Basic satellite link
A satellite can receive a signal on an uplink frequency and retransmit it on a downlink frequency. Many amateur satellites use a transponder that shifts signals from one frequency range to another.
Uplink and downlink
The uplink is the signal sent from the ground station to the satellite. The downlink is the signal sent from the satellite to the ground station. The two frequencies are often different to reduce feedback and simplify filtering.
Satellite footprint
The footprint is the area of Earth from which a satellite can be received or to which it can transmit. It depends on orbit, antenna pattern, altitude and link budget.
Orbit types
Low Earth orbit satellites pass overhead relatively quickly. Geostationary satellites remain over nearly the same point on the equator. Amateur satellites commonly use low Earth orbits, though other arrangements exist.
Doppler shift
Relative motion between satellite and ground station changes the received frequency. The shift is highest as the satellite approaches or recedes rapidly. Operators may need to correct frequency during a pass.
Satellite antennas
Directional antennas such as crossed Yagis and helical antennas are used for many satellite links. Circular polarisation can reduce signal loss caused by changing satellite orientation.
Link budget
A link budget accounts for transmitter power, antenna gain, feeder loss, free-space path loss, receiver performance and other losses. It helps determine whether a satellite signal will be usable.