[time-nuts] GPS active antenna delay ?

[Tom Holmes]

HI Tom...

Good to see you getting into Time-Nuttery!

How accurately do you need to know the delay? For absolute time, it is important, but it doesn’t really help much with position since the calculation doesn't know which direction your receiver is located from the antenna. For a time-base it probably doesn’t matter much either; the PPS will simply be out of sync with 'real' seconds. The above comments will likely get me chewed up by the gnat's-eyelash crowd but that's how we mere mortals learn.

My first guess on the prop delay of the antenna would be the reciprocal of the bandwidth, but that is just a shot at getting into the ballpark. No doubt it varies over the BW and the amplifier adds a bit. Which leads back to how accurately do you need to know.

I have some ideas for helping you out with the measurements if you want to contact me off list. My TAPR email will work just fine.

Tom Holmes, N8ZM

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Tom McDermott
Sent: Tuesday, February 17, 2015 9:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS active antenna delay ?

Hi Dave - agree that VNA is one good way to measure the delay.  If required
accuracy is less than about
0.5 nsec, then Tx antenna to Rx antenna mutual impedance starts to become
an issue. Above about
1 nsec error probably most of these can be ignored.  No access to a vector
VNA that works at 1.5 GHz.
unfortunately.

Normally I would use a difference measurement (substitute known reference
Rx antenna for Rx
under test, and difference the two), but am afraid that the (reference
Rx-to-Tx) and the (Rx under test-to-Tx)
might have different mutual Z.

Thanks for the pointer to the Keysight VNA discussion list.

-- Tom, N5EG






On Mon, Feb 16, 2015 at 6:12 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> On 7 Feb 2015 19:18, "Tom McDermott" <tom.n5eg at gmail.com> wrote:
>
> > Has anyone on the list measured or otherwise estimated the active antenna
> > delay including the amp and filters?
> >
> > -- Tom, N5EG
>
> I have never done this, but suspect that using a VNA is the best way to
> go.  With a simple passive dipole on one port, and the active antenna and
> cable on the second port, a transmission measurement (S21) is probably the
> way to go. But I was unsure of exactly how to do it.
>
> Your question prompted me to ask on the Keysight VNA forum. There's a reply
> by Dr. Joel Dunsmore - one of the worlds leading authorities on VNAs.
>
> http://www.keysight.com/owc_discussions/thread.jspa?threadID=39151&tstart=0
>
> He asks what accuracy is needed. Being time-nuts, I think the answer is "as
> high as possible". He writes
>
> "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."
>
> You might want to contribute to that forum post.
>
> Dave.
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