Antenna Polarisation for Satellite Systems.                    GM1SXX

'Or, should I worry about polarisation?'

Newcomers to the hobby of amateur radio satellites have a lot to contend with.  The mere business of  communicating via a moving satellite brings a whole lot of new problems they will never have encountered before.   These problems include Doppler shift and  tracking among the more usual issues of operating and log-keeping.   For satellites in Low Earth Orbit, the challenges in some ways are greater because pass times are shorter, satellites appear to move across the sky more quickly and on 70Cms and upwards, Doppler shift is an issue. 

Don't be put off though, you CAN manage with quite basic equipment if you are imaginative.  You DON'T NEED AZ/EL rotators. 

Just use a modest beam on a rotatable pole tilted up say 25-30 degrees (depending on the number of elements/gain of the aerial *)   and rotate it by hand as the satellite approaches and then departs.  If you fix the elevation as I've described, you don't need a lot of power and as the satellite over-flies your station, the fact that your antenna will NOT be pointed directly at it means that you don't need to worry so much about overloading the transponder.  Small beams have a wide pattern, this helps because you don't need to point them so accurately leaving you more 'brain cells' with which to operate!  Be sensible and keep the power to the minimum needed to conduct your QSO.  5  by 9 is NOT what you should be aiming for. The satellite's beacon is your power reference. Your downlink signal should NEVER be stronger than the beacon..

One other issue you will have seen writ large is antenna polarisation.  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 I 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).   To put this more plainly, if the satellite and groundstation antennae are not directly pointed at each other (Bore-sight... as down the barrel of a gun), then the signal will NOT be circularly polarised.

So, 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.

CP comes in two 'flavours' Left-handed and Right handed.  They are NOT compatible.  In fact if you use a left hand antenna on the 'bird' and a right handed antenna on the ground (or vice versa), you can expect a theoretical loss of 20Db.  In reality it's usually a bit less, but its still A LOT!   So, cross polarisation is a non-starter.  

 If CP (of any 'hand') is used on the bird and linear on the ground (either vertical or horizontal polarisation), the loss due to incorrect polarisation drops to a mere 3Db.  Many users would not be able to detect a 3Db loss.  Its pretty insignificant

 The above table shows the losses caused by various types of antenna polarisation mismatch

Be aware that these loss figures are theoretical.  It can be readily seen that using linear antennae on the ground causes a 3Db loss WHEN ALIGNED DOWN THE BORESIGHT.  In the real world, this is a rare scenario unless you have AZ/EL computer controlled antennae and  accurate tracking data feeding a control computer... AND...it also makes the assumption that the satellite is also accurately pointed at the user.  The reality is that this rarely happens.

So what about misaligned antennae.  Well, LEO satellites usually 'squint'. Squint is the antenna pointing error  (in degrees). If the squint is large, then the signal at the groundstation cannot be CP. It may be elliptically polarised but it cannot be true CP.  This is no huge hardship. The average operator need only use either a CP antenna of the correct sense or 'hand' or alternatively a normal terrestrial amateur radio Yagi (linear polarisation) to obtain acceptable results.

From the above table of data, It's obvious that apart from the situation where one end of the link has the opposite circular polarisation from the other (20Db loss), that polarisation error losses at worst will be of the order of 3Db when using linear polarisation on the ground.. 

Some users favour switched crossed yagis, (vertical + Horizontal) so they can swap between them as a satellite pass progresses.  One thing you will notice during a satellite pass is that the polariation DOES change. Thats a simple fact.   If you have seen anyone use an Arrow antenna, the Arrow is a modest crossed 2M & 70Cms linearly polarised ..(cross polarised) yagi, you will see them doing the 'Arrow Dance'... literally chasing the best polarisation at that time..while they conduct a QSO. So, polarisation changes during a satellite pass are an entirely normal event.

If you have read the above and understood it, you should now know why satellites use CP.

So, should you worry about polarisation?  Well, probably not.  I reckon you should only worry if you are using a CP antenna of the wrong 'hand'.  Alternatively, you can just use your ordinary terrestrial linearly polarised yagi's and take a slight signal loss.  That's what many folk do.

73 Al.  GM1SXX

* The number of elements in a yagi affect afffect the horizontal and vertical beam-widths).  Suffice to say that if you fix the elevation of a modest beam (6-8 elements) at 25-30 degrees you should be able to work a typical LEO pass with no problems.

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