[time-nuts] Important parameters for a GPS/GNSS antenna

Jim Lux jimlux at earthlink.net
Thu May 7 19:57:55 UTC 2015


On 5/7/15 7:23 AM, Attila Kinali wrote:
> On Sun, 03 May 2015 07:29:30 +0000
> "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
>
>>> When you post-process raw GPS data you get to include antenna phase
>>> center / gain / az/el corrections for free.
>>
>> Speaking of which...
>>
>> I wonder if anybody ever made a rotating GPS antenna to average out the
>> X-Y phase-center offset ?
>
> There is a severe mechanical problem with that. Moving contacts
> are very hard to keep electrically stable. It works for simple
> power and digital signal wires, as there the only considerations
> are resistance over the joint and sparks. If you need to transmit
> analog signals you generally convert them into a form that does
> not depend on the amplitude of the signal (either going digital
> or doing a voltage to frequency conversion).
>
> Of course, this does not really work with a gps antenna, unless you
> put the whole receiver onto the rotary table. But then you shift
> the problem onto the PPS output (note: amplitude noise translates
> into phase noise).
>
> The best system i know about, for such rotating contacts with analog
> (possibly high frequency) signals is a small pot with mercury. But
> i guess you can see the problems that causes.
>
> The second best, but which only works with high frequency signals,
> is to use a hollow waveguide. There you need "only" to ensure that
> the waveguide walls are properly connected and you have a large
> area to use for that. If you can ensure that there is only one mode,
> you can make it such, that there is no current flowing over the gap.
>

They make rotary coax joints as well as waveguide. The coax is an air 
dielectric type.

There is, inevitably, some wow and flutter in S21 (and S11)




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