FM Satellite QSOs Are Hard Work

PUBLISHED ON JUN 1, 2020 — SATELLITES

I find it amazing that with a handheld radio and 5 watts of power you can send a RF signal 300 plus miles into space and connect to a satellite. A satellite that’s roughly the size of a softball, traversing space well over 17,000 miles an hour. Once these signals reach the satellite, it’s sent back to earth allowing you to communicate with someone hundreds or thousands of miles away. It sounds relatively straightforward to establish a contact via satellite. But there is frequently more going on than merely pointing an antenna at the sky and pushing the PTT button on your radio.

KJ7NZL working AO-92.

KJ7NZL working AO-92

The Doppler Effect

One of the most formidable hurdles for hams just getting started with satellite communications is adjusting their radios to account for the Doppler effect. Doppler effect represents the change in frequency of a wave in relation to an observer who is moving relative to the wave source. You experienced this phenomenon before if you detected the sound a car’s engine change as it has driven past you. As a satellite moves towards you the radio waves are arriving at your antenna quicker than they would if the satellite were stationary. To compensate for this, you must tune to a frequency higher than the one the satellite is using to transmit on. As the satellite passes your location you need to decrease the frequency. If you’re transmitting then the adjustments are inverted. The frequency starts below the satellite’s uplink and increases as the stellite passes overhead.

Visual repersentation of Doppler effect from AO-91.

Visual repersentation of Doppler effect from AO-91.

The Doppler effect is more significant the higher the frequency. With uplinks/downlinks on 2m you won’t need to adjust your frequency unless you’re working the satellite at the edge of the horizon. With uplinks/downlinks for 70cm you’ll need to adjust the frequency +/-10Khz over the course of the pass. The higher the uplink/downlink frequency the broader range needed for adjustment. For 23cm you’ll adjust +/-30Khz of the course of the pass. AMSAT recommends frequency adjustments in 5Khz steps, however you may find it better to use 2.5Khz adjustments.

Satellite Polarization

Amateur radio satellites don’t remain in one position as the orbit the earth. They are constantly tumbling throughout space. As a result the polarization of their signals fluctuates significantly. Depending on what type of antenna you’re using, this may require you to constantly adjust the antenna. Conventional choices for satellite antennas designs include the yagi or log periodic. For both of these antennas, you’ll need to rotate the antenna’s boom to adjust the polarization of the antenna.

5 Tips For Working FM Satellites

  • Don’t Transmit If You Can’t Hear The Satellite It’s rude to interrupt someone’s QSO, you wouldn’t want it to happen to you. You need to listen to what’s going on. If you can’t hear the satellite, it doesn’t mean that it isn’t there, don’t transmit if you can’t hear the satellite.

  • Share the Satellite FM satellites are just like terrestrial repeaters; only one person at a time can speak at a time. Satellite passes at most last fifteen minutes. With this brief amount of time, people are scrambling to make as many contacts as possible. There just isn’t enough time for you to maintain a conversation. Complete your QSO and move on.

  • Minimize Repeat QSOs If you’ve already contacted someone before, then let someone else take a turn. If the person is operating from a grid square you haven’t contacted before, then it’s fine to contact them once again. Activating rare grid squares is a frequent activity among the satellite crowd.

  • Use Phonetics It’s already hard enough to tune into the satellite, so using letters that sound the same doesn’t do any good when trying to make a QSO. You don’t want to squander the precious amount of time you have during a satellite pass by constantly repeating your call sign over and over again.

  • Use Only the Minimum Power Required Satellites operate with extremely sensitive equipment often transmitting on milliwatts of power. It doesn’t take a considerably amount of power to overload the satellite, so keep your power below five watts. This is enough power for anyone with a good antenna.

The QSO Format

Sattellite QSOs are quick exchanges of information, often times it’s no more than callsigns and grid squares. A typical QSO is structured as follows:

KJ7NZL: “Kilo Juliet Seven November Zulu Lima, Delta November 31”

WD9EWK: “Kilo Juliet Seven November Zulu Lima, this is Whiskey Delta Nine Echo Whiskey Kilo, Delta Mike 43”

KJ7NZL: “Whiskey Delta Nine Echo Whiskey Kilo, thanks for the contact. Delta November 31, QSL?

WD9EWK: “QSL. Delta Mike 43, QSL?”

KJ7NZL: “QSL, thanks and 73.”

Summary

I hope you have found this post helpful. Remember there is a step learning curve to operating FM satellites, but it is possible to make QSOs through them. You have to remain patient and not give up so easily. It’s an extraordinary feeling when you make your first QSO. Remember to adjust the frequency to account for the doppler effect and listen before you transmit.