I believe that reflections inside the cable (that is, after the antenna)
are very different from reflections before the antenna. GPS receivers do
their calculations based in the different arrival times of the
satellites signals to the antenna center, so delays caused from
different paths caused by reflections on external objects really
matters. Reflections inside the cable, where phase relationships between
the signals is already fixed, does not matter, or at least, not very much.
I don mean to disregard matching, which can cause big signal level
losses if it is very bad. BTW, GPSDO's like the popular Thunderbolt,
are meant to use 75 Ohm cable and connectors but 50 Ohm cable can be
used without much difference. The PPS phase with respect to UTC, witch
is affected by the cable delay, must be adjusted with the corresponding
parameter. I think that a cable delay measurement could be more useful
in this regard.
Anyway, I can be wrong and if it is the case, I would like comments from
the experts here.
Best regards,
Ignacio, EB4APL
El 21/11/2016 a las 14:45, Dr. David Kirkby (Kirkby Microwave Ltd) escribió:
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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