On 11/21/16 2:58 PM, Attila Kinali wrote:
On Mon, 21 Nov 2016 08:22:50 -0800
jimlux <[email protected]> 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.
[...]
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.
Interesting. I haven't done the math, so I didn't think about that.
Yes, the reflections in the chamber would probably limit the resolution.
Now I wonder how the calibration data for mass produced geodetic antenna
are collected. I very much doubt they put them outside for a couple
of days to measure them exactly.
That's exactly what they probably do (put them outside) - assuming they
have an individual cal at all.
A *good* antenna design is one where if the mechanical assembly is
within manufacturing tolerances, it will have the same performance as
all the others made to the same tolerance.
ftp://www.ngs.noaa.gov/pub/abilich/calibPapers/Goerres2006.pdf
Note that the residuals after cal were biggest at 0 and 90 elevation,
and best in the mid elevations..
Attila Kinali
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