Hi all:
Well, Bill (ZV) gave a nice (brief) summary and pointed to a nice
reference. Ten Tec had (maybe it is still there) some good info also,
but specific to their radio...however, you can still use it to get a
"feel" for a what a roofing is (and what it isn't). If you're a
member of ARRL, go to their web page and search on Roofing Filter.
Don (FPR) cited some uses which I am not familiar with (data), so no
comment from me on that.
I've commented directly to the source about my "wonder" at why one
would want a variable roofing filter. As Lee said, one "wide" and one
"narrow" is enough (at least for SSB / CW only operation).
If I'm on CW and my radio is hearing squeaks, pops, and "garbage" due
to lots of LOUD signals being in my passband (say 15 or 20kc wide on
an "old" radio), I would not be able to do much...maybe cut back on
the RF gain, add some attenuation, etc. However, if I could narrow
the passband (the roof) to say 1kc, the likelihood of two (or more)
very loud signals being captured within that 1kc is greatly reduced.
If I use a 500hz roofing filter (assuming all other parameters could
remain unchanged), the odds of having two (or more) "other" very
strong signals there (besides the one I want to copy) is even
less...hence less chance of squeaks and pops masking the signal I want
to copy. BTW, the squeaks and pops = IMD Products.
Of course, if there were that many LOUD signals that close together,
I'd simply move (qsy) to a clearer (relatively) spot.
Why in the world one would want to take the time to "carefully" or
"slowly" adjust a roofing filter's bandwidth is beyond me. I would
simply dump in my "narrow" roofing filter and be done with it. Either
it works or it doesn't; why play around?
This topic is of major concern to contesters, especially (mostly) on
the low bands. I know...it is also of concern to Europeans on 40m
even NOT during a contest...true.
If you're not a SERIOUS contester or Low Band DXer (or data guy ala
Don's description?), I have a hard time understanding why you'd want
these "narrow" filters. Hey look, 2.7kc is pretty darn narrow
compared to most of what is out there. Again (as had already been
cited), even the IC7800 and FT9000 have a 3kc roofing filter as their
narrowest (which I don't think is good enough for some CW situation).
I would guess that for 95% of non-serious contesters, or non-low band
DXer (and digital guys?), you'll be VERY happy with a single 2.7kc
roofing filter for SSB and even for CW. For those who are used to IF
filters, this is just not the same. You'll still have that with the
K3....twiddle the bandwidth any way you like (narrow). Remember,
roofing filters are for CROWDED band condx FILLED with LOUD signals
(with the digital proviso still there).
And NOW, I'd like to say, I have no idea exactly how much MAGIC the
Aptos boyz have included in this radio and for all I know (not having
the radio, the specs, schematic, code, etc., they may have come up
with something that voids all of the above. Wouldn't that be great!
de Doug KR2Q
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