Making contacts via amateur 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 determine the location of a satellite at a given time is by using the Keplerian elements for the satellite. Keplerian elements are the parameters that define the orbit of a satellite, and they are widely available on the internet.
QUESTION: What are Keplerian elements? (E2A06)
ANSWER: Parameters that define the orbit of a satellite
Most amateur radio satellites are in a low Earth orbit, or LEO. What this means is that they are constantly changing position in relationship to a point on the Earth. So, to make contacts via a satellite, you have to track the path of the satellite as it travels through the sky.
Some satellites circle the earth from north to south, while others travel from south to north. When the satellite travels from north to south, we call that a descending pass; when the satellite travels from south to north, it’s called an ascending pass. satellite will rise above the horizon, It’s also important to know the direction in which it is traveling. The direction of an ascending pass for an amateur satellite is from south to north.
QUESTION: What is the direction of an ascending pass for an amateur satellite? (E2A01)
ANSWER: From south to north
There is, however, now a geostationary satellite, the QO-100, that is available for amateur radio use. A geostationary satellite appears to stay in one position in the sky.
QUESTION: What type of satellite appears to stay in one position in the sky? (E2A10)
ANSWER: Geostationary
Another thing that you need to know to successfully make satellite contacts is the satellite’s mode. The mode is a two-letter combination that tells you the satellite’s uplink and downlink frequency bands. The first letter denotes the uplink frequency band; the second letter the downlink frequency band. U stands for UHF, V for VHF. So, if a satellite’s mode is said to be U/V, then the uplink is in a UHF band and the downlink is in a VHF band.
QUESTION: What is meant by the term mode? as applied to an amateur radio satellite? (E2A04)
ANSWER: The satellite’s uplink and downlink frequency bands
QUESTION: What do the letters in a satellite’s mode designator specify? (E2A05)
ANSWER: The uplink and downlink frequency ranges
Some amateur satellites operate in the microwave bands, including the L band (23 cm) and S band (13 cm).
QUESTION: What do the terms L band and S band specify regarding satellite communications? (E2A09)
ANSWER: The 23 centimeter and 13 centimeter bands
Most FM satellites are simply FM repeaters in space. Some satellites, however, are more sophisticated. They repeat signals using linear transponders. Transponders are similar to repeaters, except that they receive signals of many different types, including FM, CW, SSB, SSTV, PSK, and packet signals, across a band of frequencies and repeat them across another band of frequencies.
In a satellite that uses an inverting linear transponder, an incoming signal is passed through a mixer, and the satellite transmits the difference rather than the sum. Inverting linear transponders reverse the signal position in the band and transform upper signals on the uplink to lower sideband signals on the downlink and vice versa. Another consequence of using an inverting linear transponder is that Doppler shift is reduced because the uplink and downlink shifts are in the opposite directions.
QUESTION: Which of the following occurs when a satellite is using an inverting linear transponder? (E2A02)
ANSWER: All these choices are correct
- Doppler shift is reduced because the uplink and downlink shifts are in opposite directions
- Signal position in the band is reversed
- Upper sideband on the uplink becomes lower sideband on the downlink, and vice versa
QUESTION: How is the signal inverted by an inverting linear transponder? (E2A03)
ANSWER: The signal is passed through a mixer and the difference rather than the sum is transmitted
One thing to keep in mind when making contacts via a satellite using a linear transponder is to keep your transmitter power to the minimum needed to hit the satellite. The reason for this is that the transponder has a limited power output and the more power that your signal has reduces the downlink power available to other users.
QUESTION: Which of the following types of signals can be relayed through a linear transponder? (E2A07)
ANSWER: All these choices are correct
- FM and CW
- SSB and SSTV
- PSK and Packet
QUESTION: Why should effective radiated power to a satellite that uses a linear transponder be limited? (E2A08)
ANSWER: To avoid reducing the downlink power to all other users
Some low Earth orbiting satellites are capable of relaying messages around the world using a technique called store-and-forward. Satellites that use this technique store digital messages sent by one station for later download by another station that is not currently in the coverage area of the satellite.
QUESTION: Which of the following techniques is normally used by low Earth orbiting digital satellites to relay messages around the world? (E2A13)
ANSWER: Store-and-forward
QUESTION: What is the purpose of digital store-and-forward functions on an amateur radio satellite? (E2A12)
ANSWER: To store digital messages in the satellite for later download by other stations
There are quite a few interesting phenomena that result from the fact that satellites rotate while they are orbiting. For example, an amateur satellite may exhibit a rapidly repeating fading effect as the satellite’s antenna rotates away from the receiving station and then towards the receiving staton. These is called spin fading or spin modulation. The polarization of a radio waves from a satellite also changes as it passes into the magnetic field of the Earth. This is called Faraday rotation. To minimize the effects of these two phenomena, most satellites use a circularly polarized antenna.
QUESTION: What type of antenna can be used to minimize the effects of spin modulation and Faraday rotation? (E2A11)
ANSWER: A circularly polarized antenna
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