[time-nuts] Cabling GPS antennas
vogelchr at vogel.cx
Tue Apr 7 12:40:31 UTC 2009
> 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
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
compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm
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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