We recently had one of our AM listeners write in and ask the following question:
> Why is the signal bad after 16:00ish and why does it stay like that untill 8am?
It’s a very good question and it has a fairly complicated answer but I thought it would make for some good Christmas reading:
AM signal propogation is finite and we use many different transmitters in order to cover the UK – this is the same with other stations. What we have to do, as an industry, is make sure we don’t overlap same or similar frequenices between coverage areas. One of the things we do is to adjust our frequency slightly in the smaller, so called “filler” regions around the UK. However this is not always practical so we need to make sure that same frequency overlaps don’t occur by getting the power levels on our transmitters just right.
Now this is where it gets a bit more complicated. During the day the D-Layer of the ionosphere of Earth has a fairly high absorption of the electromagnetc waves used in the AM radio spectrum. This allows us to crank up the transmission power and deliver a stronger signal (albeit in a more limited area). During the night, however, this absorbtion factor decreases and we get what’s known as “ionospheric reflections” where the radio signals bounce off the upper layers of the ionosphere and scatter back down to Earth over a wider area.
You’d think that was a good thing right? Well no. What happens is the same transmission power delivers the signal to too big an area and begins to cause the sort of interference I mentioned above. So we have to turn the power down to cover the same area. However, especially in the outer regions of coverage, the signal is having to travel much further as it’s being reflected first (see above) and thus the signal strength is lower by the time it reaches you.
DAB Radio broadcasts operate in the VHF (Very High Frequency) spectrum and the overall signal propogation is much lower (higher frequency = smaller wavelengths = smaller range) and ionospheric reflections are pretty much neglible. That’s bad for us (means we need loads of transmitters to get national coverage) but great for you because we don’t need to reduce power at night and the signal strength stays the same.
I hope that explains it for you. Physics is encouraging you to get a DAB radio!
Cheers,
Ben
