Television Interference Source?

Perhaps it's co-channel interferrence from WGBH Channel 2 in Boston?

Harry C.

There are many radio transmitters that do many other things than communications. In fact, the growth of wireless everything these days means the bad old days of lots of RFI will return. RF spectrum space is NOT unlimited. Even cordless phones, which are for communications, are a duplex system, so transmitters (both base and phone itself) are transmitting all the time you are using them.

Also, consider computers. General purpose computers sold to general public no longer need to be shielded as well as they once were. Modern PCs put out copius amounts of RFI :-(

I don't know any more than I did in my earlier posting in this thread, but the more I think about it, the more reasonable the following seems to me: short wave radio stations go on and off the air at specific times. You might see what you can pick up with a short wave radio at around the same time that you experience the interference. I'd guess you'd want frequencies near that of the tv channel, but maybe you'd also want to look near frequencies of which the tv channel is an overtone. You could look in some reference book to see what channels are officially on the air, but I think there might be some short wave channels that operate unofficially. If someone is operating a station like that near you, it could be the source of your problem, and you could detect it by hearing the program on the short wave radio.

Sure lots of them

I used to go around the neighborhood jamming TV signals just to watch the couch potatoes get up and attempt to adjust it.

What does the interference look like? Wavy vertical patterns, "herringbone" are caused by a carrier (signal on the same or close channel or some sub harmonic)

Horizontal sparkles or bands of interference moving slowly from top/bottom are in sync with the power line frequency (in the US). They might be arcing from a motor or other 60 cycle source.

The fact that it occurs on channel 2-4 suggests some lower freq and the harmonics are causing the IRF.

I'd also try a new piece of cable - to make sure you haven't inadvertently left a splice at the wavelength of the channel that has problems - or just unplug the antenna and see if rabbit ears can't get the channel during the time of interference . .

And they occur at a particular time - that suggests "skip" (remote station on the same freq that occurs at night or certain hours of the day) tweak the antenna aim a bit and you may eliminate that.

Try turning down the volume - your neighbor may be jamming you deliberately when he/she hears the station you are tuned to. Ditto move the TV away from the windows.

Radio stations don't have to stop and listen. There are plenty of idiots out there that leave a carrier running just to clear the band in case they want to communicate.

Likewise I know of one (sugar company in Yonkers) that would leave their CB channel 22 carrier running to clear the band - then would use it for tone signals for radio control (for unloading ships).

illegal, to be sure. But big corporations have big lawyers, and don't consider their actions beyond what they can get away with.

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Thanks for all your suggestions.

To answer some questions:

It's an antenna in the attic-- can't easily move it. No other members of the household. Not sure about daylight savings-- seemed to happen a lot last summer, little this summer. The neighbors probably all have cable, so they wouldn't observe the same problem.

If I remember right, it's horizontal lines across the screen, and it obliterates the sound. But since I see it only on videotapes (I'm not up at 12:35) it could be that there's sound that the VCRs just don't record. Oh, it's been observed on different VCRs, so it's not them.

Only happens in summer? You use the RF input from the VCR to the TV rather than the AV input? If both are a 'yes', I'd be betting on skip. Either change the channel you have the VCR output tuned to or use the AV output.

Ken

This sounds like a tracking problem with the VCR.

No, it's not between the VCR and the TV that's the problem-- the problem is caught on tape when I'm taping.

What kind of antenna in the attic? Anything special like a signal booster at the antenna or in the line? Any environmental possibilities like heat, moisture, etc.? Can you temporarily sub a jury rigged antenna that will get the channel you want as a means of eliminating a flaky antenna, matching transformer, or some interconnections?

Any chance of some improperly shielded appliance on the same power line that is transmitting RFI through the power line? Things with switching power supplies are always suspect - computers, TV's, some stereos, and digital clocks.

Electric water heater with a set of arcing contacts ditto an air conditioning unit - one might assume if that were the cause it might also happen during the day, but the contacts might only arc when some other condition is met.

And last but not least, any budding Frankenstein with Tesla coils in the neighborhood? I had one that could knock out cable TV, and loran receivers that I operated late at night so I wouldn't get the neighbor's ire.

Guy down the road with a 1KW linear hooked to his CB would also jam cable TV. He switched to a cleaner linear and that problem went away. He had a tower with rotor and obvious 11 meter beam antenna - so I knew who to talk to.

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My bad. :-)

Ken

Unlikely to be any of the above. No transmitter antennas I've seen anywhere in the neighborhood. No electric water heater, and I went from window AC units to central AC this summer, so it's not that.

As it almost always starts (when it does start) right around 12:35, my thought was that someone watches Leno then turns off their TV and does something else right then-- uses a computer, or something like that.

I've pretty much eliminated everything in my house.

No matter. I not up on how/what to do to make a formal complaint to the FCC. But that may be one of your options.

The thing I would do is make very sure I could duplicate the problem with entirely separate equipment. And if possible at a friends house. Stay up late and watch TV on the channel you want using a different antenna, equipment. That alone might provide some insight into what is happening. A fresh approach . . .

Years ago I had a problem that was similar. I had a small distribution amplifier on the TV antenna. Thing was in place for years and all but forgotten. It started oscillating when it felt like it and would knock out the TV. All hours but only if it was a hot day.

Wife had something else that only knocked out some channels and usually at night. Turned out to be a splice in the cable that was run under the house (crawl space). It lay in the dirt and eventually had enough corrosion that it caused a problem. At night moisture would condense on the connector. Only select channels 2-4, 65 and above, and a few others in between depending on how bad it was. I'd guess that the length of cable just happened to form a trap for those frequencies when the shield connection opened up. Problem was worse if it rained that day. She didn't know there were channels above 65 until it knocked out 2-4 and I started hunting for the problem.

Troubleshooting RFI problems can be difficult, and it takes time and perseverance.