Amateur Radio Single Sideband SSB

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Single sideband is the preferred mode for amateur radio voice communications on the HF bands and 50 MHz. It is also used, but to a much smaller extent at frequencies above this in the VHF and UHF bands.

Single sideband provides a very effective mode for amateur radio communication, and it is efficient in both its use of spectrum and power.

Single sideband has been in use within amateur radio operation for many years and it is available on a huge variety of amateur radio equipment. Its widespread availability is another reason for its level of usage.

Single sideband technology

Single sideband is widely accepted technology and it is used for many communications systems, both by radio amateurs and for commercial use.

As shown below single sideband utilises a basic amplitude modulated format, but removes the carrier (which only acts as a reference during demodulation) and one sideband.

This improves the power efficiency because the constant carrier which carries not information is removed, and it improves the spectral efficiency because each sideband is a mirror image of the other, and transmitting only one halves the spectrum usage without impairing the signal.

Bandwidth for amateur radio SSB

Bandwidth is an important element for any SSB transmission and no less so if it is used for amateur radio.

There are three factors that come into play. The wider the bandwidth, the better the SSB signal sounds, but against this, limiting the frequencies to those that contribute to the intelligibility improves the impact and intelligibility - often the lower frequencies have a higher power content but contribute little to the intelligibility but give it naturalness. Thus reducing the frequency response to between about 300 or even 500 Hz at the bottom and 2.7 kHz at the top can help. Also another factor is the bandwidth. The narrower the bandwidth, the less noise and interference is received. It is a real balance.

Often a very broad approximation of 3kHz is used for the bandwidth required. This figure comes from the fact old telephone communications used to limit the top frequencies to around 3.3kHz and 3 kHz is a nice round figure. The assumption is that if the top frequency is 3 kHz then this will be the bandwidth of the SSB signal.

In reality the bandwidth of the SSB signal is a little more complicated. The bottom frequencies are often limited to around 300 Hz, so this reduces the bandwidth to 2.7 kHz, and some transceivers tend to limit the frequency response even more to 2.4 kHz.

Many modern amateur radio transceivers allow the upper and lower filter frequencies to be adjusted so some experimentation is needed to provide the best SSB reception. Also remember these are the cut-off frequencies where the response has fallen, typically by 6dB, so the actual bandwidth of the SSB signal is wider than this.

Amateur radio SSB on HF

When operating an amateur radio station using SSB, there are choices for upper and lower sideband. It is generally accepted that Upper Sideband, USB is used on frequencies above 10 MHz and that Lower Sideband, LSB is used in frequencies below 10 MHz.

Single sideband is available for use on most of the HF amateur radio bands, although some bands and frequencies are narrow and do not allow the use of SSB.

On 10MHz, 30 Metres, the IARU has recommended that in view of its narrow bandwidth that SSB is not used.

Also on 5MHz, in view of the almost channelized operation care has to be taken to ensure that frequencies are adhered to and that the right dial frequencies are used. Upper sideband is used on this band of channels and it does not follow the normal convention for the selection of LSB / USB.

Ham radio SSB & speech processing

One of the issues with voice communications, is that the voice waveforms are very “peaky” and they have a high peak to average power ratio. As licence conditions and final power amplifiers limit the peak power than can be transmitted, this could result in a transmission with a low average power level that would not appear nearly as strong as it could.

To overcome this issue, speech processing is used to raise the average level and make the signal sound much stronger for the same peak level.

Most modern amateur radio transceivers have speech processing built in, and external units are not needed. Older transceivers may not have speech processors and external units were used.

When adjusting the speech processing settings, care should be taken to ensure that the optimum performance is obtained - a good level of naturalness without sounding too harsh. Over processed signals can be more difficult to read.

Although normally not an issue these days, speech processing increases the average power of the signal, and the final amplifiers need to be able to accommodate this increase in average power. Some older amateur radio SSB transmitters had difficulty accommodating this increase and occasionally it caused the final amplifiers to fail.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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