
We'd remind readers of what Al said in his 2006 article.
<Virtually all amateur radio satellites have used circular polarisation (CP). Some sources will suggest that you MUST use circularly polarised antennae at your ground-station. This is plain WRONG and I'll attempt to debunk this myth>.
There are many web resources describing how Circular Polarisation (CP) works so we won't go into that here. There is one very important fact to remember about CP and it is this.
| A circularly polarised signal is only CP down the bore-sight of the antenna(e). |
Roll this fact around in your brain for a bit before you read on.
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Definition ... (Wikipedia)_Boresight
To put this more plainly, if the satellite antenna(e) are not directly pointed at the receive antenna (Bore-sight... as down the barrel of a gun), then the signal received CAN NOT be circularly polarised. Since this situation only occurs fleetingly (if at all) during a satellite pass, it should be blindingly obvious that the signal received on the ground is almost never true CP. Don't believe us? Well just step back, take a few seconds and think for yourself about what's really going on when someone does the 'arrow-dance'. For sure, they are not doing it for the good of their health.
Better still, and this is a far better idea... go do it for yourself and get the proof first-hand.
When AMSAT launch their fifthcoming P3E bird, you might want to try the same experiment again. Oh yes, Molniyas do also suffer polarisation changes, scintillation and faraday rotation on the lower frequency bands, but their effects are pretty different from those encountered with LEO birds. Molniyas can stay Earth-pointed for long periods but do be aware that they often operate with deliberate 'squint' ... antennae offset from the ideal, for power generation reasons.
OK, it's not so easy to waggle a big yagi at the sky for a Molniya bird but we can absolutely assure you that the experiment will be worthwhile.
One last point. For those who think that CP aerials are somehow essential, consider for a second what sort of polarisation does a pair of stations 'see' during a QSO on a satellite pass? Since the bird can't be 'looking' (think boresight) at two users simultaneously, then it is reasonable to assume that if they don't live in the same town, 'at least' one cannot possibly be 'seeing' a CP signal :-)
So, to close, why may you ask, do satellites use CP? Good question. The answer is to be found in the losses encountered with different types of antennae and polarisation.
Our advice .... have fun and use that grey dollop of gunge between your ears from time to time, to come to your own conclusions. It's fine to read the books but a lot more fun to experiment.
73, Al & John.
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Well, we promised it would be a short article!