Receiving Modes

AM

Amplitude Modulation, AM, is made up of two parts. The radio frequency part that determines where the station will be on the radio dial and how strong it will be. It is this part that moves the Signal Meter, if you have one. Not having one does not make you a bad person. The other part is the audio frequency that we eventually hear after the radio has recovered it from the RF, the radio frequency part sometimes called "the carrier".

Dealing with SSB

Try this as a concept. Imagine you are able to stand on the carrier and look up and down the radio band. You will see the audio has produced two identical sidebands on each side of you that hold what we want to hear. It is the radios ability to deal with these sidebands which will determine our listening enjoyment.

If we go back to the idea of standing on the carrier wave and we are happy that in an ideal world the two sidebands are the same, we can save up to half the power by only sending one. We can make greater savings by reducing or suppressing the carrier altogether, sending only the Lower Side Band or LSB. The radio will put the carrier wave back in again to render the voices clear.

Fine tuning is required for Single Side Band or SSB work, any error showing up as a voice pitch change from Paul Robeson to Minnie Mouse. Some portables may only have an SSB button. Use this. The most used mode for point-to-point communications is Upper Sideband or USB.

Returning to the concept of standing on the carrier wave, the theory is the same for LSB but here only the upper sideband is being sent, the lower sideband and the carrier being suppressed. In reality, there is always some leakage of the unwanted parts of the signal. These are usually only one-millionth part of the whole signal, a mere bagatelle dismissed by the professionals and talked about endlessly by the amateurs. With no carrier to hold the signal meter steady, the needle responds to the energy in the wanted sideband.

Accountants cheer when they learn that if we have got rid of half the signal, then we can deliver twice the power in the wanted half. This piece of economics makes USB the mode for all professional communicators.

What's ISB?

ISB is an Independent Sideband transmission with two entirely different services, one on each sideband. As the radio is expected to work very well in SSB but also receive two stations in less bandwidth taken up by an AM station, ISB is the province of the more costly receiver. ISB is now mostly a back up to a satellite feed where two language services are sent to the same relay station. Try to find an ISB Feed these days...

Many textbooks have been written on radio theory, our mind-expanding concept of standing on the carrier peering into the sidebands tending to generalise large areas of engineering practice. Remember, if anyone offers you that kind of mushroom again, just say no.

Fax

Although this Guide deals mostly with voice circuits, pressure was put on the writer to include some fax frequencies. Most decoders use the audio output from the radio feeding a comparator to turn the high and low tones into noughts and ones. They either stand-alone (like the author) or connect to a computer. The receiver offers carrier-insertion, usually USB, to provide the two tones used by the decoder to result in readable text. Some radios now have a DATA mode to provide the correct frequency offset, but:

Careful tuning is needed and your frequency readout may differ from the figures listed at the end of The Guide. This is due to your configuration of the tone set-up. Getting it right comes with experience. As long as the decode is consistent, don't worry about the least significant figure on the display.