Most of the AM-broadcast band interference reports we receive here in the ARRL
Lab are not coming from the transmitter.
The most common cause of interference from an AM broadcast station is an
overloaded receiver. There are two ways to test this.
The first method is to install either a high-pass filter in line with the
receive antenna input. A bandpass filter can also be used. If the "harmonic"
goes away, or significantly attenuates, the harmonic is being generated within
your own receiver.
You can also put a step attenuator in line with the antenna. DO NOT TRANSMIT
into a step attenuator. If you change the step attenuator by 10 dB and the
level of a 2nd or 3rd harmonic varies by 10 dB, then it is external to your
receiver, and thus could be the broadcast station. If the harmonic varies by
about 20 dB, it is being generated within your receiver.
The same is true for intermodulation, except that for 3rd-order intermod, you
would typically see about a 30 dB change in intermodulation when you change the
step attenuator by 10 dB.
Harmonics and intermodulation can be generated anywhere, but the most common
locations are either at the transmitter site, in your receiver, or at some
location close to the receiver. We had an interesting situation at W1AW
recently. I was operating a mobile receiver and noticed that when I got to the
W1AW parking lot, I sometimes had intermod between two local broadcast
stations. After a bit of detective work, we found that when the solid-state HF
amplifiers used for bulletins were turned off and left connected to the
bulletin antennas, those antennas were picking up the two stations, conducting
that signal down to the amplifier. The amplifier output stage, when not biased
on, was acting like a diode and intermodulating the AM stations and putting the
result back onto the feedlines, to be radiated by the antenna.
The levels were low, so not technically an FCC rules violation, but locally,
the noise was strong enough to mask some signals on the band.
If the intermod were occurring at the transmitter site, it would be heard at
the amateur station and generally get louder as that test receiver was driven
towards the station. In this case, it did not, showing me that the intermod was
local to W1AW, not happening at the station.
The bottom line is that before you decide that the AM station is radiating
harmonics, run through the troubleshooting steps that determine that the
problem is not at your own QTH and/or receiver.
Ed Hare, W1RFI
ARRL Lab
________________________________
From: RFI <rfi-bounces+w1rfi=arrl.org@contesting.com> on behalf of Tom
<k8tb@bosscher.org>
Sent: Monday, January 9, 2023 8:46 PM
To: rfi@contesting.com <rfi@contesting.com>
Subject: Re: [RFI] 160m spur
My recommendations:
1. Make sure it is not your receiver. If your receiver has a passive
attenuator(resistor pad), go to a steady signal like WWV and switch it
in, and see how much attenuation it supplies. Then dial in your 1940 kHz
signal. Switch in the attenuation. If the signal drops the same level as
you saw in the WWV test, the signal is coming via your antenna. If for
example, your attenuation showed 10 DB attenuation at WWV, and you show
a 20-30 drop at 1940, your receiver is into overload.
2. Lets say it is not your receiver. Where is the signal coming from?
Possible choices:
-It is produced at the broadcast transmitter site. Plenty of things
to go wrong there. Been there.
-It could be produced by some thing some place else, including the
proverbial rusted fence.
-it could be produced by something electronic in your house or
neighborhood. These are always fun to trace down. Your receive location
is definitely in the very strong field of the radio stations.
My guess is your will have to DF this. Please be very careful to not
assume where the signal is coming from.
Of note, both AM stations are what is called a DA-2 directional.
Different patterns day and night. WKDM changes power at night (actually
goes up from 5 kw to 13 kw)
Does the level of the signal change at daylight or sunset? When it
changes, it will be a sudden change. For the most part, the stations
will go off the air for 1 second or so.
You have a bit of research to do. There is a lot of AM broadcast RF
within a 2mile radius there in the Meadowlands.
Those two AM's, should have a calibrated field intensity meter, which
could verify the level, and is also directional.
But asking for help is the key.
Tom Bosscher K8TB, retired from over 50 years of pushing the RF out on
the broadcast bands.
p.s. side note. The first AM I worked for full time was on 1300 kHz. We
had complaints of a S9 + signal on 3900 KHz (could have been kc/s back
then!). My boss and I read a very weak 3rd harmonic at our own driveway
at the transmitter site. In his driveway, we very much saw a very strong
signal. On a hunch, I looked at his downspouts. All aluminum. I rattled
them, and the signal went away. The fellow ham then asked about the S
0.12 signal left on 1300. My boss said, "use it to calibrate your rig!".
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