|fahnen/12.jpg Polarization Switching|
The Case For Polarization SwitchingWherein Jim White, WD0E, argues that the best antennas for working the most satellites are circularly polarized antennas that can be switched from right-hand-circular to left-hand-circular polarization.
The following is based on design data, direct measurements and experimentation, surveys, and anecdotal evidence from discussion threads on the digital birds and Compuserve.
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 transmitter module. Here is a list of the sense of each:
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. Many of these are in the planning or building stages at present (April '97).
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.
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