[time-nuts] Cabling GPS antennas
david at endor.com
Tue Apr 7 17:15:54 UTC 2009
I find it hard to believe myself, but the numbers don't lie. A 1.5:1
SWR junction gives 0.177dB transmission loss and a -13.98 dB
reflection. If you have a true 50 ohm antenna and receiver and use
75 ohm LOSS-LESS cable, the transmission loss will be only 0.35dB and
the doubly reflected wave will be 28dB down. Any line loss makes
things rapidly better still. 15m of TV foam-type RG-6/U will have
~3.7dB cable loss or ~4dB total loss, and a reflection 35dB
down. BTW, return loss of the cable itself is rated at -15dB @
1500MHz! And, as has been pointed out, the SWR of the receiver and
antenna will not be 1:1 either.
At 08:47 AM 4/7/2009, you wrote:
>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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