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

Bob Camp kb8tq at n1k.org
Mon Nov 21 14:36:49 EST 2016


Based on extensive testing of the line mismatch issue, the answer turns out to be “it does not matter”.

The reflection issue ahead of the antenna is a reflection of the signal from a single satellite. The multipath 
reflection makes that satellite appear to be further away than it really is. In the case that the reflected 
signal  is *stronger* than the desired signal, the multipath reflection “captures” the receiver and the net
solution is messed up. 

In the case of a mismatched cable, there is no “single satellite” issue. Everything is impacted by the mismatch. 
Even if the mismatch is pretty bad, the “primary” wave is the one that will dominate at the receiver end. The 
reflections will always be lower in amplitude. That effectively guarantees that you don’t have a multipath 
issue from the coax. 

Yes, there is more to it than this simple explanation. The conclusion is still correct. There is no significant impact switching 
coax from 50 ohms to 75 ohms and having both ends of the cable at an impedance that is not equal to the cable’s
characteristic impedance. 


> On Nov 21, 2016, at 8:45 AM, 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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