[time-nuts] How can I measure GPS Antenna quality?

Attila Kinali attila at kinali.ch
Mon Nov 21 09:54:53 EST 2016

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray <hmurray at megapathdsl.net> wrote:

> If I gave you a pile of data, how would you compute a quality number?  Can I 
> just sum up the S/N slots for each visible/working satellite?

There are multiple issues. As already mentioned, SNR is only a part of the
picture. What you are looking for is an uniform gain pattern over most
of the hemisphere, with a sharp decrease at low elevations. Then the
left vs right polarization ratio should be as high as possible over
the whole hemisphere (most antennas only have good polarization ratio
at the zenith and behave like a linear polarized antenna at low elevations).

Additionally to this comes the phase center stability. Ie that the phase
center is a fixed location, independent of azimuth and elevation. And this
is probably the hardest to measure.

Absolute (and probably the most precise) measures of these properties are
done in an anechoic chambers with a rotating antenna mount. 

The second way to do it, is to use a "known good" reference antenna and
use this as a comparison with a short (3-15m) baseline between reference
and antenna under test. For additional fancyness and to get better results
one can add the antenna onto robotic arm (like on the picture in [1]) and
get a more complete picture of the antenna. In this setup you want to have
an as fancy receiver as possible. At the minimum it needs to support carrier
phase data. The better receivers allow you to connect two antennas to the
same receiver and do a direct phase/amplitude comparison of the signals.

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation. Ie for the normal GPSDO that has a loop time constant
in the 100s to 1000s seconds and is using "normal" receivers, this is
completely drowned in the noise of the receiver's PPS output. Having good
sky view and as little multipath as possible is much more important.

				Attila Kinali

[1] https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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