Mixing Wide and Narrow Modes

Mixing Wide and Narrow Modes

Why are wide bandwidth modes like phone separated from narrow modes like CW on most
bands?

Why are modes
generally arranged
in frequency
allocation so wider modes are
more restricted in
available spectrum?

These are both
good questions. Looking at spectrum
restrictions, we find CW and other narrow
band modes are
generally
allowed to operate
any frequency within
a certain license
class while phone
or wider emissions are
more limited in available operating frequency ranges.
Only recently has
phone been expanded
to a wider areas of bands. That’s the old established regulations. This makes us
wonder, why can’t we just
allow narrow modes
and wide modes to
mix?

Some wide bandwidth mode operators, or unskilled non-technical people, have
deluded themselves into thinking we can have unrestricted segmentation within
bands. They often cite the modern spirit of deregulation, where common sense
rules. We all understand deregulation. Like the banking and mortgage
deregulation, where common sense and self control prevailed to produce a strong
stable banking system. Or deregulation that brought free trade and so many jobs
to our country, first consumer electronics in the USA boomed, then our steel
industry took off, then general, manufacturing, and finally textiles blossomed
from deregulation. Deregulating Wall Street worked very well also, as did
deregulating electrical power generation and distribution in California.

Citing the spirit of deregulation, some operators propose we can
self-regulate Ham bands. Oddly, these operators tend to be strong signal
operators who prefer the widest possible bandwidth. These operators have
convinced themselves, or at least want us to believe they are convinced, they
can hear and avoid weak signals.

Operators often do not consider the technical problems caused by interleaving
dissimilar bandwidth and dissimilar mode signals. There are some
very basic
engineering reasons
why modes are
separated. Let’s
look at how the
system really works.

Receiver Sensitivity Limits

Noise is a broadband signal. The amount of noise, or noise power, reaching
the detector in our receivers is directly proportional to overall bandwidth. The
narrower the bandwidth, the less noise power reaches the detector system. Signal
power, so long as the signal is narrower than the IF bandwidth, does not change
with selectivity.

Excluding
narrow band interference, broad
spectrum noise
almost always limits
the weakest possible
signal that can be
heard. Almost any modern receiver, even the
least expensive,
is sensitivity limited by external
noise picked up by
the antenna system.

Unless our receiver
is so poor it
overloads and
internally generates
spurious signals,
noise that limits
reception is not
reduced by front end
filters or “antenna
Q”.  Excluding the relatively exceptional case of receiver front end
overload, a given receiving antenna system and receiver can
only be “quieter”
and hear weaker
signals better when
the receiver has better
overall selectivity in the narrow IF filters or DSP filters. 

Here is an example. Let’s assume we have a 500
kHz wide front end filter on
a 2 kHz wide
IF bandwidth receiver. Let’s assume this receiver is not overloading from poor
design or exceptionally strong off frequency signals. Weak
signal performance
(signal-to-noise ratio) will be exactly the same whether antenna system bandwidth is
50 MHz or 50 kHz. This is because the receiver IF filters set the ultimate
selectivity, and that selectivity determines noise power by determining the
ultimate noise bandwidth.

Wider receivers
have less useable
sensitivity than
narrow bandwidth receivers.
Anyone
who serviced two-way
radio systems back when wideband
FM was used probably
remembers it was
impossible to have a
wideband FM set
equal the
sensitivity of a
narrowband FM set
of similar design. 
The
wideband FM receiver
typically had several dB less
weak signal
sensitivity than a
narrowband FM
receiver even when
both used identical
front ends. The reason
for this is very
simple… noise is a
very wide bandwidth
signal while the
desired signal can
be made very narrow.
The wider the
receiver the more it
fills with the broad
spectrum noise that
surrounds any
receiving location.
As a matter of fact
once a receiver is
wide enough to pass
the spectrum
necessary to convey
information, any
further increase in
bandwidth reduces
system range. This
is why Collins and
other companies
(including telephone
companies) settled
on 2 to 2.5 kHz
bandwidth for SSB
voice transmissions.
While wider
bandwidths sound
better; they
actually decrease
communications
system range or
efficiency.

We can observe
the increase in
noise power with an
HF transceiver
having variable
selectivity with
constant receiver
gain. If
we tune to a
signal-free area of the band
with a wide filter
and note noise
level, we will hear
noise drop in
proportion to any
reduction made in IF
filter bandwidth. This is
because, as we narrow
the IF bandwidths by
half, noise power
drops by half. Go
from a 6kHz filter
to a 3kHz filter
with no other
changes and noise
power drops by 3dB.

