Ham Radio Exam Preparation: Amateur Radio Satellite Communications

AOS is the exact time a satellite rises above the horizon and its radio signal can first be received by a ground station.

LOS is the time when the satellite drops below the horizon and the ground station can no longer receive its signal.

The majority of amateur satellites are in LEO, typically 400 to 800 km above the Earth, resulting in fast passes lasting 10 to 15 minutes.

As the satellite speeds toward you, the radio waves are compressed, making the frequency appear higher than it actually is. It shifts lower as it moves away.

Mode designators usually list the Uplink band first, followed by the Downlink band. V is for VHF (2m) and U is for UHF (70cm).

Any two stations located within the satellite’s footprint at the same time can theoretically communicate with each other through the satellite.

Keplerian elements are mathematical data sets that define the exact shape, size, and orientation of a satellite’s orbit around Earth.

Full-duplex allows you to hear yourself coming back from space in real-time, confirming you are successfully hitting the satellite and not stepping on another station.

An FM satellite acts much like a terrestrial repeater. Only one person can talk through it at a time.

Unlike an FM repeater, a linear transponder can support dozens of simultaneous SSB or CW QSOs across its passband.

Linear transponders share their total power output among all active signals. A strong FM signal will capture most of the satellite’s power, suppressing other SSB/CW users and severely draining the batteries.

Circularly polarized antennas (like a crossed-Yagi) help combat “spin fading” as the satellite tumbles in orbit, and mitigate Faraday rotation caused by the ionosphere.

This phenomenon causes a linearly polarized signal (like purely vertical or horizontal) to twist as it passes through the ionosphere, causing severe fading if the receiving antenna’s polarization does not match.

0 degrees is the horizon, and 90 degrees is directly overhead (zenith). A high maximum elevation means the satellite will pass almost directly above you.

Azimuth tells you which direction to point your antenna. 0° is North, 90° is East, 180° is South, and 270° is West.

Because the satellite’s “footprint” covers millions of square miles, it hears a lot of RF noise. The sub-audible tone ensures the satellite only repeats intentional amateur transmissions.

Inverting transponders subtract the uplink frequency from a local oscillator. This reverses the sideband (USB becomes LSB) and means tuning *up* on the uplink causes the downlink to move *down*.

Apogee is the highest altitude of an elliptical orbit. At apogee, the satellite moves slower across the sky, providing a longer communications window but requiring better antennas due to the increased distance.

At perigee, the satellite is at its lowest altitude, meaning it moves extremely fast across the sky, resulting in a very short, fast pass with rapid Doppler shift.

Doppler shift is directly proportional to frequency. At 435 MHz (UHF), the shift is roughly ±10 kHz, which is significant enough that you must tune your receiver down as the satellite passes. The VHF shift is usually small enough (~±3 kHz) that an FM receiver can tolerate it without adjusting the uplink.

Beacons allow ground stations to locate the satellite, determine its operational status (battery voltage, temperature), and lock onto the Doppler shift before attempting a QSO.

The Arrow Antenna (and the Elk Log Periodic) are lightweight, handheld directional antennas that allow an operator to physically point at the satellite and track it across the sky.

For example, a satellite might have a passband of 30 kHz. Any signal transmitted into that 30 kHz window on the uplink band will be translated and repeated on the downlink band.

In standard satellite band designators, “L” stands for the L-band (1.2 GHz / 23cm amateur band), and “U” stands for the UHF (432 MHz / 70cm amateur band).

FM satellite passes are very short and highly congested. Long “CQ” calls are heavily discouraged. Operators simply drop their callsign and Maidenhead Grid Square to initiate rapid contacts.