Transmission Mode

There are two main modes of transmitting over radio waves. These are Amplitude Modulation [1] (AM) and Frequency Modulation (FM). The license permits the use of either mode. There is a third mode, Digital which is basically using FM, but encoding the signal first, but that is not covered by the license, and is outside the scope of this booklet.

Amplitude Modulation is the easiest way of transmitting, and when radios were first invented, was the only way. Unfortunately, the sound quality of AM is not brilliant, so some bright spark invented Frequency Modulation. The sound quality of an FM signal is much clearer than AM and is also less likely to get distorted or interfered with, making it much more reliable. The only drawback with FM is that once the signal strength drops below a certain level, the sound simply vanishes - while AM will carry on regardless, just getting quieter and quieter. Because of this, AM will often go further than FM in the same situation.

On the other hand, AM is more susceptible to interference by the atmosphere than FM. Anyone who listens to an AM (A.K.A. medium wave) radio station knows that the reception quality goes down with the sun. Also, AM is blocked more by structures than FM - the radio goes quiet right in the middle of your favorite song as you drive under a bridge, and comes back on with Cliff Richards when you drive out again.

Because of FM's higher quality most manufacturers now only produce FM sets, which means that if you want AM sets, you will either have to look harder and pay more, or settle for second hand.

If this is the case, why would anyone want to buy AM radios any more? Because of the different ways in which AM and FM carry the signal, they cannot talk to each other. In fact if both types of set are used in the same area, they will interfere with each other. This does mean that if the rest of your team uses AM sets, then you will need AM too.

The R.A. have note restricted our license regarding modulation which means that we can use both AM and FM. As stated earlier e cannot use digital transmissions as that involves encoding the signal which is not covered by our license. This is described further later in the section Voice transmissions only.