Assume we have a
signal carrying
information in a
3kHz bandwidth. We
listen to that
signal with a 9kHz
bandwidth IF filter.
We have three times
the noise power
necessary, or 4.8dB
more noise than
necessary. If we
reduce bandwidth to
3kHz while making no
other changes, we
improve S/N by
almost 5dB. We can
hear a signal that
is almost 5dB
weaker. This
effect applies to
any mode, and is
true up to the point
where the receiver’s
ultimate bandwidth
is less than the
width of the useful
transmitted
information.

This happens
because noise is
almost always of
significantly wider
bandwidth than the
information
bandwidth of the
incoming signal. The
chart below shows
the change in noise
power as
receiver bandwidth
is changed:

Let’s assume you
and I have the same
receiver, we are
using the same IF
filter bandwidth,
and we have the same
noise level. Let’s
also assume you and
I have similar
antennas and
propagation.

We start out both
using 8kHz filters
listening to a 100Hz
wide CW signal from
a 100 watt
transmitter. 
If you switch to a
500 Hz filter, the
S/N ratio of your
receiver improves 12
dB.  The
bandwidth change of
500/8000 = .0625      
.0625 * log10 =
12.04dB

With your reduced
receiver bandwidth the station we
are listening to can decrease power
from 100 watts to
6.25 watts. He will
have
the same initial
signal-to-noise
ratio to
you, but I will now
receive with 12dB less
S/N!

If you further decrease
bandwidth to 100Hz,
you now gain another
7dB of S/N over my
ability, for a total
difference of 19dB.
You can detect a
1.25 watt signal
almost as well as I
can hear a 100 watt
CW signal!

This illustrates
one major problem that
occurs when
dissimilar bandwidth
systems are mixed,
even when they are
the same modulation
type. This is a
valid technical
argument in favor of
rules or bandplans that sort signals by
bandwidth. Sorting
by bandwidth
prevents a wide
bandwidth
“alligator”
from not hearing
what is going on and
accidentally QRM’ing
narrow band signals. It isn’t
a matter of how
careful the operator
is. The problem is
rooted in the fact
that wide bandwidth
receivers cannot
hear as well as
narrow bandwidth
receivers. Keep
this important
effect in mind when
we discuss
transmitter
bandwidth and splatter!

By the way, a
“white paper”
referenced by ESSB
operators claims
readability improves
with increased
bandwidth. That’s
absolutely correct,
except the paper
assumes zero noise
floor or infinite
signal-to-noise
ratio! Indeed if we
have unlimited
signal-to-noise
ratio a bandwidth increase
beyond normal
communications
channel bandwidth
will often increase
our ability to
recognize the
difference between
certain sounds, but
this is only true
when the noise floor
is far below the
level of the weakest
part of the speech
spectrum. Under
normal
communications
conditions any
increase in bass and
treble actually
decreases range, and
it can make copy
nearly impossible
when stronger noise
or weaker signal
levels are present. 

Claiming we can work
more DX or enhance
communications range
by using wider audio
bandwidth might be
good salesmanship,
but it is far from
true. Collins and
other communications
system leaders
weren’t headed by
dummies, and a white
paper written by a
company that sells
broad bandwidth
announcement systems
for offices that
says wide bandwidth
improves the system
is not exactly and
unbiased or 
reliable technical
resource.

Transmitting
Bandwidth

How wide are
typical medium
quality SSB
transmitters? Here is the
actual spectrum bandwidth
of an old IC-751A on
SSB at 100 watts
with normal speech.
The transmitter
filter was measured
at 2.7kHz at -6dB
points.

This display is
1.2kHz per division.

 99% of the
transmitted power is within 2.04
kHz of bandwidth.
2.4kHz below and
1.0kHz above peak
emission frequency,
the signal is
-43dB.  -43dB
bandwidth is less
than 3.4kHz.

The
occupied bandwidth
is 2.04kHz.

A non-HiFi Viking
Valiant
with modifications
to remove modulator
and PA modulation
linearity flaws with
90% modulation has
an occupied BW of
about 8.6kHz under
the same recorded
voice stimuli. The
-43dB bandwidth is
more than 18kHz!
Many people blame
the bandwidth on
audio system
distortion, but a
major cause of
needless power level
at wide bandwidths is
non-linearity in the
modulated stage.

