Film, Media & TV0 min ago
AM Radio
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Why is it that medium wave radio reception deteriorates when darkness falls? I'm thinking of Capital Radio and Talksport in particular.
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The Heaviside layer is made up bands of ionised gases in the upper atmosphere that reflect and refract radio waves. Over the 24 hour period, these rise and fall and become more and less dense depending on the amount of sunlight falling on them. At night, these changes will allow radio signals (and radio noise) from more distant parts to be picked up on your receiver, and if the station you want has got a weak signal, they can drown it out.
To add to Heathfield's answer regarding ionospheric reflections:
Even if the station you wanted to listen to was the only one in the world which was transmitting, you would still suffer interference after dark. During the day, the Heavyside layer is at a height whereby any signals which strike it, midway between transmitter and receiver, will do so at an angle that does not permit reflection. After dark, however, the Heavyside layer is lower. Signals strike it at a shallower angle and are therefore reflected towards the receiver. This means that your radio's aerial picks up TWO signals from the SAME transmitter. (One signal comes direct from the transmitter. One comes via reflection from the ionosphere). If these signals arrive 'in phase', the resulting signal is amplified. If they arrive 'out of phase', they cancel each other out. This explains the 'fading' which is frequently experienced with MW reception. (i.e. the station is effectively providing interfernce for its own signals).
VHF transmissions aren't significantly reflected by the ionosphere and therefore don't experience these problems. (The only exception is during solar flares. The effect of these is to cause the ionosphere to 'bounce' radio signals which would normally pass straight through. When this happens, it's not unusual for, say, Radio 2 to suddenly disappear beneath a stronger signal from, say, a Spanish station).
Chris
Even if the station you wanted to listen to was the only one in the world which was transmitting, you would still suffer interference after dark. During the day, the Heavyside layer is at a height whereby any signals which strike it, midway between transmitter and receiver, will do so at an angle that does not permit reflection. After dark, however, the Heavyside layer is lower. Signals strike it at a shallower angle and are therefore reflected towards the receiver. This means that your radio's aerial picks up TWO signals from the SAME transmitter. (One signal comes direct from the transmitter. One comes via reflection from the ionosphere). If these signals arrive 'in phase', the resulting signal is amplified. If they arrive 'out of phase', they cancel each other out. This explains the 'fading' which is frequently experienced with MW reception. (i.e. the station is effectively providing interfernce for its own signals).
VHF transmissions aren't significantly reflected by the ionosphere and therefore don't experience these problems. (The only exception is during solar flares. The effect of these is to cause the ionosphere to 'bounce' radio signals which would normally pass straight through. When this happens, it's not unusual for, say, Radio 2 to suddenly disappear beneath a stronger signal from, say, a Spanish station).
Chris
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