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 performance.
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 environment. 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 >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. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.