Re: CAR RADIO

[Doug Craigen <dcc@cyberspc.mb.ca>]

David Abineri wrote:
> Thanks for all your help, Dick.
>
> I have always taught that the reason a car radio fades in and out when
> one moves
> just a few meters (or less) is that the signal is reflecting off a
> building (or
> other refelctive object) nearby and arriving out of phase with the
> original signal
> at the antenna.
>
> However, with all the available objects for a signal to reflect from
> before
> arriving at my antenna, this explanation does not seem correct.
>
> Can anyone illuminate this phenomenon?

The normal spread in intensity over the region of one wavelength is
around 15 dB.  This is precisely because of "all the available objects
for a signal to reflect off of". This is referred to as "multipath"
interference.  ("Shadowing" effects are distinct, but also contribute.) 
Look at it this way, each reflected signal arrives in a different
direction with a different phase at each spot.  Over a region of a few
wavelengths in length and width a complex standing wave pattern will
occur.  For a large number of similar magnitude sine waves arriving at a
spot, the statistical intensity distribution is a Rayleigh one. The
bottom is arbitrarily close to zero - perfect destructive interference
between all rays, and the maximum is N^2*I_average corresponding to them
all being in phase.  The 15 dB spread corresponds, if I recall
correctly, to 2/3 of the cases.


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Doug Craigen          "Technology with purpose"
                       http://www.dctech.com