Grant, NQ5T wrote:
"It seems less likely that something went west .. or south, than it does
that perhaps there is some set of conditions (AGC setting for example)
that is different, or noise characteristics or something else that was
different that day it seemed to perform so well."
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I've tried every possible combination of AGC selection and NR setting.
The band noise on 160m does not vary during the daytime each day (which
is when I checked the NR and it worked so fantastically well).
The day after the radio was delivered, I tuned in a very weak carrier on
1830kHz (3rd harmonic of a local BC station on 610kHz). I know that I
had the DSP BW set to 100Hz and the 250-Hz roofing filter was engaged.
The signal was barely discernible without the NR turned on. When I
turned the NR on (at a setting of "1"), the signal was noticeably better
copy. When I increased the NR to "9", it startled me how much the signal
jumped up out of the noise. It was like the difference between S-1 and
S-9!!
Now the NR is absolutely no help at all on that same signal under the
same conditions. When the DSP BW is that narrow, it doesn't work at all.
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"I just spent a while listening to weak signals on the low end of 40 and
playing with bandwidth and NR (AGC on med). My impressions are that as
bandwidth is reduced the apparent reduction in background noise (which
admittendly isn't terribly high this morning) decreased as bandwidth
decreased."
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As you decrease the BW, the apparent S/N ratio of the receiver will get
better. That's why I prefer using narrow filters when DXing on 160m.
PY1RO first put me onto this back in the early 70s when he sold me onto
the idea of buying the MFJ CW Filter. That was an audio filter that had
a BW of 80Hz and really worked well with my SB-300 and homebrew
transverter on 160m. Then I bought the Drake C-line and installed the
Sherwood 125Hz xtal filter in it, which was far superior for hear the
weak ones than the 500Hz Drake filter.
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"And in fact the "apparent" noise actually seemed to increase somewhat at
bandwidths much below 500 hz. At the same time however, the signal
itself tended to become more distinct (with a slightly harder edge) and
come UP out of that noise, be louder, and be easier to copy. Very
apparent effect which got more pronounced at the narrowest bandwidths."
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Why the apparent noise seemed to increase as you narrowed the BW below
500Hz is beyond me, hi. The main thing is that the signal became easier
to copy.
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"Thinking it might just be a level increase overall, I compared that with
just increasing the volume level with NR off, but on very weak signals
there was a striking advantage to signal readability with NR on."
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Well, if you notice a striking advantage for copying weak signals with
the NR on (and a very narrow DSP BW), then something definitely went sour
with my Orion's NR since I've had it. My NR works great on wide DSP BWs,
as in SSB.
73, de Earl, K6SE
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