[time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

Scott Stobbe scott.j.stobbe at gmail.com
Mon Nov 21 20:53:05 EST 2016

I haven't used one personally but a spirent gps simulator would let you do
a try it and see.

It will be interesting to see if out of the growing sdr community an open
source gps simulator emerges.

On Mon, Nov 21, 2016 at 1:01 PM Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> 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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