Message-ID: <004801bf0c5b$78aca1c0$23088cd4@default>
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: "rsgb_lf_group" <rsgb_lf_group@blacksheep.org>
Subject: LF: re Dave's Comment on filters
Date: Fri, 1 Oct 1999 21:31:05 +0100
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Hi Dave, I must admit to being a little confused now. I am told a 500Hz
filter is not good enough for 136, but the theoretical advantage of a 300Hz
filter is only a couple of dB better, I believe. A 100Hz filter would have a
theoretical advantage of 7dB (I believe) this should be the same as going
from 2.5kHz to 500Hz (where I find the practical advantage more like 3-4dB).
Yes there is a lot of QRM on HF cw and you need to isolate a signal to
listen to.
My experience (with 'inexperienced cw ears' !) is that I don't get the
theoretical advantage by going to narrow filters. Am I missing something? or
am I maybe not sufficiently practiced to take full advantage of the
difference? I guess as Toni says that 1 to 2dB can make the difference
between working a station and not working it.

I can hear, and copy, most of the signals on 136 with a 2.5KHz SSB filter,
and I don't get trouble from DCF39. I use lower sideband and put the carrier
at 138.05 or 138.10. With a cheaper rx like the Lowe HF150 the SSB filter
does not have steep enough skirts (and there is no cw option) and the
carrier of DCF39 is only 60dB down, and I get a light trace of the signal on
the waterfall display. Now listening like this does mean you have to tune
the 'grey matter filter' to morse at frequencies up to about 2kHz. My tally
of calls was about 20 or so listening like that, and it gave me the
incentive to get better gear. On a receiver with a decent SSB filter shape
factor the signal from DCF39 is 90dB down and no trouble (provided the front
end has enough dynamic range)

I'm keen to get a few tens of dBs advantage on weak signals over the band
noise but I'm afraid I dont see it as a reality yet. The problem seems to be
that those last few dBs are beginning to get expensive now. Thanks for the
Icom filter number, Toni, I will look that one up.

I hope this is not regarded as 'cage rattling', or is getting too boring.
There are a lot of experienced ears out there, and even after 45 years in
the hobby there are things to be learned (at least by me). If you have the
time to impart your experience, I'm all ears (and so I suspect are a lot of
other readers of the reflector.) After all, a 'sage' in my youth declared
"if you can't hear 'em, you can't work 'em" and another "communication is
95% listening"

Cheers de Alan G3NYK
Alan.Melia@btinternet.com