> 2 meters is a little different since most G.E./Motorola radios have a
> little better sensitivity, & the noise floor of 2 meters is
> much higher. I
> once measured the 2 meter noise floor here to be ~3000 K, or
> -122 dBm in a
> 16 kHz bandwidth. That's wny I almost never recommend a
> preamp ahead of a
> G.E. VHF receiver, but always for a UHF.
>
> Bob NO6B
I agree, and to compound the problem, it's difficult to get enough T/R
isolation in most 600 kHz duplexed situations to see any increase in
usable sensitivity even at a quiet site.
A few years back I got called to look at a 2m repeater with a "deaf"
receiver that had just been relocated to a new site. As it turned out,
the new site was beneath a high-voltage power line. Using the spectrum
analyzer you could "see" the notch response of the duplexer without
using a tracking generator - the noise level was *that* high. Effective
sensitivity for 12 dB SINAD was measured/calculated using a lossy tee at
a little over 5 uV, even though the Micor Rx was working just fine at
0.3 uV by itself when connected directly to the service monitor.
They moved to another site a few months later...
At high-rise sites in downtown Philly I've mesured effective sensitivity
of 2 uV on 2m and 5 uV on 6m. In contrast, I have quiet 440 sites that
will break squelch and decode PL at 0.03 uV (Angle Linear PHEMT's doing
the job).
--- Jeff
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