In
order to have low
distortion the power
output of a plate
modulated RF
power amplifier has to follow
the square of anode
voltage change.
Tetrodes don’t ever
do that very well
because the screen
voltage has such a
large influence on
power. The solution
is to modulate one
of the grids and the
anode at the same
time. If we
carefully pick the
ratio of audio
voltage applied to
the anode and grid
(generally the
screen) we can
greatly improve
modulation linearity.
Unfortunately it is
almost never close
to perfect. As such,
most tetrodes make
very poor modulated
stages so far as
distortion products
are concerned.

This creates a
problem. We not only
have the original
audio bandwidth to
contend with, we
have distortion
products extending
out for a
considerable
distance. While
amateur AM
transmitters are the
worse offenders, SSB
transmitters are not
without problems.

Mixing Wide and
Narrow modes

If we move the
IC-751A
transmitter’s signal
(or any other
reasonably clean SSB
rig) close enough
to a normal
bandwidth SSB receiver’s
frequency without
overloading
anything, we will
start to hear the
other signal on
modulation peaks.
Part of the receiver
passband is
overlapping the
extended parts of
the transmitter’s
passband. The
occasional peaks are
heard as short
“spits” of
sharp noise.

If we have two
normal SSB BW systems,
each system has equal footing.
The noise floor,
sites being equal,
is identical. As we
move the operating
frequencies closer and
closer together, and if the
systems have
about the same IM
performance and
noise floor, each
operator will hear
the other at about
the same time. It’s
very easy for either
of the operators
to know when they
are bothering the
other operator when
the systems have
equal radiated power,
bandwidth,
and local noise.

When we mix a SSB
system (or any wider
system) with a CW system
(or any
significantly
narrower bandwidth)
system,
a much
different situation
develops.

Let’s look
at a CW signal (it
could be PSK or any
other narrow mode)
and a normal SSB
signal (it could be
AM or ESSB).

The CW transmitter
has the majority of
its energy within a
few hundred hertz,
the exact bandwidth
dependent on the
rise and fall times
and slope of the
rise and fall. In a
properly engineered
transmitter very
little energy
extends outside of a
few hundred Hz total
bandwidth. By the
way, it is important
to note this
bandwidth is NOT set
by the speed of the
CW. It is set by the
shape of the rise
and fall of the
carrier and other
transmitter
characteristics.

The SSB receiver,
on lower sideband,
can be set so the CW
signal is either
just a few hertz
above the dial
frequency or 3kHz
below the dial
frequency of the SSB
transmitter. In most
cases very little if
anything will be
heard from the CW
transmitter. This is
not the case for the
narrow mode
operator.

The CW receiver,
having somewhere
around 10dB less noise
power due to its
narrower bandwidth,
has 10dB more
ultimate sensitivity than the
SSB bandwidth
receiver. This means
the narrow bandwidth
receiver can detect
or be bothered by 10dB weaker SSB
distortion than a
regular SSB receiver
could hear.
If a good
clean CW
receiver is 5kHz
away from the same
IC751A, it will be
bothered by frequent
spits from the SSB
transmitter. The
same
spits would be
masked by the
increased noise
floor of a wider
receiver.

I have a very
quiet rural
location. On 160
meters I sometimes
have difficulty
copying weak DX as
far as 8kHz from
strong SSB stations.
Often this is not
because the SSB
operator is
“overdriving”
his transmitter, but
rather because
transmitters (even
when not HiFi or
overdriven) 
have fairly wide
bandwidth low-level emissions. Some of
the very wide
spurious emissions are only
reduced 50 to 60dB
from peak power. Of
course the SSB station
generally
has no idea his
transmitter is
bothering anyone, since
the stations he
bothers are well outside
his receiver’s passband.
Also, 
for the same site
background noise, he
has a higher noise
floor because of his
receiver’s increased
bandwidth. He can
hear as weak a
signal as a narrower
receiving system can
detect.

I ran a daytime
test on several S-9
75-meter AM amateur
stations, and in my
quiet location I
could detect spits
that would bother
weak CW signals up
to 15kHz up and down
from their
carriers.  This
problem wasn’t
nearly the same
severity when using
SSB bandwidth because the
wider receiver
bandwidth increased
effective noise
floor by 10dB. The
10dB higher noise
floor of the wider
receiver filter
masked the splatter
with broadband noise.

