Single Sideband (SSB) Explained: How It Works and Why Hams Use It

Almost every voice contact on the HF bands is made in single sideband, yet few operators could explain what the term really means. We press the push-to-talk, speak, and the radio does the rest. Understanding what happens inside is not essential for operating, but it explains many of the habits that make for good contacts: why you need to tune accurately, why speech processing and the ALC are treated with care, why upper and lower sideband are used on different bands, and why a sideband signal seems to have no carrier when you tune across it. This guide explains how SSB is derived from ordinary amplitude modulation, how a radio makes and receives it and what the benefits and the limits are.

Starting From AM

To see what SSB removes, start with the older mode it replaced, described in the Wikipedia article on amplitude modulation. The Wikipedia article on single-sideband modulation explains that in conventional amplitude modulation, an audio signal controls the amplitude of a radio-frequency carrier, producing a carrier plus two mirror-image sidebands. Each sideband contains a complete copy of the original information, while the carrier conveys none. An AM signal therefore occupies a bandwidth equal to twice the highest audio frequency, and a large share of the power goes into the carrier.

The numbers for ordinary AM show how large that share is. For a carrier that is fully modulated, two thirds of the transmitted power is in the carrier and only one sixth is in each of the two sidebands. Since either sideband alone contains the entire audio signal, the information is carried by one sixth of the power, and the rest is overhead. The opposite sideband adds nothing new, and the carrier adds nothing at all, other than a convenient reference for a simple receiver.

What SSB Removes

SSB is derived directly from AM by removing this redundancy. It transmits only one sideband and usually suppresses the carrier. According to the same article, compared with AM, SSB requires approximately half the bandwidth and uses transmitter power more efficiently. A voice signal that would occupy about 6 kilohertz as AM takes up about 3 kilohertz as SSB, and all of the power that the transmitter produces is spent on the information. That is why a station with 100 watts in SSB can be heard much further than a 100 watt carrier of AM, and why SSB became the standard for long-distance voice work. The article adds that, in amateur radio, most HF voice operation moved from AM to SSB before the arrival of widespread digital voice.

The frequency of the SSB signal depends on which sideband is used. The article gives an example: a 2 kilohertz audio tone modulated on a 5 MHz carrier produces a frequency of 5.002 MHz if the upper sideband is used, or 4.998 MHz if the lower sideband is used. In the lower sideband the spectrum is inverted compared with the audio signal, which is why a receiver has to be set to the correct sideband. Our guide to upper and lower sideband conventions lists which one is used on which band.

How a Radio Makes SSB

There are several ways to generate a single-sideband signal, and every one of them is a way of getting rid of the unwanted sideband. The filter method generates a double-sideband suppressed-carrier signal and then removes one sideband with a very sharp filter, often a crystal filter. That is the classic way and the one used in many analogue transceivers. The phasing method works by cancellation: it combines two paths of the audio signal with a 90 degree phase difference with two carrier signals in quadrature, so that one sideband adds and the other cancels. The Wikipedia article attributes this approach to a 1928 patent of Ralph Hartley, and amateur experiments with phasing transmitters appeared in the late 1940s.

A third method, described by Donald Weaver in 1956, uses low-pass filtering and two stages of quadrature frequency translation. Modern radios that use digital signal processing usually generate the signal mathematically, which is a digital form of the phasing and Weaver methods. In every case the SSB is made at a low power level, then amplified in linear stages, because any non-linearity in the amplifier creates distortion and splatter. This is the reason why the output stage of an SSB transmitter is linear and why overdriving it is a serious fault; see our article on key clicks and splatter.

How a Receiver Decodes It

An AM receiver can recover the audio by detecting the envelope of the signal, because the carrier is there to define it. With the carrier suppressed, there is no envelope, and the receiver must supply the missing carrier itself. A product detector mixes the incoming signal with a locally generated carrier, produced by a beat frequency oscillator, and the difference is the audio. The Wikipedia article puts it simply: because the carrier is suppressed, reception requires reinsertion of a locally generated carrier and greater frequency stability than AM.

That stability explains a familiar experience. If your receiver is a little off frequency, the audio of an SSB signal sounds higher or lower in pitch, like a cartoon character, and it becomes more natural as you tune to the correct frequency. The same article says that amateurs are sometimes tolerant of frequency errors that cause unnatural-sounding pitch shifting. A shift of 50 to 100 hertz is still readable, but accurate tuning gives a more natural sound, and some listeners find it easier to copy. Our guide to AM operating shows the opposite side of the coin, where the carrier makes tuning forgiving at the cost of bandwidth.

Power in SSB: Why PEP Matters

An SSB transmitter puts out no power when you do not speak, because there is no carrier. When you speak, the output rises and falls with the voice, and the highest instantaneous level during a syllable is the peak envelope power. That is the value that matters for the transmitter and for the licence limits. The average power is much lower, often only a small fraction of the peak for natural speech, and a speech processor raises it by reducing the difference between the peak and the average. Our article on peak envelope power and wattmeters explains how to measure it, and our guide to SSB audio settings shows how to adjust the gain and the compression without splatter.

