Hunting down sources of intermod on the AM MW BC band
 

I think I'm seeing intermodulation on the
AM broadcast band here in northern NJ. A station WMTR 1250kHz apperas to be
getting interference from WABC 770 and WINS 1010, two very strong stations
here.

3rd order inter-modulation between two strong signals follows this equation:

Fb > Fa

2Fa - Fb is a 3rd order IM product below the frequency of Fa. And 2Fb - Fa
is another 3rd order distortion product which falls above Fb.

2*770 MHz - 1010 MHz = 530 MHz.

2* 1010 MHz - 770 MHz = 1250 MHz, the WMTR frequency.

However, third order IM can be generated by any junction of dissimilar
metals. This is now called Passive Inter-modulation.

Receivers with high RF selectivity reject most strong undesired signals but
I don't think any are still around. That required a tuner RF amplifier and a
3 gang tuning condenser.


the intermod was getting really bad, as it's been raining in NJ the past few days. So I figured it would be a good time to hunt it down. Using a portable AM radio to sniff around, I hunted around various cables around here. Turns out it was my ham radio setup. The antenna is a vertical designed to work from 3.5 to 30MHz, and is mounted on a metal pole in the ground in the back yard. And the ham radio transciever has an external 12VDC power supply, with a green wire safety ground tied to the negative 12V terminal. So now I have a large loop of wire, the antenna coax cable from the antenna and its ground about 40 feet (I used RG8 for less attenuation), to the transciever, and then there is the safety ground conductor on the house wiring running something like 50 feet to the electric servive entrance ground. Break this loop and the intermod goes away. I now think that there was a loose coax connector on the back of the rig, and some corrosion acting like a diode, and not a protection diode in the rig minus 12V line. As I didn't want to run the power supply with a disconnected safety ground, I found that a short piece of wire to commect the power supply safety ground to the antenna coax outer shield, to jumper over that "diode", also killed the intermod. Later on, tightening the loose connector also kills it.

You dam Hams...Screwin' up TV's & other peoples' radio's for YEARS... Orta report you to the Sheriff's Dept or sumpin...(grin)

Very interesting. So the two powerful NY stations beat against each other, superhet style, yielding a signal the same as your local, and the "imperfect contact" detector, early wireless style, completed the "radio."

This sounds like the flag pole setup on Hogan's Heroes. I always wondered how something "apparently grounded" could actually transmit....

"WGN, WGN, all I hear is WGN - and I don't even own a radio!" - comical clip played from time to time by long-past WGN morning host Wally Phillips. There were reports of some homes in the subdivision that grew up near the transmitter with WGN audio coming from the air ducts.

That's what I thought at first, but I have a friend who does stuff like this professionally,, and it's one station's frequency (the higher of the two) doubled minus the other station, though in this case it comes out the same.:scratch2: Looking at the algebra, it looks to be in fact the same, first station minus the 2nd, then add the frist station. Same as twice the first station minus the 2nd. I ain't that swift at math, that I could spot that quicker...

Another thing, the transceiver was hearing, albeit weakly, some SW broadcast signals, when I had the antenna switch switched away from teh transceiver (and to another SW set). Once I tighened the loose connector, that weak reception went away, and now I get reception on the transceiver only when the switch selects it. I haven't transmitted lately, it might make a horrible mess of the SW bands around here if I did with the loose connector.

..frequency doubled, minus the other station, then add...my head hurts! :no:

There are stories around the country about hearing radio stations with no radio. One involves a machinist who had a few filings from work in his fillings that acted like a coherer and he could hear music in his head. Others have picked up strong stations in the vicinity in fence wires, gutters and downspouts, and so on. WLW out of Cincinnati in the thirties when they were running 500KW was the premier example of this.

Yeah, it happened on Gilligan's Island, too, remember ?!? (grin)

There are some old radios still around and in use that are very good at rejecting unwanted signals. I own one, a Zenith C-845 table model. 6BJ6 RF amp and three gangs on the AM tuning capacitor. I live within a couple of miles of a 1kW day/0.5kW night AM station; it never bothers my Zenith one bit, as far as images or otherwise appearing at more than one spot on the AM dial are concerned. The AM digital tuner in my Aiwa solid-state bookshelf stereo system, however, receives the station at its fundamental frequency (1460 kHz) and 0.9 MHz down the dial, at 560 kHz. This tuner must have been extremely poorly designed, but not as bad as the AM tuner in my old Zenith IS4041 stereo system. The AM tuner in that system could pick up short wave between about 1100 and 1160 kHz at night. I always thought the AM tuner in that stereo wasn't much better than a crystal set.

A professor in school (MD-DC area) claimed that townhouses built in the near-field of WTOP-AM 1500 had "talking drywall screws":smoke:

Yeah, I remember talking to a SWLer back in the 1980's on the old Fidonet BBS system where he told the story that when he went to medical school in Mexico, it was next to a 100 kW AM station tower. The signal was so strong that it made florescent tubes glow without any electricity at all and when it was time to "turn off the lights," they had to put the tubes in their dorm closet.

My dad lived in Cincinnati in the 1930s and told me stories about hearing WLW on the bedsprings, in the silverware drawer, on the kitchen stove and the
basement furnace. Stores with fluorescent tubes could not turn off the lights!

He also said that the tower lighting transformers kept burning up at WLW and were finally replaced with a generator mounted on the tower turned by a motor mounted on a stand 15 feet away connected by a paraffin impregnated wooden shaft.

Cliff

Wow, that's nuts! And so the legend of The Nation's Station continues.

That can happen if you are near any 50kW AM radio station, even these days. Here in northeastern Ohio, the transmitters and towers for all AM, FM and TV stations serving Cleveland are located in the southwestern Cleveland suburb of Parma, Ohio. I'm sure some if not most folks living in the shadow of the 50kW AM towers located in that city have the same problem now that cbenham's dad had in Cincy in the 1930s. The signal from a 50kW AM radio tower (let alone two or more) at a mile or two or less is tremendous, and can wreak all sorts of havoc. My grandmother had a cottage that was located about five miles from the transmitter towers of a 50kW AM station. That station came in very well on 1220 kHz, its fundamental frequency, and also could be heard anywhere from about 600 to 640 kHz at the low end of the dial as well. In the 1970s, I lived in a Cleveland suburb that had a local FM station on 92.3 MHz, 27.5kW ERP. I lived on the next street over from where the tower and studios were located (I could see the tower's red lights from my bedroom window after sundown), and the station literally boomed in -- on my stereo FM radio, at 92.3 and also between other local stations, on channel 6 of my Silvertone roundie color TV (yup, that's how strong the signal was), and even on my dad's Ampex "Micro 88" solid-state stereo cassette tape deck.

I now live in a very small town about five miles from a 1kW AM station. The signal from that station doesn't bother anything in my apartment (I have an electric stove here; however, the station doesn't come in over the burner coils!), but it does come in at two spots (1460kHz, the fundamental, and 560kHz) on the AM/FM digital tuner in my Aiwa bookshelf stereo. I chalk that up to just plain poor design of the AM tuner's front end. I had a Zenith stereo that would pick up short wave (no kidding) on the AM broadcast band after dark; I chalked that up to poor front-end design as well. In fact, the AM tuner in the Zenith stereo wasn't all that great at receiving local stations in the daytime either, and I lived in a suburb at the time that always has very good reception of all Cleveland stations on AM and FM. To this day I'm convinced that the AM tuner in my Zenith integrated stereo system was not much better than a crystal set.

IIRC, WLW was 500 kW for a while back in the late 1930's/early 1940's.