[time-nuts] Delay through GPS antenna splitter/amplifier -- an answer, and a question

[John Ackermann N8UR]

Tom Van Baak wrote:

> John, next time you can borrow that instrument blow
> some hot/cool air on the antenna and see what changes
> and by how much.
> 
> I've heard that the older GPS antennas, the ones with zero
> or less RF filtering, were much better for timing applications
> but have never seen data to prove it.

That would make sense.  The HP splitter has a single filter of some sort 
(probably SAW) on each output port.  It is probably very similar to the 
filters used in current antennas.

Just a swag, it's probably reasonable to assume that each stage of 
filtering adds about 10ns of delay.

The tempco of the antenna amplifier/filter is certainly another factor 
to consider.  If I can get hold of the analyzer again, we can do a hair 
dryer test on the splitter to see how it's affected.  My mantra has 
become "every resonator is a thermometer".

> For example, do you have one of the old Motorola hockey
> puck ones you could test? If not, I can send you a bunch
> of different GPS antenna to try.

My hockey puck died and has been replaced by a Timing 2000 antenna.  I 
do have a couple of others sitting on the shelf that I could test.

But, I'm not sure how to non-destructively test an antenna, as the 
measurements we did require two-port analysis.  I'm not sure if we could 
  radiate enough signal into the antenna to get a usable output.

By the way -- for anyone who's considering similar tests of amplified 
GPS antennas or splitters, note that common network analyzer output 
levels will drive the amplifier into compression and cause messed up 
readings.  You need to shove as small a signal as possible into the 
device; 0dBm will definitely hose the readings (though it probably won't 
damage anything).  I think we found that -30dBm was about the maximum we 
could use without seeing gain compression.

John