Why don't radio stations interfere?

John - It depends on the frequencies we're talking about. As I mentioned earlier, medium wave has about 120 channels available for use across Europe. It's all coordinated internationally so that some countries have a preferred frequency for high power stations and then other countries can use these frequencies again, but only on low power.

So for example, if we were to take radio 5 live which is on medium wave across the UK, it has two primary frequencies, 693 and 909 kilohertz. These are high power channels, so these would have maybe half a dozen transmitters on each of these frequencies, and that can cover the UK with a few fillers around and about, on different channels. That means that these frequencies could be used by countries, maybe like south of France or Spain at low power, so long as they don't interfere with the national service.

In terms of local radio, nearly all the local radio frequencies we use on medium wave are re-used frequencies. So in other words, another country has these frequencies as its high power allocation and we're allowed to use it on a non-interference basis. What that means is, we mustn't interfere with them. That doesn't mean to say that they mustn't interfere with us because they are the primary users and as I said earlier, the after dark problems, sky wave propagation means that quite often, you might be listening to radio Norfolk on 855 kilohertz, you can hear a Spanish station quite strongly in the background.