[time-nuts] 75Z vs 50Z for GPS receivers

[Dr Bruce Griffiths]

I think that the simplest explanation for the BIPM's recommendation that 
the antenna cables be matched to the antenna output impedance and the 
GPS receivers input impedance is as follows:

If one has gone to the trouble and expense of installing an antenna that 
is relatively insensitive to multipath effects, then using an unmatched 
cable to connect the antenna to the receiver in effect reintroduces 
multipath signals into the receiver input, reflections on the cable due 
to antenna and receiver mismatch are no different in character to 
multipath signals.

The magnitude of the effects of cable mismatch will depend on the degree 
of mismatch at the antenna and at the receiver as well as the cable 
attenuation, and the cable length
For the less accurate timing receivers such as the Trimble Thunderbolt 
such effects may be insignificant as other the effect of other timing 
error sources are much larger.

Bruce