[time-nuts] Trueposition Antenna Location

[Mark Goldberg]

On Fri, Nov 30, 2018 at 8:56 AM Attila Kinali <attila at kinali.ch> wrote:

> On Thu, 29 Nov 2018 21:22:44 -0700
> Mark Goldberg <marklgoldberg at gmail.com> wrote:
>
> > How much could I expect ADEV to improve if I move the antenna to a better
> > location with clear view of the sky? The things I am testing need to be
> > better than 8e-10, so this at just under 3e-10 may be "good enough".
> Moving
> > the antenna would involve drilling a hole in the outside wall, so it is
> not
>
>
> As Bob already wrote, antenna position is only one thing. But if your
> receiver is going into holdover at all, then you are already in a place
> where you will not have good enough reception to get down to your
> requirement.
> Antenna position, ie sky view and multipath, are the two things that
> kill most of the performance of any GPSDO. Bad sky view can result
> in several µs of phase variation and multipath in several 100ns.
>

I definitely have a bad sky view. The antenna is in a ground floor window
of a two story home with a metal covered RV parking area next to it. I can
only see maybe 30 degrees of sky. I am in Arizona. The view is basically
from about straight up to 60 degrees from the horizon to the south.

>
> After that, you will need a local oscillator with sufficient stability
> to average over a long period. To get below 1e-10 you will need to average
> over at least 100s (given good receiver and perfect antenna position),
> better make it 1000s to 10ks.


I made the measurements using the setup described at:

https://sites.google.com/site/spectrumlabtesting/

and the other sites linked there. There could be some anomalies due to the
peak frequency measurement algorithm.


> This means, the local oscillator has to be
> stable enough to stay below 1e-10 over that period. Which means you
> need to use a good OCXO at the very least, in a temperature stable
> environment. No-one of the GPSDOs you mentioned fullfills that requirement.
> (The NV47* of the TruePosition is decent, but falls slightly short.
> Leo's GPSDO uses a TCXO)
>
> I would recommend you to get either a Thunderbolt (first choice)
> or a Star4 (second choice) from ebay. Alternatively, get one
> of Nick Sayer's GPSDO, modify it to use a good OCXO and crank
> up its loop time constant.
> Do not get one of the Trimble UCCM, they have quite horrible
> frequency jumps.
>
> Unfortunately, I have what I have for now. I have basically three devices,
the TruePosition and Bodnar GPSDOs, and an 8642A generator with the OCXO
option. None are optimal I know, I just want to do the best with what I
have.

Thanks for the suggestions though.

Regards,

Mark