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

Scott Stobbe scott.j.stobbe at gmail.com
Tue Nov 22 10:50:50 EST 2016


Ouch! Relay stepped attenuator? Or solid state components?

On Mon, Nov 21, 2016 at 11:00 PM Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> There are packages you can put on a fairly standard HP signal generator
> that will
> do the mismatch stuff without spending all the money Sperient wants for
> one of
> their machines. One suggestion: If you *do* go with the HP solution,
> running the
> attenuator up and down to simulate fast fades (think urban canyon) will
> fry the generator
> in about 30 days … I have empirical data ….
>
> Bob
>
> > On Nov 21, 2016, at 8:53 PM, Scott Stobbe <scott.j.stobbe at gmail.com>
> wrote:
> >
> > 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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