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

Mon Nov 21 11:22:50 EST 2016

```On 11/21/16 6:54 AM, Attila Kinali wrote:
> 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.

I'm not sure about whether an anechoic (which is really "hypoechoic")
chamber is going to get you the data you need.  Calibrating the chamber
to the needed level of accuracy might be harder than doing field
measurements.

It might just be because there's a ton of analysis software out there,
but the folks who really, really care about 0.1 mm shifts in phase
center seem to use field data in a well characterized site, and
accumulate it for a number of days.

The GPS antenna folks at JPL, when they're testing a spacecraft antenna
for things like precision orbit determination (a basic choke ring sort
of thing) go out with the antenna and a test receiver on a cart in a
parking lot.

Looking at it in terms of numbers:
1mm is 1/150 wavelength, or about 2-3 degrees of phase.

sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3
cm different path length (modulo wavelength) and 30 dB down will give
you a 1mm phase center error.  0.1 mm is -50dB.

Now, it's true that if you had a good spherical near field range, with
time gating, you can probably get rid of the reflections from the
chamber (and, in fact, you can do the measurements in a regular lab, or
your garage). But even there, it's tricky, because the probe calibration
has to be very good, and the structure supporting the scanning probe
also has to be accounted for.  You might be able to do it by doing
transmit/receive measurements on something like a spherical target of
appropriate size.

I've done measurements on what was essentially an interferometer with a
2 meter baseline, in a conventional chamber on a conventional pedestal
(JPL Mesa 60 ft chamber  http://mesa.jpl.nasa.gov/60_Foot_Chamber/).
You could easily see -40dB specular reflections as the array rotated.
(and you could also see things like the ladder on the positioner behind
the antenna we accidentally left in there, even though it was behind the
horn antennas in the array)

I think a good test using satellites and a very well characterized
comparison antenna in a open air test site is probably the easiest, and
most accurate, way to do it.
Arranging your test on a post well above the terrain, and making sure
that the surface is flat is easy.

>
> 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
>

```