[time-nuts] Safely getting the electrical length of a connected antenna feedline

[Chuck Harris]

If you have *the* active antenna, it is pretty easy to
make a little quadrapole circularly polarized antenna out
of a couple of pieces of 141 semi-riged coax, and to transmit
signal into the active GPS antenna, and measure the signal
coming out of the active GPS antenna.

If you don't have the actual antenna being used, you could
get close by using another of the same type and manufacture.

I tested all of my GPS antennas that way for gain, as a way
to be sure that they were functioning properly, but there is
no reason that you couldn't use a VNA to test them for phase
delay, group delay, whatever you desire.

You could even modulate your sweep generator with a pulse,
detect the pulse with a diode, and measure the delay with
your oscilloscope.

-Chuck Harris

Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> On 8 Aug 2016 21:23, "Bob Camp" <kb8tq at n1k.org> wrote:
>>
>> Hi
>>
>> An even more significant question:
>>
>> Is it worth doing?
> 
>> Your antenna and module could easily have delays
>> in the 40 ns range. It has no impact on a “frequency” GPSDO. It is one of
> a number of static offsets in a time transfer system.
>>
>> Even the NIST level outfits seem to have issues coming up with a purely
> mathematical answer to “what is the offset”.
> 
> I thought i might be possible to do this with a VNA, and a test antenna
> located a known distance apart. So I asked in the Keysight forum on 8th Feb
> 2015
> 
> https://community.keysight.com/thread/23082
> 
> There's a replay by Dr_Joel, who is a VNA guru. Dr. Joel Dunsmore,
> suggesting the use of the group delay function. In one sentence he wrote
> 
> "What is the level of delay accuracy you are looking to achieve.  With this
> method, 1 nsec is reasonable, but if you need 100 psec or 10 psec, then we
> will have to be much more careful."
> 
> The thread never got a complete solution, but it might help if other look
> at that, and perhaps start another thread on a similar topic, as that one
> is very old. However, if you don't have a VNA, I would not bother asking,
> as you are not going to get a response (no pun intended), to a non-VNA
> question on a VNA forum.
> 
> But IF it was possible to determine the delay through the antenna/filter
> within a ns, it would make measurements of coax length useful, whereas as
> Bob said, it is pointless unless you know the characteristics of the active
> antenna.
> 
> I don't know if there would be a way of generating a pulse and feeding that
> into two antenna
> 
> 1) Active one.
> 
> 2) Passive one at the same distance, and same length of cable.
> 
> The pulse should arrive at the same time if the two antennas had equal
> delay. But the signal from the active one will arrive later due to the
> delay. That might be possible to see on a scope.
> 
> I think assuming that the delay in the active antenna can't be measured is
> maybe an assumption that is untrue. You could perhaps do better if you
> built your own antenna, and characterized the SAW filter separately.
> 
> Dave
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