Putting Together Your Receiving System

An optimum radio amateur satellite receiving system consists of antennas, rotor, preamplifier, receiver, and a modem for the digital modes. The receiver is usually the most expensive piece of equipment, so let's start by analyzing the characteristics an ideal receiver should have. The table lists types of receivers and all of the satellites that can be received with each receiver.

• It should cover 135-145MHz, 435-437MHz, 1.2GHz.
• If it's a transceiver, it should be able to receive and transmit simultaneously.
• Tune increments of 10-20Hz should be available.
• The microphone should have UP/DOWN button tuning.
• It should have a 9600 Bps digital I/O connector.
• Non standard FM offsets should be available.
• You should be able to change the IF filters to wider ones if necessary.

The receiver doesn't have to have all of the above characteristics, but the more it has, the more modes and more data you'll be able to copy.

Preamplifier

Adding a preamplifier at the bottom of your antenna is an easy and cost effective way of improving your satellite reception. As the received frequency increases, so do the losses in signal strength caused by the coax cable connecting the antenna with your receiver. A preamp placed just underneath the antenna will amplify the signal received making up for the losses induced by the coax cable.

Antennas

There are several types of antennas that may be used to work with amateur satellites, such as turnstiles, eggbeaters, helixes. The most used one for frequencies below 1.2GHz is the cross Yagi, with both horizontal and vertical elements. To work digital satellites a 12-14dB gain antenna is preferred, for FO-20 a 14dB gain is fine, and for the AO-10 satellite the longer the antenna the better…

A rotor capable of azimuth/elevation movements is also a desirable thing. You are then capable of aiming the antennas at the satellite as it passes overhead.

Modems

As you can see from Table 1, each digital satellite mode usually requires a different kind of modem ($100-$150). If however you buy a DSP modem you'll pay more, but you'll have all the modes in one box.

What do Amateur Satellites Transmit?

The analog satellites have voice and CW (Morse code) signals, as well as occasional RTTY (Radio TeleType) and SSTV (Slow Scan Television) signals. All satellites also transmit telemetry signals, such as how much power the satellite is using to transmit, the satellite's temperature, solar cell current and many other interesting things. This telemetry is transmitted as AX.25 (packet data). Some data is also sent using ASCII, CW or RTTY.

Receiving satellite pictures from weather satellites

There are two kinds of weather satellites. They can either be geostationary or spinning in polar orbits around the earth. Geostationary satellites are satellites 36000km above our heads, whose position in space relative to us on earth never changes. Once you point your antenna at them you won't have to move it again. The European Meteosat satellite and the American Goes satellites are geostationary weather satellites. Meteosat transmits continuously on 1.69GHz, and can be received using small satellite dish, a receive converter which converts the 1.67GHz into 137MHz, a receiver capable of receiving 137MHz with a 30KHz bandwidth, a simple interface to convert the receiver's analog data into digital data which is fed into the computer through the serial port. Shareware software like JVFAX 7.0 will then decode these signals and give you a photo on screen just like those in the weather forecasts you see on TV. The dish runs for $50-$100, the receive converter is about $150-$200, the receiver ranges from $100 to $500, and the software is shareware.

There are also polar weather satellites. Among these are the Russian Meteors and the American Noaas. These satellites have very low orbits (~1000Km) compared to those of geostationary satellites (~36000Km). Their photos are thus a lot more detailed, but will cover only a small portion of the earth. They transmit at around 137MHz, and can be received using a couple of simple round dipole antennas. The equipment needed is the same as for the Meteosat, with the difference in antennas and the unnecessary receive converter.

Tracking Satellites With Your Bare Eyes

It is possible to watch satellites with your bare eyes, given the right conditions. Using LogSat just select a very low orbiting satellite, as could be the Russian space station MIR, or the space shuttle during one of its flights, see when it passes overhead, go outside and with a bit of luck you should be able to spot it. The key is choosing a good time of the day. The best chance of seeing one is a little after sunset, when you're in the dark, but the satellite is still illuminated by the sun, which will reflect onto the satellite body, allowing you to see it.

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