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RHCP or LHCP or Linear



Hi all,
I have followed the discussion on circular polarisation over
the last week.
I have used a 16 turn helix (G3RUH) for many years. This antenna is
RHCP.
A month ago I changed it to a 19 el. TONNA 70 cm antenna - and at the
same time added LHCP to RHCP relay to my 2 meter antenna.
My experience is, that there is not thouth out there. It depends upon
what you want. Also you must consider the different types of satellites
before you jump to conclusions.
For FO-20 and FO-29 with their RHCP antennas one should think that RHCP
is the best. That is not the case. The attitude of the satellites can
be upside down seen from your QTH - thus turning the signal to be LHCP.
I had experienced that often when using the helix. This situation
give deep fades.
Switching polarity can mean the difference between a 59 signal and almost
nothing with the two satellites.
Therefor using circular antennas with these two satellites - and if you
want the best signal at all times - you need a switch.
BUT the penalty in terms of operating time is not all that big.
One problem with using RHCP only is, that you may increase your power
to compensate for the loss thus running too much signal into the satellites.
The disadvantage of using switching is that you have one more thing to
look for.
The easy way out is to use linear antennas.
Now what about satellites with linear antennas on 2 meters ? I found that
most of the time it does not matter wether you use LHCP or RHCP. 
If a few cases the is a difference. 
What can one learn from that ?  In my oppinion that the signal comming
from these satellites are elliptically polarised.
Most of the satellites we use are very small compared to the wavelength
therefor the body of the satellite can have a big influence on the actual
polarity of the transmitted signals. Current will flow in the structure thus
modifying the polarity. Since the attitude stabilisation is "poor" we can
expect 
any polarity.
You can experience the same thing listening to the beacon from AO-10. In some
cases it is better to use LHCP than RHCP.
For AO-27 with its linear antennas it is an advantage to use circular on
ground - but
again the loss of operating time is not big if you use linear antennas.
If your starting point is that you want to use P3D when it is up, use RHCP
since all the antennas are RHCP (not 24 GHz to my knowledge). If you also
want to use other satellites - switching may be the best solution.
Sorry I wanted to keep it short - but !!
73 OZ1MY/Ib

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