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Re: circular polarization sense



At 02:48 AM 10/10/2000 -0500, Jon Ogden wrote:
>on 10/9/00 10:00 PM, MERRETT, David at david.merrett@baesystems.com wrote:
>> My first problem is I dont know which Sats actually transmit in circular
>> pol.
>
>ALL of them.

Sorry, but I believe this is wrong. It depends on the antenna used on the spacecraft. For example, the omnidirectional antennas on AO-10 are simple whips, which are linearly polarized, whereas the gain antennas are turnstiles, which are circularly polarized on-axis. On RS-10/11, the antennas are dipoles, which are linear. The Microsat and UoSat platforms do use a circularly polarized downlink antenna, but the uplink antennas are linear. Fuji platforms are circular in both directions. (Reference: The Radio Amateur's Satellite Handbook)

For circularly polarized antennas, it also depends on the angle between the antenna's axis and the line between the satellite and the groundstation. When these are the same ("on-axis") then usually you see pretty good circular polarization in the intended sense. When the antenna is pointed in some other direction, you can get most anything, including linear or even the opposite sense of circular polarization.

>  The satellites are typically stabilized (from what I
>understand) by being spun.

This doesn't really say anything about antenna polarization.

There is a connection, though. If a satellite with a linearly-polarized transmit antenna is spinning (and the antenna's physical axis doesn't match the spin axis), the direction of the linear polarization will change with the spin. If you're trying to receive this signal with a linearly polarized antenna, it can only be aligned properly for one polarization (assuming you don't spin your antenna!). When it is properly aligned, you'll get a strong signal. When it's 90 degrees off, you'll get a deep null. In between, the signal will vary. The result is wicked fading, called "spin modulation". You can make this fading go away by using a circularly polarized antenna at either end, at a penalty of 3dB in peak signal strength. This is really why circular polarization is cool for satellite links.

There is some residual spin modulation, due to imperfections and asymmetries in the antenna pattern, such as those caused by other antennas on the spacecraft or the funny shape of the spaceframe itself.

Also, note that many satellites are not spun for stabilization. The Microsat platform satellites, for instance, are spun (slowly) mainly for thermal balance. They are stabilized with permanent magnets. Another example, Phase 3D in its final configuration won't be spinning at all. (One rotation per orbit to keep the antennas pointed down doesn't count as spin.)

>> An article I read recently claimed that polarization will reverse sense
>> after passing overhead (IE SAT goes from approaching to receeding). Implying
>> that if an operator has a (say LH.) circular polarized RX ant, then he will
>> only have good sigs on half of the pass

This happens mainly because the angle at which you see the satellite's antenna is changing. This happens to some extent with any satellite, but it's especially interesting with Microsat platform spacecraft, which tumble during each orbit as the Earth's magnetic field acts on their permanent magnets. As I mentioned above, when the satellite antenna isn't looking right at you, you won't get exactly the intended polarization. Indeed, the common turnstile antenna tranmits the opposite circular polarity off the back of the antenna.

>You may be interested to know that TV stations (and I think FM Radio as
>well) transmit in elliptical polarization (for TVs this is in the horizontal
>plane).  I believe they found that the elliptical polarization helped to
>combat a lot of the fading and multi-path interference that would happen
>with a linear antenna.  There's also other reasons that escape me right now.

The story I heard was that FM radio was transmitted with vertical polarization back when everybody's car radio antenna was a whip. When car manufacturers started installing horizontal dipoles embedded in the windshield, it became necessary to transmit horizontally polarized energy for those listeners. So today FM is all circular (or elliptical) so it doesn't matter at what angle your antenna is mounted.

>And anyhow, like
>I said most birds are spin stabilized so the resulting signal out of the
>bird is CP anyhow.

That doesn't follow. You can't make circular polarization by physically spinning a linear antenna.

73  -Paul
kb5mu@amsat.org

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