[time-nuts] Do reflections up/down the antenna cable cause a problem, with GPS? (Bob Camp)

John Haine john.haine at haine-online.net
Wed Nov 23 08:40:04 EST 2016

Hello all, I'm a recent recruit to this list, it looks very interesting 
and I hope I can make a contribution.  Though now retired I'm an 
electronics & radio engineer and until last March worked for u-blox 
(though not on the GNSS side of the business).

Looking at their product integration guides they don't mention antenna 
match and cable length.  They do suggest inserting a SAW filter between 
the antenna and module especially with an active antenna, and of course 
the group delay of a SAW filter makes it equivalent to quite a long 
length of cable - at least 30 cm a nanosecond.  Multiple reflections 
will be attenuated at each reflection - if the return loss is say 6 dB 
then you lose that much at each reflection, plus the attenuation in the 
cable itself.  So the effect of the cable plus mismatches will be to add 
an extra channel impulse response that will be quite short as the 
various echoes decay.  Given that the system is working with wanted 
signals well below thermal anyway, a couple of reflections plus a dB or 
two of cable loss would make the largest echo ~15 dB below the wanted 
signal.  I suspect that this will cause rather small error in the 
device's estimate of pseudo-range  but this will be the same for each 
satellite.  Overall I doubt that the effect will be serious either for 
positioning or timing - if it were I think (knowing u-blox) they would 
make recommendations in the guide.  By the way they don't even mention 
it in the context of their high-precision devices.

(And I've just found an application note where they do discuss matching 
but only in the context of maximising SNR, i.e. power transfer.)

But if one is really concerned then the obvious thing to do is mount the 
module/chip right on the back of the antenna so there is no cable.

On 23/11/2016 12:00, time-nuts-request at febo.com wrote:
> Re: Do reflections up/down the antenna cable cause a problem
>        with GPS? (Bob Camp)

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