[time-nuts] How can I measure GPS Antenna quality?
kb8tq at n1k.org
Mon Nov 21 11:38:49 EST 2016
> On Nov 21, 2016, at 9:54 AM, Attila Kinali <attila at kinali.ch> 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?
If “sum the S/N” gives you a difference you should immediately ask why.
Normal receiving antennas are a gain = directivity sort of beast. There is
not a lot you can do about that. For a GPS antenna, you want to be able to
receive over a hemisphere. You don’t really know in advance where the antenna
will be used, so that’s how it’s done.
Early on the designs had more gain straight up than at the horizon. That’s a
bad thing. If anything, you want more gain at the horizon. Signals are strong
from straight overhead (short path, less atmosphere) and weak(er) at the
horizon. They could easily give you a better “sum the S/N” number while
actually performing worse in a location with sat’s mostly overhead. A “straight up”
antenna might be wonderful at a location on the equator. It’s probably a disaster
at a location on the arctic circle.
The real answer for signal to noise will always be location dependent. If I’m in
an urban canyon the only “sky” may be straight up. If I have a lot of terrestrial
broadband noise close to the horizon, again straight up might be the answer.
If my antenna is on top of a pole and I have a clean view 360 degrees around and
down to < 5 degrees elevation, a straight up antenna is very much what I do
Even more complex: If I have a bunch of transmitters at a wide range of frequencies running at the
same site as the GPS, I may want *really* good filtering ahead of the preamp. Those
filters likely will have a temperature sensitivity.The filters create loss ahead
of the preamp so the noise figure (and thus S/N) take a hit. I get something I desperately need
and trade it off against degraded performance in other areas.
Lots of variables.
> 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 ) 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
>  https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html
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