Abstract
Mobile FM Radio reception is particularly vulnerable to a phenomenon known as multipath distortion, which degrades or distorts audio quality.
Multipath distortion is caused by radio signals reflected from nearby or distant objects combining with the direct radio signal. Comparing the reflected signal delay time to the transmission signalling time is possible to divide the multipath in two classes: short and long time delay multipath.
Short delay multipath takes place when the direct signal combines with a reflected signal from nearby objects like buildings at specific points in space. As the path difference between the signals is relatively small, the signals show a higher correlation.
If the reflected signal is phase reversed, compared to the direct one, and has a comparable amplitude then cancellation of the two signals results: there is no signal, just noise.
Long time delay multipath occurs when the direct signal combines with a reflected signal from a faraway object . As the path difference between the signals is large, the correlation between signals is lower. Hence the superimposition of similar amplitude signals does not result in cancellation but in uncorrelated information which creates audio distortion.
In either case, the car antenna goes through points in space where there is no signal or a distorted signal that degrades audio quality.
In order to decrease the effect of multipath on mobile reception diversity systems are employed .
They consist of two ore more antenna placed at some distance on the vehicle, plus an electronic control system which elaborate signals coming from different antennas.
Based on the algorithm implemented in the electronic control system several types of diversity are possible (switched antenna diversity, linear combination antenna diversity, maximum signal/noise ratio diversity etc.) To evaluate the real performance of a diversity system it is necessary to consider the whole system
(antennas and tuner) trying to relate the audio quality with the radio frequency field distribution.
A possible method to do this evaluation is to simulate a multipath field distribution in anechoic chamber and measure the effects on the vehicles audio system.
This paper describes the experimental methodology developed to measure the effectiveness of on vehicle diversity systems.