[time-nuts] How can I measure GPS Antenna quality?
davidwhess at gmail.com
Mon Nov 21 13:19:02 EST 2016
When I was doing VHF and UHF direction finding antenna design, I would
drive out to the highest readily accessible hilltop for testing. Once
I came up with a low sidelobe design, I started picking up things like
lamp posts, trees, and bushes in the parking lot, aircraft over LAX
and John Wayne airports 50+ miles away, etc. which limited testing
While a perfect test environment is handy for design, a GPS antenna is
going to be subject to all kinds of environmental limitations so I
would accept field testing which includes considerations like
multipath, temperature variation, and a generally hostile RF
On Mon, 21 Nov 2016 08:22:50 -0800, you wrote:
>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
>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
>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
>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.
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