[Elecraft] Do I need to upgrade my QRP-K2/ Clean Signals...

[George, W5YR]

The best that could be done would be to select a keying waveform and
receiving filter that comprise a matched set; i.e., matched filtering at the
receiver. As a practical matter in the amateur service, this is not an
applicable solution, but it gives us some basis for selecting a next-best
approach and some guidance in the selection of transmitted signal bandwidth.

We aim to approximate matched filtering - in the face of "noise" whose
statistics may not shape up well with conventional matched filter theory -
by shaping our keying waveform, et al in an effort simply to limit the
bandwidth of the transmitted signal. At the receiver, we perform a
complementary operation by choosing a filter bandwidth that maximizes in
some subjective manner the signal to noise ratio combined with preserving
enough of the transient content of the keying to satisfy the operator and to
allow manual decoding at higher keying rates.

So, the important metric, it seems to me, is the signal energy density
within the receive bandwidth compared with the noise power density in that
bandwidth. A keying waveform that generates a relatively wide distribution
of signal energy will have  less energy density within the passband of the
receiver, if the passband is chosen to minimize noise, for example, than one
which generates a more compact spectrum at signal levels above some
reference level.

If the transmitted signal energy is maintained within a bandwidth compatible
with the usual passband used for signal separation and noise reduction -
typically 500 Hz - then the S/N ratio within that bandwidth will be improved
over the case of a transmitted signal that has significant energy outside
that bandwidth.

So, after all that, my conclusion is that reducing the bandwidth of the
transmitted signal should in fact increase the S/N ratio and hence the
"readability" of marginal signals (read QRP, at times) when narrow passbands
are required for noise reduction,  adjacent signal separation,etc. The
notion of a matched filter approach is loosely applicable to this question,
although the wide variety of signal waveforms encountered in amateur
operation and the non-statistical nature of the competing signals and noise
defeat its application directly.

Interesting stuff . . . I agree with Dan that until there is better
correspondence between the transmitted signal spectrum and the receiver
bandwidth, passband shape, phase response, etc. there is unlikely to be
significant improvement except under conditions of noise-limited reception
(rare on 40 meters!) and weak signals.

But, setting aside all the theory, at any power level one can create
interference of some degree to one's frequency neighbors. As good neighbors,
we should strive to limit our occupied bandwidth to the minimum necessary or
feasible.

One need only witness the sound of the Pack in a Fox Hunt to appreciate the
fact that relatively weak signals can indeed interfere with one another when
closely spaced in frequency. Excessive transmitted bandwidth only
exacerbates the problems.

73, George W5YR
w5yr@att.net
http://www.w5yr.com



----- Original Message ----- 
From: "Dan Barker" <dbarker@visioncomm.net>
To: "Elecraft" <elecraft@mailman.qth.net>
Sent: Thursday, February 26, 2004 6:25 AM
Subject: FW: [Elecraft] Do I need to upgrade my QRP-K2/ Clean Signals...


> Earl, I believe Trev's question was "with the energy that used to be
wasted
> in clicks now in the correct bandwidth, is there more punch to the clean
> signal?"
>
> You answered "the energy outside the bandwidth doesn't help much".
>
> I imagine the "punch" difference between properly shaped and poorly shaped
> CW is very slight, but possibly not negligable. However, unless it's ten
> percent or more (very unlikely), the receiving station will never be able
to
> tell.
>
> Dan / WG4S / K2 #2456
>
> <snip>
> Q: "Now, one question I have is what is the impact when running QRP?
> </snip><snip>
> A: A wider BW CW signal is no stronger than a clean
> </snip>
>
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