Extra Class Question of the Day: Amateur satellites

Working the satellites is a very popular amateur radio activity. There’s even an organization dedicated to launching and operating amateur radio satellites – AMSAT (www.amsat.org).

Perhaps the most important thing you need to know when trying to communicate via satellite is where the satellites are. One way to predict the location of a satellite at a given time is by calculations using the Keplerian elements for the specified satellite. (E2A12)

Amateur radio satellites are not in a geostationary orbit. That is to say they are constantly changing position in relationship to a point on the Earth. The type of satellite appears to stay in one position in the sky is geostationary.

When determining where a satellite is, you might want to know its orbital period. The orbital period of an Earth satellite is the time it takes for a satellite to complete one revolution around the Earth.(E2A03)

It’s also important to know the direction in which it is travelling. The direction of an ascending pass for an amateur satellite is from south to north. (E2A01) The direction of a descending pass for an amateur satellite is from north to south. (E2A02)

Next, you need to know what mode the satellite is in. The term mode as applied to an amateur radio satellite means the satellite’s uplink and downlink frequency bands. (E2A04)

We use a combination of letters to denote the mode. The letters in a satellite’s mode designator specify the uplink and downlink frequency ranges. (E2A05) If it were operating in mode U/V, a satellite’s receive signals would be in the 435-438 MHz band. (E2A06) U stands for UHF, V of VHF. With regard to satellite communications, the terms L band and S band specify the 23 centimeter and 13 centimeter bands. (E2A09)

Satellites repeat signals using transponders. Transponders are similar to repeaters, except that they receive signals across a band of frequencies and repeat them across another band of frequencies. The most common type of transponder is the linear transponder. All of these choices are correct when talking about the types of signals can be relayed through a linear transponder (E2A07):

  • FM and CW
  • SSB and SSTV
  • PSK and Packet

One thing to keep in mind is to keep your transmitter power to the minimum needed to hit the satellite. Effective radiated power to a satellite which uses a linear transponder should be limited to avoid reducing the downlink power to all other users. (E2A08)

There are quite a few interesting phenomena that result from the fact that satellites rotate while they are orbiting. One reason the received signal from an amateur satellite may exhibit a rapidly repeating fading effect is because the satellite is spinning. (E2A10) To mitigate the effects of this of fading, you might use a circularly polarized antenna. A circularly polarized antenna is the type of antenna that can be used to minimize the effects of spin modulation and Faraday rotation. (E2A11)

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