I saw a rather expensive GPS antenna made by one of the
big-name GPS survey equipment mfgrs that was mounted
on top of a 12 or 15 inch diameter disc about 3/4 inch thick.
Turns out that the disc is made of some sort of RF absorbent
foam covered by a weather-proofing coating of some sort.
If one were to try to home-brew something like this, where
would a small piece of the RF absorbent material be obtained
without having to spend too much $$ for this experiment?
You might have been looking at a Stealth ground plane antenna.
http://facility.unavco.org/science_tech/dev_test/testing/zephyrperformance.pdf
I recall seeing a long time ago a CAD image of a ground plane disc (non
metallic?) with lots and lots of holes. The hole to material ratio
increased with radius I guess to match the free space impedance at the disc
edge and eliminate diffraction.
These guys had a different idea
http://webone.novatel.ca/assets/Documents/Papers/gps_pinwheel_ant.pdf
page 19 and 20 show a gps antenna on a robotic arm getting calibrated
http://www.oregon.gov/ODOT/HWY/GEOMETRONICS/docs/presentations/1-11-12_OGUG-Improvements_NSRS.pdf?ga=t
Then of course you can also correct for the phase center variation of the
transmitting antenna.
http://www.iapg.bv.tum.de/46790-bD1lbg-~iapg~forschung~gnss~antenna.html
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