interference on 146.58?

Every Monday-night simplex net I've had the same problem: there's some sort of local interference.

I can hear the interference on 146.58 very faintly in the background if my radio's in the right place and tilted at the right angle, but when anyone transmits, it completely overtakes their signal. Even strong, clear signals become impossible to make out.

Today I was able to track down the interference. I originally suspect it was a broadcast station (it sounded like people talking and music, but I couldn't make it out either—it was doubling with other people's signals in weird ways so neither one was intelligible).

Apparently, it's WFIU (103.7MHz). From google's cache, I learned the following:

By 1953, the station had settled down to routine operation, with a broadcast day extending from 12:55 p.m. to 10:15 p.m. Due to the location of WFIU’s 100 foot transmission tower in the center of campus and its impressive 75,000 watt signal strength, small “interferences” persisted. Keith Klein, who worked on the WFIU staff as a student, remembers, “In the days when WFIU’s antenna was adjacent to Read Center, it was legendary that people who lived in the surrounding dormitories could pick up WFIU on their dental fillings, eyeglasses, electric shavers, and such. Some people found that they could hear WFIU on their stereo speakers, even when their stereo wasn’t turned on!” John Harrell, WFIU’s Chief Announcer in the late ‘50s and now a lecturer in the Department of Telecommunications, recalls, “I was living in what is now Ashton Center, and I had a reel-to-reel tape recorder in my room. Well, I listened to WFIU on my tape recorder. I had no need to buy a radio.”

William Kroll notes that the strong presence of WFIU’s signal on campus occasionally caused problems at special events, too. “It was anticipated that our signal would interfere with the dedication ceremonies at the Lilly Library. The university wanted to take the station off the air completely during the ceremonies, but our director, Elmer Sulzer, managed to strike a compromise, and we played an afternoon of very soft music instead. Then there was the time when the rock and roll group The Turtles came to campus to perform. They rehearsed in the morning, and they returned that evening for their concert. When they turned on their public address system, they heard WFIU, loud and clear.” This type of problem was resolved when the WFIU transmitting facility moved to its current location on Sare Road (then the very southeast edge of Bloomington) in 1968. Apparently the only people not satisfied with this solution were the boys of SAE fraternity, who frequently scampered up WFIU’s tower late at night to hang bed sheets scrawled with creative messages.

So I assume that the antenna seen on google maps on Sare Rd. is indeed WFIU's antenna. If so, that's far enough from my QTH that I really shouldn't be expecting any interference, right?

I find that if I don't use my J-pole (and instead my dual-band rubber duck), it's not as big a problem. But then I get worse reception of the other stations on simplex, and don't get much of a signal out, so it's not worth it.

Does anyone have any ideas or suggestions on what might be causing this and how to deal with it? Has anyone else noticed anything similar? Might it be a problem with my radio?

I'll be leaving Bloomington soon and will be most likely be gone for the summer. When I come back in the fall, I'll be moving about two times closer to that antenna (though I'll be nowhere near the awful AT&T building), so I want to start working on this problem now if possible.

Jonathan, kd5cfx

I should add that I

I should add that I identified the interference by tuning a broadcast receiver to find the same signal.

kb9jhu's picture

Hrrmmm.. This is an odd

Hrrmmm.. This is an odd one..

First of all if your radio is susceptible to interference on a particular frequency, then adding a higher gain antenna (like the jpole) will just magnify that reception. The question is why WFIU. You're really too far away for that to be interfering in the ham band, in my opinion. It is quite possible that the radio you are using is just vulnerable in that spot. Easy way to test that theory is to have someone swing by with another radio and plug it in, see if the same signals are picked up.

Other questions:

Does it happen every time you tune to that frequency? Day & night? Is it always the WFIU signal or does it change?

There could be other factors at play here; ie, someone in your building is using a set of wireless speakers and they like WFIU, that signal interfering with you.. etc.. Its a tough one.

Take a trip to the WFIU/WTIU tower and tune in to 146.580 with your rubber duck and see if the interference sounds the same. If so, you just might be weak to it at that spot.

-Corey KB9JHU

k9sql's picture

Your problem is most likely

Your problem is most likely not RFI, but IMD in your radio. The interfering signal is probably the NWS transmitter on 162.45 MHz and/or perhaps another signal in the 154 MHz region.
IMD can work like this: 162 - 154 = 8 MHz mixed signal. 154 - 8 = 146 Mhz, right smack dab in the middle of the 2M band. HTs are designed to work with short low-gain antennas. Put a high-gain, high profile antenna on one and the RF front end of the HT can become easily overloaded resulting in intermodulation distortion. Unfortunately, your options are either live with it, don't use the J-pole, or get a different radio that is not suspectible to IMD.

73 de
Bob Poortinga K9SQL

The only time I've noticed

The only time I've noticed this problem is during the simplex nets, where it usually makes other stations really hard to make out, even if the other stations' signals are strong (when there are pauses in the talking and music, I can hear the other stations just fine).

I also notice that I sometimes hear WFIU in my analog landline telephone, usually in the afternoon (which could just be because of when I use the phone). I had this problem in Louisiana, where especially one of our home phones was sometimes impossible to use because it picked up a local radio station (97.7MHz iirc) so well.

I haven't noticed this

I haven't noticed this interference on other frequencies, and have only witnessed traffic on 146.58 on Monday evenings ;)

At some point I'd like to test some other frequencies, and maybe get someone over here with another radio to see if they get the same interference.

One of the problems with testing anything is that the interference is really only noticeable when another station is transmitting on the same frequency.


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