Intermodulation Interference - Don't let it happen to you!

[KK4YY]

One day while tuning the band I came across a strange signal at 3640Khz. It was hard to understand but sounded like two people talking at once on AM. I listened a while. Then a commercial ran. OK it's a BC station... or two. Tuning the BC band I found the voices matched up to two local AM BC stations. One was Sports Talk the other God Talk. Why was I hearing both on 3640? I ran the numbers and it turned out to be a mixing of their frequencies -- the fundamental of one and a harmonic of the other. It hadn't noticed it there before and day after day it got a bit stronger or weaker.

I wondered where the mixing was taking place. Was it front end overload in my receiver? No, still there with the attenuator on. A rusty bolt on one of their towers or an old chain link fence around the transmitter site? I didn't think so. There were too many miles between them, but maybe. Eventually it went away so I didn't think of it too much.

Then one day, while doing antenna work, it came to me. I hadn't soldered the connection of one leg of the dipole to the feedline... just twisted the wires together as an expedient after a branch had taken it down. I had neglected to get back to make a proper connection. The weather must have done the rest, making a nonlinear device of corrosion where signals could mix. Even a very tiny signal that's generated right on the antenna wire itself would be strong in the receiver. That's my best explaination, at this time, but I don't know for sure. I wonder what kind of stuff it generated while I was transmitting? Errr.

Anyway, I may find out the answers someday as I don't seem to be getting less lazier as I get more older and expedient antenna repairs are terribly alluring.

Don
KK4YY

[steve_qix]

So, did you take the antenna down a solder the offending connection to see if the IM products go away?  That would be the proof!!

Regards,

Steve

[KK4YY]

Steve,

The problem went away some time before I "caught" the poor connection. I think the conditions (temp, humidity) for it to exist had to be just right as it's strength varied over the course of days. No way to know unless it returns.

Pointing to this connection as the cause is half based on reason and half on a desire to blame myself, I guess. When it first started I wanted to blame the BC stations for it. Then, finding that it may have been my own fault, I learned (yet another) lesson in humility. Just as our neighbors first blame every incidence of RFI on our transmitters, I had had the same instictive reaction. So it proves I'm human... as if I felt the need to be reminded!

I had put this antenna up, on the cheap, as a temporary antenna at my apartment. It's been there for 4 years now! So I posted this anecdote for all those who may have done similar. Technology can be facinating. Human nature can be mystifying. When you mix the two, you get intermodulation interference!!!

Cheers

[Jeff W9GY]

Don, I've seen this stuff happen when I used to field test auto receivers (way back when I used to work for a living HI).  A little different, but same principle: two BC signals mixing in bad or dirty power line hardware.

[WB2EMS]

We had a similar problem here that affected a local repeater.

The CARC repeater runs on 146.61. For a while, we were plagued at certain times of the year by a weird noise. The repeater would be quiet for hours - but after being keyed would stay up until it timed out with a "hammering" sound in the audio. This would persist till around sunset, and then go away and be fine till next morning. Then the cycle would repeat - quiet repeater, kerchunk, and then hours of hammering and timeouts. We figured out that the hammering noise was due to a delay line in the audio path, and that the repeater was apparently hearing itself during certain times of the day, but we didn't know why but quickly came to suspect intermod.

We finally figured out that it only happened during the daylight broadcast hours of one of the local AM radio station. In the day, that station is close to the repeater and also to a gravel pit with a lot of rusty old equipment. At sunset the station switches to a remote site with a directional array about 10 miles away, removing the strong local signal from any mixing possibilities. This station is on 870 Khz.

The other local station, just over the hill behind the gravel pit, runs on 1470, and drops power and becomes directional at sunset also. 1470-870=600 Khz, the repeater input/output offset!!

Fortunately, whatever was causing the mixing seems to have changed because the problem seems to have largely abated. We didn't think we'd have much luck convincing one of the AM stations to move 30 Khz or so. 

If it hadn't gotten better, we might have had to move to a non-standard split on the repeater to keep it running.

[DMOD]

You were fortunate Don.

I have a 5kW ClassB AM about two miles from my QTH and when they go to their nightime pattern, I sometimes get some mixing but the receiver's attenuator seems to kill it. 

To make it worse, I have a 43' vertical of the same polarizaton.

Phil - AC0OB

[WA2ROC]

That may be the same cause as the intermod on my NC-300,

If there are strong AM signals at the upper end of 40 meters, I can also hear a broadcast station that transmits on 7490 KHZ.  That station (WWCR) is putting out 100KW into an antenna system with 14 db of gain, aiming at 85 degrees out of Nashville Tenn. 

Basically directly at me and my antenna.

But it is only on the air Saturday mornings.  Just when I'd like to get on 40 AM....

[KB2WIG]

K,

" If it hadn't gotten better, we might have had to move to a non-standard split on the repeater to keep it running. "

No go on a PL tone??

klc

[WB2EMS]

No go on a PL tone??

I think we looked into the PL tone pretty early on - and I don't think it helped much. What i would do is run the transmitter with no PL, and require PL on the receiver so the transmitter couldn't get back in. I think the particular intergrated uniden repeater box didn't have that option. If we enabled PL, it set it on on the transmitter as well as requiring for the receiver. Our other repeater is more flexible that way, we can elect to use or not use PL independently on TX vs RX, and even pick different tones if we wish.

[KK4YY]

Kevin WB2EMS,

That sounds like the classic repeater jammer of an oscillator running on the offset freq near the repeater. Mixing makes the machine jam itself... so I'm told. 


Phil AC0OB,


Sounds like you have front-end overload. I think there are bandstop filters available that may help. Or you might try to notch the BC station out with a tuned filter. A reallllly long coax stub, maybe. But a stub will notch out the harmonics of it's tuned freq so be careful.


Someone I know... once used a coax stub with a trimmer cap on the end to notch out the freq his cable company used for talking to the set top box. When they turned HBO on as a promotion he then put the filter in line. When the promo was over they couldn't turn it off! He got HBO for an extra month until the box timed-out and went black from not hearing from home. It took a few hours of having the box connected without the filter for it to come back on. Too clever by half. 


Dick WA2ROC ,


A receiving loop with a null steered at WWCR might do the trick especially if your receiving them on groundwave. For skywave signals the loop would be less directional, as I understand it. I built a 75M shielded loop some years ago and it was fun to play around with. 


Good luck to all,
Don