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

[Bob Camp]

Hi

Relay based attenuator. Oddly enough I have data on more than one generator :)

(ouch indeed if it’s not being paid for on somebody else’s credit card)

Bob

> On Nov 22, 2016, at 10:50 AM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> 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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