John Green wrote:
Mark Sims wrote:The quality of the results match the quality of the antenna.
A geodetic/survey grade antenna gives results to within a few inches. A
conical timing antenna within 8 inches. A cheap patch antenna around a
foot.
I took one of those conical timing antennas apart only to find that it has
the same ceramic patch as most cheap GPS antennas. How is a survey grade
antenna different? I would like to be able to know one when I see it.
John:
The "survey grade" antenna usually incorporates "choke rings", which are
ring structures
surrounding the antenna, designed to suppress signals arriving at
extremely low or
negative angles, which is where most of the multi-path is assumed to
occur. The antenna
itself could still be a "patch antenna."
If you mount your antenna low, with respect to surrounding structures,
then you could
have multipath (reflections) arriving from high angles, and this kind of
antenna can't save
you.
But when the antenna is mounted with a clear view of the sky, and
nothing much above
it for signals to bounce off of, then the survey antenna will (mostly)
ignore signals arriving
horizontally or from below the antenna, which are by definition, bounce
or distorted.
--- Graham
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