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RF Interference

K

KB3HEY

If this is not the appropriate place to post please forgive me.

I live in the shadow of a couple AM/FM radio station antennas. A couple I can see just a couple streets away and two more are on the mountain just miles away. I am getting interference through out my receiver. Is there anything I can to to eliminate this?

Thanks,

Bob
 
You don't specify what kind of receiver you're using, or what frequencies you're listening to or are causing the interference. It sounds like you're getting front-end overload. If you're using an external antenna, a trap or filter that rejects those stations' frequencies may be required.
 
Me bad! I have a Denon 1500RDS receiver. The frequencies are pretty much in the middle part of the band. We have a local station on 94.9 and I can hear it seems like all over. I hope this helps.

Thanks,

Bob
 
KB3HEY said:
Me bad! I have a Denon 1500RDS receiver. The frequencies are pretty much in the middle part of the band. We have a local station on 94.9 and I can hear it seems like all over. I hope this helps.

Thanks,

Bob
Click to expand...

Is this with external antenna or can you hear the problems with NO antenna connected? If you disconnect the antenna and you DONT hear anything, you MAY be able to install a trap for 94.9 inline and see if that clears it up...they may not be cheap (40-75 bucks depending on vendor, etc)...but worth it...note; DONOT put a FM trap inline unless it is TUNABLE...a standard FM trap sold by RS or others is wideband and kills the entire FM band...you just want to notch one freq.
73 and GL.
 
I'd also recommend the notch filter... also, any sort of RF amplifier is just going to make the problem worse. Microwave Filter Company makes really nice 75-ohm filters... i've used a bunch of them, but they are somewhat wide, so plan on losing everything within a 1 or 2 MHz swath.
 
Try grounding the chassis of your receiver. Loosen one of the screws on the cover, and run a wire to an 8' grounding spike. Read part 15 FCC Regs... and good luck with any consumer-type equipment when you are operating it near transmitter plants :)

-A
 
Microwave Filter Company (google it) sells a 5kfm series that goes inline between your antenna and tuner. Make sure you use real shielded cable not the aluminum foil junk someone sells at their computer store. They insert and have f connectors on each end.

Use these as a Godsend to trap individual or multiple frequencies.

With an 88.1 fm and 87.75 Channel 6 we have used these within a few hundred feet of a site to eliminate FEO.
If you place an order regular i thin they are $30 and arrive within a few weeks. Multiple discounts on multiple orders over 10. No, I don't work for them but their stuff works.

Part 15 junk is the main culprit. No shielded cases, just plastic maam.

Other cases of FEO are eliminated by simply replacing the old aluminum foil sheilded cable.

This doesn't help anyone with a boom box because the interference will come in through the plastic.
 
Another thing to try is to insert a 10-15dB pad in the antenna line. While this may seem counterintuitive, the pad will reduce all incoming signals which will quite often reduce rf levels in the front end circuitry below the point where the overload is produced. I have done this many times on communications receivers and sometimes the results can be quite remarkable. These coaxial pads used to be available at Rat Shack, don't know if they still have them or not.
 
phatdaddy said:
Another thing to try is to insert a 10-15dB pad in the antenna line. While this may seem counterintuitive, the pad will reduce all incoming signals which will quite often reduce rf levels in the front end circuitry below the point where the overload is produced. I have done this many times on communications receivers and sometimes the results can be quite remarkable. These coaxial pads used to be available at Rat Shack, don't know if they still have them or not.
Click to expand...

I believe Radio Shack still sells a variable in line pad. I know that Winegard has one which should be available from various electronic distributors. The variable attenuators are handy because you can reduce the signal to just past the point where the overload goes away, and no more. There are also in line 10, 15 and 20 db "F" connector pads available from similar sources. I've found them in stock in most All-Tex stores in Texas. I'm sure that Mouser and MCM have them as well.

Like you, I've had quite a lot of success using this type of device. It issue isn't really interference in the usual sense that something else is broadcasting on the same frequency you are trying to listen to. Usually it is caused by the very strong RF field that overloads the broad band input of your receiver. The common term is "swamping" the receiver. Attenuating the entire input to the tuner frequently cures this problem.

Swamping can be an even bigger problem with any kind of amplified antenna system, like a master TV antenna, or even an amplified mast mounted TV antenna. Once the systems amplifier is overloaded by the undesirable signal, it becomes useless as an amplifier of the desired signal. Frequently just removing the amplifier and reverting to a totally passive system helps clean up the problem.
 
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