OK a further update. Reception is affected when driving as the ariel is being buffeted and moves so I get rubbish reception. If I hold ariel through sunroof it's ok so. I know it's the ariel base as the female screw thread has a crack.
AM and MW are the same thing. But it does sound like it's using the aerial to receive this frequency.
The reason radios generally use a coil on a ferrite former to receive AM/MW, is due to the longer wavelength than FM. This long wavelength requires a very long aerial, so it's normally wound up on a bobbin inside the unit. So to get good AM/MW on a short aerial, the receiver needs be very sensitive, with heavy pre-amplification. If your FM aerial is broken, and it's being used for AM/MW too, then that is likely to be the problem.
The standard FL1 aerial is a compromise for receiving FM and would be bad at receiving AM. A full wave FM aerial needs to be a little over 3 metres long, for maximum signal power reception, the FM wavelength being a little over 3metres long. However a 3 metre aerial stuck on the car roof would be completely ridiculous. So the aerial is shorted, often to 1/4 wavelength , giving 75 cm, which is about the length of the factory and much more practical for car use. This length is still a compromise, as the aerial needs matching perfectly to the wavelength, which of course changes with the frequency being received.
AM/MW is a different kettle of fish altogether, due to MW's very long wavelength. For MW, the minimum aerial length for good reception at the high end of the scale would be about 170 metres long, which is even more ridiculous on a car. This is why AM/MW aerials are often made of long wire rolled around a bobbin.
So if the Becker nav unit does get it's AM/MW signals from FM aerial, it's definitely not optimized for the job, not even close.
