[time-nuts] Cabling GPS antennas

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Apr 7 12:47:09 UTC 2009

The reason that such cable mismatch isnt a problem is that the VSWR of
the Thunderbolt input is relatively high.


Christian Vogel wrote:
> 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.
>          Chris
> --- 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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