A few days ago I had the urge to go "pedestrian mobile" (ham term for carrying a radio) and listen to my AM station around the yard. As I set up the system and tuned the pocket radio I heard a loud heterodyne and some other station right on top of my signal. This was early evening and my transmitter is some distance away from my computer where I was located so I thought that a strong skip signal from another station was coming in. But the heterodyne should not have been heard since my transmitter is within 3 Hz of the nominal frequency. I turned off my transmitter and the band was silent...no other station to be heard.
The interference only happened when my transmitter was on and I was near my computer and when I moved away about 6 feet the interference stopped.
It is not unusual to hear strange noises on an AM radio near a computer but this one had clear and loud programming of music and talk. It is also not unusual to have artifacts if the signal from a part 15 transmitter is very strong but in this case my transmitter signal was fairly weak due to the transmitter being in my back yard. This was not co-channel interference being received.
My best guess is that there was some strong RF signal produced by my computer which was of a frequency which mixed inside the receiver and acted as a local oscillator causing it to hear another station on a different frequency. This is rather strange since my computer is a stock Dell with all panels in place. The other thing which was strange is that the interference only happened when my fairly weak AM signal was turned on. It could have been due to some high order mixing of several signals and it may never happen again.
Ever had an "ah-ha" moment while describing something? I just did. I just realized while typing this that I had an Arduino board operating on my desk just about a foot from the computer and the radio. This little device is a computer with a 16 MHz clock and it has absolutely no RF shielding. Maybe this was the culprit. This would explain why I have not previously had this problem with the pocket radio near my big computer.
Digital devices can produce some strange artifacts.
Neil
Mysteries on radio can take a lot of solving.
It would be interesting to find out what the "station in the background" was. If you knew its frequency you might be able to derive a mathematical formula that brings all the pieces together.
I seem to recall from years ago that a Zenith Transoceanic Radio, depending on where it was tuned, put out a sweep oscillator that interferred with other radios once in awhile.
One mid-day about two years ago I picked up a strong signal at about 1mHz (1000kHz) which identified itself as the infamous Family Radio Shortwave station in the Florida swamps. When I checked the shortwave channel for the station, it was booming in and their website indicated some really high power level, either 500kW or maybe even a million Watts if that's possible. But it never happened again.
Yes, if I knew the station ID and frequency it could be possible to make some sense of this but at the time I didn't want to wait around for the very infrequent station IDs practiced by so many.
I'll try to replicate this and maybe be more patient the next time.
Neil
Neil, next time you get that background station, use another radio to tune the dial until you find a matching signal. That has worked for me when an unknown signal was coming in at 530kHz, and it turned out to be 1430kHz, somehow generating the "false" signal at 530.
and thank you.
Neil
I wish I could have been there.
Something was mixing somewhere,
and producing strange output products -
just under those proper conditions.
How interesting.
It remnds me of the time I heard an
FM station on my audio mixing board cue bus.
(I think when my Part 15 AM transmitter was on.)
And that sure doesn't make any sense at all.
Bruce, DOGRADIO
I have had the pleasure of seeing
the pictures of your radio set-up -
and it just struck me:
Do you have any wire antennas
that terminate near the point where
you experienced the interference?
Bruce, DOGRADIO
Bruce,
There are two antenna terminations near by. One is into the back panel of the FRG-100 monitor receiver shown at the top right of the pic. The other is a 2 meter roof top antenna connection to the 2 meter rig which is just barely visible behind the computer monitor in the lower left part of the pic.
I suppose your thinking is that these could be picking up a strong signal which then couples to the receiver. This is possible.
I am going to try to recreate the situation, identify the station, and go from there.
Neil
Recreating the situation sounds like fun.
I have had portable radios that - when
tuned to the AM BCB - receive shortwave
BC stations in that range. Under certain
conditions anyway.
My 150 foot long wire for my crystal set
(the wire is down now) would couple
to some radios. I hadn't really paid
attention to that - now I wish I had.
Bruce, DOGRADIO
Did some more sleuthing and found something interesting. I was able to reproduce the interference. The heterodyne is not coming from any of the digital computer equipment and it is loudest when the portable is placed next to a CCTV monitor (upper right in the picture on ALPB) which is connected to an outdoor camera. The video line is grounded at the point of entry to the building but it is still apparently acting as an AM antenna even with the CCTV and camera power off. The audio is a co-channel station which becomes louder when I turn off my AM tx. Strangely, the heterodyne is still there with my tx. off so it is coming from somewhere else.
A check with another portable radio found no heterodyne at all when moved about in the same area as the first portable. The squeal appears to be an artifact in the first portable. It is a Grundig mini 300 with a digital frequency display. This radio has been a disappointment from day one with its tendency to drift requiring constant retuning (literally every five minutes or so) and now this nonsense. I seldom use this radio since it is basically junk but chose it the other day on a whim.
This explains why I did not hear the heterodyne on the other receivers earlier and first heard it on the Grundig since it is coming from inside the Grundig.
Things are not always as they appear at first.
Neil
Neil, I had one, too. I loved the way
it looked and that was all. It was
not as good an the Grundig Mini 100PE
I had.
While walking the dog (one of the
DOGRADIO dog's) in the evening I
took out with me a wide variety of radios.
(This was several years ago.) Anyway -
I used these many different radios to
either track the DOGRADIO signal, or
DX 1230, 1240, 1340, 1400, 1450, or, 1490 kHz.
To those who haven't tuned those frequencies,
they are the AM BCB local channels, and I love
the challenge of listening to them. (Still.)
Here's the thing. Somewhere near the top
of the AM BCB my Grunig Mini 300 was
generating images of other stations.
Shortwave BC, AM BC - I don't know.
As DOGRADIO evolved, it changed
frequencies. But mostly it was on 1690 or
1700 kHz. Since the Grindig 300 had images
up in that range, it was not good for tracking
my Part 15 AM transmitter.
Speaking of portable AM, SW, FM, or
public service band radios - those have always
been a favorite of mine. I started out
AM BCB DXing with a couple of 'substandard"
multiband portables. I heard things that
were complete mysteries to me. WWV, CHU,
the ARRL's W1AW (without a BFO) marine
stations, 2 MHz marine telephone
operators, 2 meter FM repeaters (wow, did
that blow me away) and images of UHF TV
channels that weren't even supposed to
be there. Oh! And hams on 160, 75, and
40 meter AM. I couldn't figure out why
the guys on 75 seemed to be "a little off,"
but the guys on 160 and 40 seemed fairly
normal.
I could start a thread on portable radios,
and even their role in the Part 15 game.
Oh my.
By the wya (and this is the most
important part of this note) - it's great
you recreated the mystery situation, Neil.
That is neat.
Bruce, DOGRADIO