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

[Bob Camp]

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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