Because of the way the power is distributed, an SSB signal does not look like a steady tone on a power meter. A meter that is made for AM or FM will read low, and a meter that responds to peaks will jump about. This is normal. The way to judge your signal is by the ALC meter, the reports from other stations and, if possible, an oscilloscope or a monitor that shows the waveform.

A Short History

The idea is older than most amateurs realise. The Wikipedia article records that the first United States patent application for SSB modulation was filed on 1 December 1915 by John Renshaw Carson, and that SSB first entered commercial service on 7 January 1927, on the longwave transatlantic radiotelephone circuit between New York and London. Telephone companies used the same principle in the 1930s to put many voice channels on one line by frequency-division multiplexing, spacing them about 4000 hertz apart. Amateur radio operators began serious experimentation after the Second World War, and the article notes that a US military air command adopted SSB as the radio standard for its aircraft in 1957, after which it became the de facto standard for long-distance voice radio. When you tune an SSB signal today, you are listening to a technology that took about forty years to travel from a patent to a hobby.

Tuning an SSB Signal Step by Step

Finding the right setting on a sideband signal is easy once you know what to listen for. First set the receiver to the sideband that the other station is using; if you choose the wrong one, the voice sounds like gibberish with an unnatural, inverted quality, even though the signal is strong. Then adjust the tuning slowly. The voice will sound deep and muffled on one side of the correct frequency, and high and thin on the other. The best setting is the one where the voice sounds natural, and the pitch of the speaker seems familiar. If you tune with a radio that has a clarifier or RIT, you can correct the small error without moving the transmit frequency, which is the polite thing to do when you answer a call.

When you call, remember that the other station hears you at the frequency that you transmit, and that a small difference will make your voice sound unnatural to them. If both stations are within a hundred hertz of each other, the contact is comfortable. Modern radios are stable and accurate, so large errors are rare, but old equipment may drift. If a station tells you that your voice sounds high or low, check the frequency readout against a known reference, and note that the problem may also be their receiver.

SSB Compared With Other Voice Modes

SSB is not the only way to send voice, and the choice depends on the band and on what you want to achieve. FM is used on VHF and UHF, where the bandwidth is not scarce and the noise-resistant behaviour is welcome, and it gives better audio quality than SSB at the cost of a wider channel and a threshold below which the signal vanishes. AM remains in use for historical reasons and by enthusiasts who like its sound. Digital voice modes, such as FreeDV and others, send compressed speech as data and can work in conditions where SSB would be unreadable, though they have their own demands on the equipment. On the weak-signal side, CW and the narrow digital modes can get through where SSB does not, because they use much narrower bandwidths and concentrate their energy. Our guides to FreeDV and RADE and the earlier comparison of digital modes show where the technology is going, although SSB still dominates the HF voice segments.

Advantages and Limits

• Efficiency. All of the power carries information, so the signal travels further for the same transmitter power.
• Narrow bandwidth. More stations fit on the band, and the receiver filter can be narrower, which also reduces noise.
• No heterodyne whistles. With no carrier, two signals close together do not produce the whistle that AM stations create.
• Tuning demands. You must tune within a small error and choose the right sideband.
• Linearity required. Amplifiers and audio chains must be clean, or the signal becomes wide and distorted.
• Fidelity. The narrow audio passband gives communications quality, not high fidelity; the receiver filters cut the low and high ends of the voice.

The Bottom Line

Single sideband is amplitude modulation with the redundant parts removed: one sideband is sent and the carrier is suppressed. The result uses about half the bandwidth of AM and puts all of the transmitter power into the information, which is why it became the standard for HF voice contacts after the Second World War. Radios generate it by filtering, by phasing or by digital signal processing, and receive it with a product detector and a local carrier that must be accurate. Tune carefully, pick the correct sideband for the band, keep the audio and the amplifier linear and judge your power in peak envelope terms.

Frequently Asked Questions

What does single sideband mean?

A form of amplitude modulation in which only one of the two sidebands is transmitted and the carrier is usually suppressed.

Why is SSB more efficient than AM?

Because it does not waste power on the carrier or on the duplicate sideband, and it needs about half the bandwidth.

Why does SSB sound strange when tuned slightly off?

The receiver supplies the missing carrier, so a frequency error shifts the pitch of the voice. Tune until the voice sounds natural.

Do SSB transmitters send a carrier?

Normally no. The carrier is suppressed, so with no audio there is almost no output.

What is the difference between USB and LSB?

USB is the sideband above the carrier frequency and LSB the one below it. By convention, amateurs use LSB on the lower bands and USB on the higher ones.

Can I receive SSB on an AM radio?

Not properly. A receiver needs a product detector and a beat frequency oscillator to restore the carrier, and a plain AM receiver has neither.