Why do AM and FM radio stations in the UK (and presumably worldwide) have to change their frequencies every 50 or so miles rather than keep one for the whole country?

Baloo55th

Very simple with FM - FM works on line of sight tramsmission. Go too far and you just lose the station. That's why national stations have different frequencies for different areas. A frequency can be reused once you're far enough away from the first one not to cause interference. AM stations that broadcast locally are low powered and if a station covers a wider area than one tramsmitter can cover, they will have two frequencies. Also, if they have a large blind spot, they may have a different frequency to cover that area. Then it has to be different from the main one to avoid overlap interference. Most national AM stations have one frequency, except for blind spot coverage. If you get two transmitters broadcasting the same signal over the same area, you get beats set up in the reception as the signals arrive at slightly different times. Not nice for the listener. When in a car, most new radios have RDS which works on FM by searching for the station you want when the received signal falls below acceptable limits. Previously, you had to twiddle the knob when the radio went fuzzy. Now, when you're on Radio 3 it looks for Radio 3 on a nearby frequency. The FM signals carry station data (and time signals) as well as the music or whatever. Feb 21 06, 5:30 PM

Baloo55th

It doesn't only work on trams - please excuse tramsmissions. Just been celebrating National Pub Week. What will they think of next.... Feb 21 06, 5:40 PM

satguru

That's many years of wondering finally solved, not many people in the past (none I asked in fact) seemed to know that, believe me! Feb 21 06, 6:44 PM

Baloo55th

I mentioned this question to GingeryNutt's husband. His reaction: "***** obvious, that!" Then again, he is a radio ham and repair/build expert. My electronics training is nothing compared with his. Feb 22 06, 1:03 PM

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