[CW] Speed vs Bandwidth

Sat, 13 Mar 2004 09:42:33 -0500

One way of thinking about it is to say that "bandwidth is proportional to
the transmitted intelligence" - the more useful information, the wider the
bandwidth.

In theory, the bandwidth of a constant carrier is zero but as a practical
matter, you can't transmit forever, at least you had to start sometime! - so
it isn't infinate.  Also in all carriers there is an amount of hum which
modulates the signal.

One practical way to illustrate the need to provide wider bandwidth to
faster CW is to make a signal that has low bandwidth - one that has VERY
soft keying.  Soft keying produces a narrower bandwidth than hard keying.

Adjust the keying circuit in a transmitter so that a bell shaped note is
heard and can be copied at 25 wpm.

Now speed up the rate of transmission - adjust the code speed so that code
is sent at 35 wpm - you will notice that the code becomes mushy and very
difficult to copy.

As the code speed is increased not only does the bandwidth go up because of
the code speed, but to be able to copy it well, we must increase the
"sharpness" of the waveshape of the keying.  Also the receiver must be wider
to receive the code - it is similar to listening to orchestral music on A.M.
with a 2 kHz receiver bandwidth - you can "copy" the music, but it doesn't
sound good at all.  Change to 15 kHz bandwidth and now it sounds good.

Same thing with CW.  If you copy a 55 wpm CW signal in 1.2 kHz mode you will
be able to hear the code much better than if you copy it in 150 Hz
bandwidth.  In fact 55 wpm code is nearly impossible to copy in 150 Hz
bandwidth for me, but 20 wpm is easy to copy in that bandwidth - simply
because the modulation (keying speed) of 55 wpm is over twice that of 20
wpm.

Many Old Timers copy CW with 1.5 kHz filters because they use the high
frequency "clicks" to copy the code with.  Try it, you will find that you
can often copy a station better with 1.5 kHz filter than a 250 Hz (or
smaller) filter.  The only time I find I need the sharp filter is when there
is a strong signal within 300 Hz that is blowing the other signal away and I
must eliminate that signal so I can even hear the other one. If the signal
is fairly weak, I find if I EXPAND the receiver bandwidth I receive the
interfered with signal much better.

73

David Ring, N1EA

----- Original Message -----
From: "Buck" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 13, 2004 3:28 AM
Subject: [CW] Speed vs Bandwidth

> For some unknown reason, I have been bugged lately about the theory I
> learned 25 years ago that the faster a cw signal is sent, the broader
> the bandwidth of the signal.  I didn't get my license by learning the
> questions and answers.  While I am not a CET or equivilent, I have
> studied electronics and had to get my license by learning theory.  (This
> isn't a put-down to those who used the q/a method, its my background so
> readers get a feel for my understanding of electronics).
>
> When I studied it, I had no choice but to accept the theory that the
> signal width (band-width) is actually greater, the higher the speed of
> the cw.
>
> In practice, I see that the more power I receive from a station, ie the
> greater the signal strength, the greater the tuning band-width that I
> can receive some noise of interference, as it may be.  However, in cw,
> whether its a carrier or someone sending 60+ wpm, at s-5 or so, I can't
> tell the difference between the width of either station.
>
> Has anyone actually tested this theory on a spectrum analyzer?  Is this
> a theory we just have to accept or is there any real basis for the
> statement.  You folk might have to forgive me, but I can't immagine that
> there is actually a cw speed that could consume the entire 80 meter ham
> band.  I can imagine a cw speed that is so fast that on 80 meters it
> can't be copied reliably but, and maybe I understand it wrongly, I can't
> think of anything that will key on and off so fast that my rig will
> actually transmit two or three times the signal width in it's specs.
>
> The world won't come to an end if I never understand this, but if
> someone can explain, I would appreciate it.
>
> Thanks
>
> Buck
>
>
>
>
>
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