When a radio signal is broadcast from a transmitter, some of it听disappears upwards into space and is lost. However, during certain weather conditions, the 鈥渓ost鈥 signal can be bent back towards earth and received much further away than normal.听Potentially, this听signal can travel great distances -听up to many hundreds of miles.
This is known as tropospheric ducting and it can distort, or prevent radio reception.
What causes tropospheric ducting?
Normally, the higher you go into the atmosphere, the colder it is. However, high pressure can cause a temperature inversion.听This is where a layer of air in the troposphere will be at a higher temperature than the layer below it. Some of the radio signals from a transmitter bounce back towards the ground and potentially up again. The听inversion effectively creates听a duct for signals to travel along.听
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When does it happen?
Tropospheric ducting is relatively common during summer months and especially during evenings in periods of stable and fine weather. It tends to happen when we have very hot days followed by rapid cooling at night. This is why it is common near the coast.听
When we are aware of this happening, we will put up a news item on听.
What effect听does it have?
As the signals can travel further afield it can become possible to receive services you can鈥檛 normally get. Your aerial can听pick up the听signals from distant transmitters as well as signals from your local one or, if the signal is very strong, you will pick up the more听distant radio service.
If they are on the same frequency, it can cause co-channel interference.听This will cause a temporary loss of reception or two radio services to be heard at the same time.听These additional signals can sometimes cause problems听with relay transmitters in our network.听
How long does it last?
This is hard to estimate as there are many variables that cause tropospheric ducting. It often appears at the same time on consecutive days and then can disappear for several months at a time.
What should you do?
When this happens there is nothing you can do but to wait until the weather changes.听You should not re-tune your听radio.听
