[time-nuts] Precision Time Protocol – Windows 10 implementation

[Achim Gratz]

Adam Kumiszcza writes:
> I've made a ca. 10.5 cm metal disc cut from an old car radio chasis and put
> it under my gps antenna. It does not hang now, but lays on the window sill,
> facing up.

That way you changed three variables in one go.  You'd be better off
changing one single thing each time so you can see what changes cause
which results.  You do have a patch antenna (rectangular flat shape),
not a quadrifilar helix, do you?

> I cannot see any difference between these positions. I've made 10
> measurement for each position, getting 12 SNR values for satellites for
> each measurement and then calculated the average of these.

Looking at your gpsmon pictures you seem to see the low elevation sats
at very similar SNR as (most) high elevation ones.  Any RF reflecting
things around in your neighbourhood?  Also, as David already mentioned,
thermo-pane windows are actually damping RF quite significantly and even
reflect them at low incidence angles.  You might try a few minutes with
an open window and otherwise unchanged antenna location to see what
happens.

> Hanging antenna had 25.18 SNR average (2.21 standard deviation). Antenna on
> a ground plane had 26.10 SNR average (2.08 standard deviation).

The constellation providing your fix (four satellites minimum) should
probably all be well over 30dB, peaking in the mid 40dB SNR range.

> If I take into account single high elevation satellite with good SNR
> (satellite 26 in the examples), the new placement and ground plane even
> lowered it :(

You may have to experiment with antenna position a bit more.  I have
seen more than 15dB changes from rather small changes in antenna
placement.  It's definitely worth experimenting a bit to find one that
is good over the full cycle of the GPS constellation.

> I guess the real benefit of the ground plane would be if the antenna was
> outside, or my measurement method was wrong?

No, the ground plane (if your antenna needs one, not all do) is there to
improve the gain in the forward direction and make the elevation pattern
more uniform.  Outside oir inside doesn't matter, but antennas are
influenced by many things in their near-field region (at around 1.5GHz,
a few wavelengths at most, so ~30…50cm).  Again, you may be too close to
the window pane for instance.


Regards,
Achim.
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