Linear Transponder Satellites: SSB and CW Passband Operating

FM satellites are where most people start, because a handheld and a small beam are enough to talk through them. But the quieter and more rewarding side of amateur satellite operating is the linear transponder satellite. Instead of a single FM channel, it relays a whole slice of spectrum, so many stations can make SSB and CW contacts at the same time, each on its own frequency, and every signal keeps its natural sound. The price is a little more skill: you have to tune two frequencies, correct for Doppler shift and listen to yourself coming back from space. This guide explains how a linear transponder works, why full duplex is so useful, how Doppler affects your tuning, what AO-7 can teach a beginner, and which habits keep a crowded passband pleasant for everyone.

What a Linear Transponder Is

The ARRL guide to amateur radio satellites describes the difference simply. Linear transponder satellites retransmit an entire slice of spectrum, typically SSB and CW, rather than a single fixed channel. Multiple stations can be on the transponder at once, at different frequencies inside the passband, and the experience is similar to tuning across a band rather than using one repeater channel. The reference article on amateur radio satellites adds that since 1965 most OSCAR satellites have carried a linear transponder for two-way communication in real time, and that amateur satellites may act as repeaters, as linear transponders and as store-and-forward digital relays.

Inside the satellite, the transponder receives a block of frequencies on the uplink band, amplifies them, shifts the whole block to the downlink band and retransmits it. Because the whole block is shifted together, a signal that is 5 kHz above the bottom of the uplink passband appears at a fixed offset in the downlink passband, and two signals keep their spacing relative to each other. Voice stays voice, and CW stays CW. That is the reason it is called linear: the transponder does not demodulate anything, and its output is a faithful copy of its input.

FM Repeater Satellites Compared

The two types of satellite behave very differently for the operator. The table compares them.

The amateur satellite reference notes that frequency modulation is more tolerant of Doppler shift than single sideband, and that FM is much easier to tune manually. Linear satellites are more demanding for that reason. If you have not yet made your first FM satellite contact, start with our introduction to amateur radio satellites, and come back here when you are comfortable with tracking a pass.

Why Full Duplex Matters

On a linear transponder you transmit on one band and listen on another at the same time. If your radio can do this, you hear your own signal coming back from the satellite, a few moments after you speak or send. That is very valuable, because it tells you three things at once: that your signal is reaching the satellite, how strong it is compared with others in the passband, and whether you are on the frequency you intended. The ARRL notes that linear satellites require full-duplex operation, transmitting and receiving simultaneously to hear yourself through the transponder, and more careful tuning, which makes them a natural second step once FM satellite basics are comfortable.

Full duplex can be done with two radios, with a single dual-band transceiver that supports satellite mode or with a receiver and a transmitter that are separate. Use headphones in all cases, because a speaker near the microphone causes feedback and picks up your own audio. If your station is limited to half duplex, CW makes it practical: you can listen between elements, but the experience is harder, and you must tune by calculation rather than by hearing yourself.

Doppler Shift on a Linear Pass

The satellites move at high speed, and the frequencies you hear and the frequencies you must transmit on change during every pass. The amateur satellite reference explains the pattern. While the satellite is moving toward the ground station, the downlink frequency appears higher than normal, so you must tune your receiver higher. At the same time, the satellite receives your uplink at a higher frequency than normal, so your transmitted frequency must be lower for it to arrive at the correct place. After the satellite passes its closest point and begins to move away, the process is reversed. The downlink appears lower and the uplink must be adjusted higher.

The amount of correction depends on the band. Doppler shift is proportional to frequency, so it is larger on 70 cm than on 2 m, and larger on 2 m than on 10 m. On a linear transponder, even a small error moves you away from the station you want to talk to or onto somebody else’s conversation. The reference points out that these corrections are normally calculated by tracking software, and that many modern transceivers have a computer interface that allows automatic Doppler correction. Manual correction is possible, but it is difficult to stay precisely near the right frequency. Our guide to Gpredict, tracking and Doppler control shows how to connect software and radio so that this is done for you.

A Simple Manual Method

If you are tuning by hand, a common technique is to choose the downlink frequency you want to listen to and to leave it where it is on the dial, adjusting only the uplink to keep your signal on the right spot. Another approach is to fix the uplink and tune the downlink. Either way, the principle is the same: tune one, correct the other, and check yourself by listening. Practice on a high-elevation pass in daylight, with the radio, the logging and the tracking all set up before the satellite rises.

AO-7: The Oldest Linear Satellite

For many operators, the first linear satellite is AMSAT-OSCAR 7, usually called AO-7. According to the reference article on AO-7, it is the oldest operational satellite, whether government, scientific, military or commercial, and it carries two amateur radio transponders. The Mode A transponder has an uplink on the 2 meter band and a downlink on the 10 meter band. The Mode B transponder has an uplink on the 70 centimeter band and a downlink on the 2 meter band. It also has beacons on the 10 meter, 2 meter and 70 centimeter bands.

The reference gives details that matter for planning. AMSAT reported AO-7 still operational in 2015, after a period of 21 years during which it was silent, and the satellite has reliable power only from its solar panels. It is eclipsed on every orbit during the northern summer and autumn, and during the rest of the year it is in continuous sunlight and alternates between mode A and mode B. In other words, whether you can use AO-7 on a given day depends on its schedule, so check the current mode before you plan a pass.

