AM Radio

[Dudley]

Why is it that medium wave radio reception deteriorates when darkness falls? I'm thinking of Capital Radio and Talksport in particular.

[vehelpfulguy]

Not sure, but you can listen to Talksport on Freeview or Sky on your TV for digital quality reception.

You can also listen to it on your PC, got to the Talksport website and press Listen Now.

[Dudley]

Thanks for that VHG but it is in the car I most notice it and vehicle manufacturers seem to be unduly backward in respect of DAB radio in cars.

[spacechimp]

Could it be something to do with transmission licencing laws?

[Mouse0_0]

Just a guess, but perhaps the radiation emitted from the sun somehow reflects or amplifies radio-waves back towards the earth during the daytime. I know that MW reception is better on a cloudy day because of reflection, but maybe EM radiation from the sun has a similar effect...?

[heathfield]

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.

[Dudley]

Thanks for those very interesting answers.

[Buenchico]

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