Fig. 1: A
communications receiver tuned to 1440 kHz.
It reads like a
Sherlock Holmes mystery novel.
Tim Laes, corporate
director of engineering for Midwest Communications, received
occasional listener complaints about an unusual noise heard on
WNFL(AM) in Green Bay, Wis., in November and December 2013. WNFL is a
DA2 with 5 kW day and 500 watts night into three towers on 1440 kHz.
However, AM radio
constantly battles noise from fluorescent lights and a hundred other
sources, and listeners often don’t know how to explain what they
are hearing, even when it is a valid complaint.
Laes is a ham, call
W9EE, so he got off to a good start in unraveling this mystery. Hams
often think farther out of the box than most people, and Laes
discussed the issue with both local engineers and other hams.
The WNFL signal in
Green Bay was clean, so it was not a studio or transmitter problem.
Then Laes determined that most listener complaints were coming from
south of town. He lives north of Green Bay but was able to hear the
interference, just barely, using a ham receiver and listening in the
quiet environment of his ham shack. There it was, something like
Morse code at about 15 words per minute, but it didn’t decode into
letters and words. Instead, it was a 1 kHz tone of random dits and
dahs, as heard in the AM mode of his receiver. That would be A2
modulated CW, which is almost never used.
(Hear a one-minute
sample of the audio online at
Was it a pirate
station, deliberate interference or a switching power supply gone
The plot thickens.
confirmed that the interference became more audible as he traveled
south. There didn’t seem to be a definite source, and it was nearly
impossible to track because KFIZ broadcast on 1450 kHz in that area.
It causes normal adjacent-channel modulation interference, since it
is only 10 kHz up from WNFL. To make matters worse, the problem was
only intermittently heard on the air.
In the spirit of
cooperation, broadcast engineer Steve Konopka, N9FOY, got involved.
He contacted Steve Brown, W9APL, chief engineer for Woodward
Commutations in Appleton. Konopka had two radios going at once and
noticed that the Morse-like sound seemed to happen in cadence with
programming on 1150 kHz WHBY, some 24 miles at 171 degrees away from
WNFL. Konopka listened at home while Brown briefly shut off his 25 kW
transmitter on 1150 kHz and then on his 1 kW 1570 kHz station. Brown
had recently installed a new STL system; it was interrupted as well.
All tests showed
Woodward was not the problem.
Laes and Konopka
decided to do some tuning and discovered the same tone was audible 9
kHz above and below 1450 kHz. They could only hear the unwanted
signals by tuning receivers up and down in frequency to get the
unknown signal inside the 3 kHz bandwidth of their receivers:
KFIZ(AM), in Fond Du Lac, Wis., is 1 kW day and night into a single
tower on 1450 kHz, a local channel. It is some 53 miles down the
road, at 206 degrees from WNFL.
Now Laes and Konopka
contacted Stu Muck, who does engineering for KFIZ and was in the
process of replacing large electrolytic capacitors in the station’s
Gates One transmitter.
Muck noticed that
the transmitter’s occupied bandwidth was, as he said, “starting
to work its way out of the NRSC mask.” Unfortunately, that repair
did not fix the 9 kHz problem. The strange sound went away when he
disconnected audio from the input on the station’s Orban 9100 audio
processor. Muck discovered the noise issue was present only when
program audio was passing through the processor. Further analysis
showed the processor’s number four card was in oscillation at about
9 kHz, but only when there was programming audio present. Who’d
ever heard of that before?
Fig. 2: 4.5 kHz
Bandwidth on NRSC Test
Remember how the 9
kHz tone seemed to come and go with modulation on WHBY? Well, that is
because KFIZ was running the same nationally syndicated talk show as
WHBY while some of the listening tests were being performed. Talk
about confusing the detectives with a red herring!
So, the 9 kHz
oscillations on 1450 kHz were heard as a Morse code-like 1 kHz note
on receivers tuned to 1440 kHz, because it was within the receiver
bandwidth when tuned to 1440 kHz. There is no nearby station on 1460
kHz to be interfered with, otherwise that would have been another
Under current FCC
rules, any station could legally transmit a 9 kHz tone at full
100-percent modulation and cause problems for RF neighbors. Muck does
not have a record of how high the unwanted 9 kHz modulation was, but
not much is required for people to take note when a steady tone, or
worse yet a Morse code-esque tone, appears at 1 kHz, mixed with audio
No complaints were
received from KFIZ listeners. Why? Well, receiver manufacturers
reacted to customer noise complaints years ago by restricting audio
bandwidth in AM receivers to about 3 or 4 kHz. Very few AM receivers
today can reproduce audio to the full NRSC 10 kHz audio bandwidth.
SO MUCH FOR 10
I did frequency
response tests in my shop on a few AM receivers. They were a Sherwood
RX4109 component stereo, a Sony CFD-10 boom box and a Bose AWR1-1W
The test setup
featured an AM stereo generator, set to monaural, with an NRSC
pre-emphasis equalizer and an NRSC low-pass filter between an audio
generator and the exciter. No dynamic audio processing occurred. The
exciter has a sample transmitter output, which produced enough signal
for receivers to hear it on the bench. Receiver tone controls were
set for standard response with no bass or treble boost.
Test results were
what we expected, or at least suspected. The surprising part is that
the receivers had very similar response curves averaging 2 dB down at
3 kHz, 7 dB down at 4 kHz, 12 dB down at 5 kHz, 21 dB down at 7 kHz,
and 32 dB down at 9 kHz. I understand that car radios are about the
same. So much for 10 kHz audio!
On a side note, I
recently sold an Orban 9300 AM audio processor to a client. Before it
was installed, I optioned the software to allow only 4.5 kHz of audio
bandwidth. The station sounded loud, in part because there was no
audio power transmitted and wasted from 4.5 to 9.5 kHz. No
complaints, just happy listeners who could hear that talk station
better. Fig. 2 is a spectrum analyzer display of 4.5 kHz AM audio
Years ago, I
remember troubleshooting a pure 1 kHz tone that listeners heard on an
AM station. The cause was a second adjacent channel (20 kHz away) AM
station that was rebroadcasting a stereo FM station using a monaural
FM receiver. The 19 kHz stereo pilot from the FM went right through
the FM receiver, audio processing, transmitter, and out the antenna.
That was before NRSC 9.5 kHz low-pass filters were required. The 19
kHz tone was just 1 kHz from the station being interfered with and
was plainly audible to many listeners in an area halfway between the
two station locations.
I’d like to make
it clear that the 9 kHz and 19 kHz signals that caused 1 kHz tones in
receivers were not mixing products between stations. They were
“signals on the band” heard by receivers.
Yes, some AM
transmitters are capable of modulating to even 20 kHz, but the
receivers are not there today to take advantage of it. With
increasing noise levels on the AM band, I do not advocate increasing
AM audio bandwidth. In my opinion, we must make the best of what we
have to get viable programming to listeners. It makes perfect sense.
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Persons, WØMH, is a Certified Professional Broadcast Engineer who
has more than 30 years’ experience. His website is www.mwpersons.com. Find
past articles under the Tech Tips tab at radioworld.com.