What is Doppler Effect?
Doppler effect is an apparent shift of the transmitted frequency which occurs when either the receiver or transmitter is moving. It becomes significant in mobile radio applications towards the higher end of the UHF band and on digitally modulated systems.
When a mobile receiver travels directly towards the transmitter each successive cycle of the wave has less distance to travel before reaching the receiving antenna and, effectively, the received frequency is raised. If the mobile travels away from the transmitter, each successive cycle has a greater distance to travel and the frequency is lowered.
The variation in frequency depends on the frequency of the wave, its propagation velocity and the velocity of the vehicle containing the receiver. In the situation where the velocity of the vehicle is small compared with the velocity of light, the frequency shift when moving directly towards, or away from, the transmitter is given to sufficient accuracy for most purposes by:
fd = V/C fi
where
fd = frequency shift, Hz
ft = transmitted frequency, Hz
V = velocity of vehicle, m/s
C = velocity of light, m/s
Examples are:
• 100 km/hr at 450 MHz, frequency shift = 41.6Hz
• 100 km/hr at 1.8 GHz – personal communication network (PCN)
frequencies – frequency shift = 166.5Hz
• Train at 250 km/hr at 900MHz – a requirement for the GSM pan- European radio-telephone frequency shift = 208 Hz
When the vehicle is travelling at an angle to the transmitter the frequency shift is reduced. It is calculated as above and the result multiplied by the cosine of the angle of travel from the direct approach
In a radar situation Doppler effect occurs on the path to the target and also to the reflected signal so that the above formula is modified to:
fd = V/C fi
where fd is now the total frequency shift.
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