Tom wrote:
I'm testing several Trimble Thunderbolts here with several different
antenna setups: indoor/outdoor, low-gain/high-gain, patch/helix,
plain/choke-ring.
Has anyone done a comparative analysis of antenna make/model/type
against signal levels for this receiver?
Or is there a rule of thumb for what is consider "good" vs. "bad" reception?
Be careful when you say "signal levels" in connection with a
Thunderbolt. The figure reported by the Thunderbolt is NOT the RF
signal level, it is the carrier to noise ratio. The c/n is mostly a
function of the antenna location, and only a weak function of the
antenna and LNA. So mostly you switch antennas and things don't seem
to change much unless the new antenna is in a different
location. Unless you are looking at the antenna output with a
spectrum analyzer or frequency-selective voltmeter/power meter, you
will not know the relative signal levels from the different
antennas. But c/n is what is important, so that's all good.
Lots of Lady Heather screen shots have been posted on the list
showing accumulated signal strengths (commands SAS or SAD). To be
"good," you want to have large swaths of the plot in cool colors
(blues and greens) (c/n over 40 dBc), and preferably mostly greens
(c/n over 45 dBc). Look at Warren's screen shots -- his antenna
location is pretty good. Contrast with Dave's screen shots -- his
antenna location is adequate but not really very good.
Finally, note that not all similar c/n plots represent equally good
reception. Multipath can look like "carrier" to the Thunderbolt
rather than "noise," so an antenna location with substantial
multipath can look just as "good" as a location that is free from
multipath. Depending on the direction from which the reflections
arrive, an antenna with good multipath rejection (e.g., a choke-ring
design) may give superior results without looking any better on the c/n plot.
Best regards,
Charles
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