There are a number of plausible explanations:
(1) The tuner in Glen's car may not be calibrated correctly. This might cause the search to lock onto the wrong signal, or to report an inaccurate frequency. (Digital readouts are preset to certain frequency bands -- an analog tuner might show the correct frequency.)
(2) There are a number of atmospheric conditions that effect radio propagation.
- In hills, radio waves that rely on line-of-sight propagation may be intermittently lost.
- Clouds, especially water, can absorb some radio waves and weaken the signal. Or they might scatter a signal. In either case, a closer, stronger signal may override the scattered or attenuated signal.
- Reflection on certain surfaces (e.g. thermoclines, etc...) can reflect incoming waves in ways that refract or diffract the incoming waves thus weakening the signal.
- The signal may be frequency shifted by some combination of reflection, refraction, and/or diffraction.
Glen is driving through the highlands and tunes his radio to listen to the DJ broadcasting at 98.7 MHz. Instead of hearing the frequency he has tuned to he hears the frequency 98.9 MHz.
The reason behind this is what is known as the Doppler effect. If an observer is moving towards a source of electro-magnetic radiation the frequency of radiation emitted by the source as recorded by the observer is higher than the actual frequency of radiation emitted by the source. If the observer records a frequency f' and the actual frequency emitted by the source is f, the two are related by f'=f*(v/(v+s)) where v is the velocity of emitted radiation and s is the velocity at which there observer is approaching the source. The velocity of electro-magnetic radiation is 299792458 m/s m/s approximately.
Substituting the values in the formula given earlier:
=> s = (98.7/98.9)*299792458 - 299792458
=> s = 606253 m/s
Glen is driving away from the radio station at approximately 606253 m/s.