Occupied BW of CW

Occupied BW of CW

Home Up

There seems to be
some lingering
debate about keying
speed affecting
occupied bandwidth
of a transmitter.
I’m fortunate enough
to have equipment
that directly
measures occupied
bandwidth. The FCC
accepts this
currently certified
equipment as proof
of bandwidth.


The following
printout is from an
unmodified IC-751A
running at 100 watts
carrier output on
3.5000 MHz.

The data is taken
at 10, 25, and 40
dots per second with
50% duty cycle. This
is normal CW weight.



The occupied
bandwidths are:

Dot speed per
OBW 99% power
10 490 Hz
25 430 Hz
40 500 Hz

We see the actual
measured occupied BW
of a relatively good
CW transmitter is
essentially the same
and does not track
CW speed. By the
way, the small
ripple in occupied
BW is caused by a
small envelope shape
change. This is an
artifact of the ALC
system in the 751A.

In  the
continuing e-Ham
thread, one person
claimed the reason
the IC-751A did not
track bandwidth with
speed was the fact
an IF carrier
oscillator was
keyed. The fastest
way to solve that
argument is to test
another totally
different rig. The
following is data
from an FT1000MP MKV
with keyclick
reduction mods.
These mods are
outlined on my

1000MK V page

The following
is the occupied BW
of the FT-1000MP MK

The measured
occupied BW’s

Dot speed OBW 99%
780 Hz
10 460 Hz
20 550 Hz
40 460 Hz

If you look at
the chart and plots
above, it might
appear the 5 dps is
a flyer or error.
Actually it is a
good reading and
cross checks with a
selective level

Dots  at 5
dps are wider
because the ALC
releases between
dots. The ALC alters
the waveform,
producing a very
sharp roll-over at
the leading edge of
every dot. The sharp
edge on the attack
substantially widens
the bandwidth. At
higher speeds the
ALC stays constant
and does not sag
between dots. This
allows the shaped
waveform to flow
through the system
without additional
sharp edges.

This is
additional proof the
edges are everything
for determining
bandwidth, and the
speed only has an
incidental effect as
it modifies the
waveshape of the
rising or falling
edges at radio
performance speed

I’ve measured
dozens of radios and
they all follow this