Transmitter
bandwidth is a
second compelling
reason to not mix
voice and narrow
signals of any type.
The bandwidth issue
stacks
on top of the noise
floor disparity
caused by receiver
bandwidth.

Mixing Modes

Here’s a good
test. At a time when
few people are
operating, use your
own receiver to
learn the difference
between detecting
cross-modes. Find a
very weak CW signal
with no one else
nearby, and switch
to AM without
changing bandwidth.
With all things
equal, you will find
it  very
difficult to tell a
weak but readable CW
signal is present.
At best you only
hear a little change
in background noise
level as the
transmitter is
keyed. If you try to
detect a very weak
carrier, you turn on
the local oscillator
to get a beat note.
It has always been
known things work
this way!

PSK and FSK are
even worse, there
are no lengthy
breaks in carrier
level.

The detection
disparity adds
another level of
problems to mixing
modes. Some modes
just do not detect
other modes real
well.

This effect adds
a third level of
disparity between
modes!

Cross-mode
Communication

Figure this one
out. You are on CW
and a SSB operator
comes on the same
frequency and
politely asks,
“Is this
frequency in
use”.

What does the CW
operator hear?
Nothing but
undecipherable
noise! Even if
through some miracle
the CW op was using
a SSB filter, the
SSB station would
have to be nearly
zero beat with the
CW BFO frequency to
be understood. The
CW operator would
have to stop his
QSO, switch to SSB,
and hope the SSB
station could hear
him say “yes,
the frequency is in
use”.

What is the
common reply when
you are not
Johnny-on-the-Spot
with an “it’s
busy” reply?
The other guy says
“I asked first,
you should have said
something
sooner.” 

What if you are
on PSK, and a CW op
comes on? You might
not even know how to
copy or send CW! How
would you alert the
CW station, or how
would a CW station
alert the PSK
station? How would a
SSB station talk to
a digital op, or a
digital op talk to a
SSB station?

What about FM?
Under normal
conditions AM, CW,
or SSB receivers
cannot decode FM.

What about the
operator who cannot
copy CW? How does he
recognize an ongoing
CW contact?

We now have found
a fourth problem
layered on the other
three. People cannot
readily communicate
between different
modes.

The Operator
Problem

The final problem
is not so much the
mentally deranged
operator, they are
few and far between,
but rather the
selfish unbending
operator who only
follows the letter
of the law. Without
enforceable bandwidth
guidelines, a
selfish operator could
park a transmitter
that would wipe out
weak narrow signals anywhere
he wanted. He could
claim he didn’t hear
you, and you could
not prove
differently. 160
meter weak signal
operators are all
familiar with a
group of W5’s who
parked on 1824 SSB
just “because
they could”.
The JA stations only
have 1810-1825, many
other DX stations
could not go below
1810 or above 1825,
and this was a
frequent spot for
DXpeditions to
operate CW. 
They clearly and
intentionally
violated the
bandplan…because
they felt they
legally could.

It took years and
ultimately required
FCC intervention to
get them to move to
another frequency in
an otherwise clear
band!

They aren’t the
only group. WA0RCR
runs wide AM
broadcasts on 1860
KHz, wiping out SSB
up and down 5kHz. In
the 1970’s W8LZM,
W8ETO, W8LAD, and a
few others formed a
“Window Shade
Net” with the
sole intention of QRM’ing weak DX. One
of the original
“Window Shade
Net” members is
still alive and
actively QRM’ing DX
today! He has a
history of over 40
years of willful QRM
and violating
bandplans.

One person can
easily wipe out the
pleasure of hundreds
when we depend on
bandplans. They only
need a desire to
cause QRM, without
enforceable
bandplans they have
the means.

A workable
non-regulated band
requires all
operators to be
willing to sacrifice
and compromise. It
requires everyone to
respect bandplans,
and to use good
judgment.

We often hear
“160
works without
segmentation”, yet
when a petition was
filed to segment the
band several hundred
people filed in
favor of
segmentation. Only a
handful filed
against forced
segmentation, and
those people were
all wide bandwidth
mode operators. 
“160
works” for
those who work wider
modes with strong
signals, and it
works exceptionally
well for those who
like to cause
intentional
QRM.  

Summary

There is logical
technical evidence
to support this
statement. Mixed
modes and mixed
bandwidths are both
clearly problematic.

We need to
encourage
regulations that
separate or segment
areas of bands by
signal bandwidth.