The Case For Polarization Switching

  
The sense (right or left) of the circular polarization of the four original 
Microsats changes depending on the transmitter in use. There was no attempt
to make a particular transmitter any sense. The way they came out was based
on how the semi-rigid cable and components could be designed into the transmi
tter module. Here is a list of the sense of each: 



Satellite  Transmitter    Frequency   Polarization
  AO-16       RC          437.050 MHz    RHCP
  AO-16       PSK         437.025 MHz    LHCP
  DO-17       xmtr A      145.825 MHz    LHCP
  DO-17       xmtr B      145.825 MHz    RHCP
  WO-18       RC          437.100 MHz    RHCP
  WO-18       PSK         437.075 MHz    LHCP
  LO-19       RC (&CW)    437.125 MHz    RHCP
  LO-19       PSK         437.050 MHz    LHCP
     

The AO-16 PSK transmitter and the WO-18 PSK transmitter have problems and are
not presently in use. We normally run transmitter B on DOVE because it is 
slightly more efficient.

LO-19 has been switched occasionally between its two.
So it is strictly coincidence that three of these are RHCP virtually all the 
time. Problems with any of the transmitters presently in use could result in
a switch, which would change their downlinks to LHCP. 

As long as you are receiving strong signals from these satellites, mismatched
sense will not reduce the margin enough to cause missed data. However, if the 
signal is weak, because of a marginal receive arrangement or low power from
the
satellite, mismatched sense will cause significant loss of data. Uplinks are
not a problem because they use monopoles and the receivers are very sensitive. 


FO-20 is circular and has a fixed sense both up and down, but because of its
orbit geometry and motion, continuously good signals through it for an entire 
pass can only maintained if sense is switched, usually several times and on
both uplink and downlink. This is particularly noticeable in analog mode. 


FO-29 antennas and stabilization are similar to FO-20, so sense switching is
also necessary as with FO-20. 

The UOSAT series use monopole antennas. I have never noticed a difference in
link efficiency when sense is switched. 

I have no data on which IO-26 transmitter has which sense. But this is a
Microsat design, so changing transmitters will change sense. It tends to run 
very low power, so the correct sense will make a large difference in
throughput on the downlink. 


AO-27 uses a monopole for the downlink so sense makes little difference when
the amateur transmitter is on high power. However, when receiving very weak
signals from it (normal mode), switching sense when using a circular antenna
on the ground is necessary to be able to even detect the signal at times.
Uplink is not an issue, as a monopole is used with very sensitive receivers. 
Future satellites based on the Microsat design will very probably have
downlink characteristics like the AO-16 through 19 series. Changing sense will
be necessary to achieve maximum throughput depending on which transmitter is
in use. 


VOXSAT will use monopole antennas, so the link polarization and sense needs may
be similar to AO-21. 

AO-10: Gain antennas are RHCP. But the motion of this bird is unknown and of
course can't be predicted. Experience shows changing sense on both uplink and 
downlink is necessary at times to have a decent SSB or CW contact. 
P3D will use all RHCP for its circular antennas. Since it will be nadir
pointing  at all times after stabilization is achieved, sense switching should
not be necessary. This assumes all goes as planned. 
I may have left out a bird or two, and have not hit all those being planned.
Nor have I attempted to exhaustively document all situations or those of
interest only to command stations. However, there are numerous situations
in the foregoing where LHCP is necessary or favored. Using the wrong sense
will reduce efficiency and/or enjoyment. In a few cases it may make operation
impossible.
There is certainly much enjoyment available to a satellite enthusiast with
fixed RHCP antennas (or even linear). But I hope I have shown that full
utilization of our satellite resources requires switchable circular. 



73's XE2EKY
                                _\\\//_
                                ( 0 0 )
                            .ooO  (_)  Ooo.
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XE2EKY Hector Solorzano          Grid Square: DM22
QTH: Av. Veracruz y 7a. #1500    E-MAIL: xe2eky@cinco.ampr.org
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