Thanks for the quick replies so far guys. So from what you've told me we have a signal being broadcast in such a manner that it is amplitude modulated at 30Hz, which I understand now so that's cleared that up. Next question I've got is how this relates to the phase of this signal. What I understand is that if we look at the signals received by two different a/c at an instant in time then the amplitude of this signal depends upon the magnetic bearing from the station but I know the receiver equipment compares the phase difference to work out QDR so how does this varying amplitude relate to the phase of the signal and its spatial dependence?
Thanks very much for your time.