Amateur satellites are radio relays that orbit the Earth, built by volunteers and universities and carried into space as secondary payloads. Their gift to the hobby is height: from orbit, two stations thousands of kilometres apart can both see the satellite, so a handheld radio can reach a station on another continent.
The orbit sets everything
Higher orbit: slower, longer lap, wider footprint, longer pass. Height is exaggerated in the drawing.
The higher the satellite, the slower it moves, the longer each lap takes, and the more of the Earth it sees at once. Low Earth orbit (LEO) is the sweet spot for amateurs: close enough for weak signals, high enough to see a few thousand kilometres. The price is a short pass: the satellite crosses your sky in minutes, then you wait hours for the next. Some satellites fly in higher or elongated orbits and stay in view much longer; one placed at a particular height over the equator hangs over one spot. See working satellites for how a pass actually goes.
What is on board
Most amateur satellites carry one of a few payloads:
Payload
What it does
FM repeater
A repeater in the sky: one user at a time, simple radios work.
Linear transponder
Relays a whole slice of spectrum, so many users and any mode (SSB, CW, digital) share it.
Beacon and telemetry
Broadcasts the satellite's health: battery, temperature, orientation.
Digipeater or store-and-forward
Packet relay, or messages held and delivered on a later pass.
An amateur satellite receives on an uplink band and retransmits on a different downlink band, so it can listen and talk at the same time. Satellite "modes" name the pair, uplink first: U/V is 70 cm up, 2 m down.
Mode name = uplink band / downlink band.
A linear transponder does not decode anything; it shifts the whole passband and amplifies it, often flipping it so that tuning your uplink up moves the downlink down (an inverting transponder).
Downlink = oscillator minus uplink. The whole band is flipped.
Facts that cause surprises
Satellite equipment, frequencies and even which satellites are alive change constantly. Look up current status rather than trusting a printed list.
On a full-duplex pass you can hear yourself on the downlink: a good check that your signal got through.
Shared transponders are shared power. Keep your signal no stronger than the beacon or the other users on the passband; see Working satellites.
The antenna on a satellite spins or tumbles, so ground antennas for satellites are often circularly polarized.