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

David 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 
>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.

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