57. Two Receiver method for SSB

This short article describes how to listen to Single Side Band (SSB) transmissions using two standard AM radios. It requires some practice, basic understanding of SSB theory certainly helps, but no modification of the AM radios is necessary.

Why this method?

There are less and less active AM stations, because a lot of stations close down or switch to other technologies like Internet, FM, or DAB+. On Shortwave, there are still a great variety of SSB stations that can be received, but older AM sets cannot reproduce the sound audibly. All you hear on an AM set is some mumbling; it can be a nice sport to guess the language, but understanding the speech is impossible for most.

The Two Receiver method, illustrated by the photo on the right, allows you to overhear SSB transmissions on an AM receiver, using a second set as a helper. Thus you get more fun out of your vintage AM sets, especially for sets with Tropical Band (1.6 to 5.8MHz) because few AM stations work in this band.

How to operate

Let us call the set that will play the station, the Primary, and the helper set, the Secondary. It will be necessary to tune the Secondary extremely precise, so preferrably your Secondary has bandspread or fine tuning.

A standard AM transmission is composed of a "carrier", which is just something like a high frequent tone, combined with the sound information. In an SSB signal, the carrier is left away, and the method replaces it with a high frequency tone produced by the Secondary. Indeed, each radio contains a Local Oscillator, producing a high frequency tone, usually about 450kHz higher than the frequency that is tuned. Normally, this tone is unnoticed, but it isn't difficult to detect it with another set, or to employ it as the missing carrier for an SSB station.

SSB Sounds
Hello Alfred
SSB Mumble
Coarse tuning
Fine Tuning
Volmet (with Toekan)
The two radios must be placed close to each other. Tune the Primary, for example in the 40m band around 7100kHz, until you hear the typical mumbling of an SSB station. Then tune the secondary at a frequency 450kHz lower; in the example of a 40m band station, this will be around 6600kHz. Because the Local Oscillator of the Secondary works at 450kHz above the tuned frequency, the oscillator will interfere with the reception of the Primary. You can hear this by wriggling the tuning knob of the Secondary.

Next, tune the secondary very precisely to make the station sound understandible. For a Lower Side Band station (as in the 80m, 40m, and 160m Ham bands), tune the Secondary a little higher than the point of maximum interference. If the sound is pitched too high, tune down the Secondary and if it sounds too high, tune up. For Upper Side Band stations (VolMet, ships, military, 20m and 60m Ham bands), it works the other way around.

Alternatives

Radios built to receive SSB, like this Tecsun PL-680, Trio 102, or Philips D2999, have an extra built in oscillator to provide the missing carrier. Also, the bandwidth of these radios is narrower, and they allow to control the strength of the SSB signal. Ease and quality of reception are often better then can be achieved with the Two Receiver method.

It is possible to modify a vintage tube radio by building a BFO that works at the Intermediate Frequency; I did this in an old scrap set and obtained very satisfying results.

The famous Grundig Satellit 2100 could be extended with external BFO, called the SSB Zusatz. This unit also works at the Intermediate Frequency.

Another external BFO is the Toekan, a small set top box that produces an IF signal, but inserts it through the antenna circuits. Succes requires a stable hand and some luck. The Kaketoe is basically a secondary receiver with everything stripped off but the oscillator; it operates exactly as described above.

What can you hear?

For your first attempts, try the 40m Ham band; with stations between 7050 and 7200 kHz, your Secondary must tune 6600 to 6800kHz. All Shortwave radios will suit as Primary and Secondary. You'll hear German, English, Italian, Dutch, Russian, and a lot more.

For the 80m Ham band, your Primary must tune 3600 to 3800kHz, and the Secondary must go down to 3150, so it should include the 90m band. There is also a 160m Ham band, with little traffic, so it's quite a sport to receive some Hams there. For the Secondary you must use the upper Medium Wave.

There is a 60m Ham band, around 5300 to 5400kHz, but it is rarely used. The 20m band, from 14100 to 14350kHz, is more challenging to receive, because it is difficult to tune the Secondary sufficently precise at these high frequencies.

Volmet stations provide weather information for airports. They are found at many frequencies, including 3413, 5450, 5505, and 8957kHz.

Gerard Tel.