The same article explains an interesting regulatory point for United States operators. The Mode B uplink around 432.1 MHz predates the 1979 international allocation of 435 to 438 MHz to the Amateur Satellite Service. The reference says that the IARU band plan marks the 432.1 MHz range as weak signal in all three Regions, and that accessing the Mode B uplink is permitted in the United States under a waiver from the FCC. If you operate from another country, check your national rules for the uplink segment before you transmit. Our article on the 70 cm band summarizes the band plan basics for that part of the spectrum.

Power and Etiquette

A linear transponder has a limited output power, shared among all the signals in the passband. If one station transmits a strong uplink, its downlink can take a disproportionate share of that power and suppress weaker stations. The result is a transponder that sounds crowded with a few loud signals and many that cannot be heard. The widely accepted practice among satellite operators is to use only as much power as it takes to hear yourself at a level no louder than the satellite beacon or the average signal in the passband. If your downlink is louder than the beacon, you are using too much power.

These other habits make a passband a pleasant place to operate:

• Listen first. Spend a minute or two tuning across the passband before you transmit, to find a clear space and to hear the style of operating.
• Keep contacts short. A pass lasts only a few minutes, and a brief exchange of call signs and grid squares lets more people make contacts. Our article on grid squares and the Maidenhead system explains the locator that is usually exchanged.
• Use CW if you can. CW signals are narrow and can be copied through noise and interference that would defeat SSB, and they take less of the transponder’s power.
• Avoid calling CQ on top of others. Move away from an active conversation and leave a gap between stations.
• Respect the beacon. Do not transmit near the beacon frequency at the edge of the passband.
• Log carefully. Record the satellite name, mode, time in UTC, call sign and locator of the other station.

The split between transmit and receive frequencies on the same radio is related to the technique of split operation used on HF pileups. If you want a refresher on the controls involved, read our article on split operation, VFO A/B, RIT and XIT.

Station Requirements

A linear satellite station can be modest. A multimode radio capable of SSB and CW on the downlink band, another for the uplink or a dual-band radio with a satellite mode, a pair of small Yagis or other directional antennas, a way to point them and tracking software cover the basics. The antennas need not be large, but they need to be directional at the higher bands, and many operators use handheld Yagis held by hand or small antennas on a rotator. Circular polarization on the antenna, which is common for satellite work because it reduces fading as a satellite rotates, is helpful, although many stations work with linear antennas and live with the fades. Keep the feed lines short, the connectors weatherproof and the preamplifier close to the antenna if you use one on the downlink band.

The mode and the band combination will determine the equipment. A satellite with a 70 cm uplink and a 2 m downlink, such as AO-7 in Mode B, needs a UHF transmitter and a VHF receiver. A satellite with a 2 m uplink and a 10 m downlink, such as Mode A, needs a VHF transmitter and an HF receiver with a suitable antenna for 10 meters. The geostationary option is different, because it does not move; see our guide to the QO-100 geostationary satellite if you prefer an antenna that stays fixed.

Your First Linear Pass: A Checklist

1. Update the satellite elements in your tracking software and check the pass time, the maximum elevation and the current transponder mode.
2. Set the radios to the correct modes, using the sideband that suits the transponder. Check whether the transponder is inverting or non-inverting, because that changes the direction of tuning.
3. Connect headphones, test the microphone or key and confirm that the tracking software controls the radio frequencies.
4. Start listening a minute before the satellite rises, find the beacon and tune across the passband.
5. When you hear stations, transmit a short call on the uplink at low power and find your own signal on the downlink.
6. Reduce power until your downlink is comparable to the others and make a short contact.
7. Log the contact, and note the time, the satellite and the settings that worked.

The Bottom Line

A linear transponder satellite relays a whole passband of SSB and CW signals, which makes it feel like a small band in space. To use it well, listen to yourself in full duplex, correct for Doppler shift with software or a clear manual routine, and keep your uplink power low enough that your downlink is no louder than the beacon. AO-7 is a historic first satellite for many operators, but check its current mode and your national rules for the uplink. Start with a short, well-prepared pass, and your next contacts will be easier.

Frequently Asked Questions

What is a linear transponder satellite?

It is an amateur satellite that retransmits a whole slice of spectrum, typically SSB and CW, so that many stations can use it at once at different frequencies within the passband.

Do I need full duplex?

It is strongly recommended. Hearing your own signal through the satellite confirms that you are reaching it and lets you adjust your frequency and power.

How does Doppler shift affect me?

The downlink frequency appears higher as the satellite approaches and lower as it leaves, and the uplink must be corrected in the opposite direction. Tracking software or a radio with computer control can do the correction automatically.

Which bands does AO-7 use?

Mode A has a 2 meter uplink and a 10 meter downlink. Mode B has a 70 centimeter uplink and a 2 meter downlink. The satellite alternates between the modes during part of the year.

How much power should I use?

Use the least power that lets you hear your downlink at a level no louder than the beacon or the average signal. A louder signal takes power from other stations.

Can I use a handheld radio?

For FM satellites, yes. For linear satellites you need SSB or CW capability, full duplex and better antennas, so a handheld alone is not usually enough.