[time-nuts] Cabling GPS antennas

Christian Vogel vogelchr at vogel.cx
Tue Apr 7 12:40:31 UTC 2009

Hi Steve,

> these frequencies. I was thinking outside the square and see that
> dual/quad screened RG6 is cheap and plentiful now but of course it's
> 75Ohm and there would be a big fat impedance mismatch using this. I
> thought about looking at fitting impedance matching baluns at each end
> but that is not cheap and there are losses involved with this approach
> anyway.

the manual of the Trimble Thunderbolt GPSDO recommends doing exactly  
that. 75Ohm cable obviously is cheaper, due to it being ubiquitously  
used for TV and video installations and has lower loss due to the  
higher impedance (impedance/resistance ratio). They (Trimble) claim  
that reflections will not introduce any problems.

One can argue that, to cause interference effects, a wave has to  
travel back from the GPSO to the antenna and back to the GPDO (where  
it interferes with the direct signal). This will be attenuated by the  
SWR twice, and dampened by the cable loss twice.

The issue at hand has been discussed at length one (two?) year(s) ago  
and included a link to an article that analytically analyzed the  
effect of multipath (of which the reflections are a specific case) on  
GPS accurady, if I remember correctly.


--- Quote from the Trimble manual (ThunderboltBook2003.pdf), Page "3-5" ---
Note ? RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet  
antenna are
compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm  
cable, 75
ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and a
better quality cable for the price. Mismatched impedance is not a problem.

Note ? The input impedance of the ThunderBolt RF input & its antenna  
is 50 ohms.

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