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FM Band Rejection Filter

Posted by mxyplx (My Page) on
Sat, Feb 14, 09 at 12:35

We have FM radio interference (FM 104.3) on our TV. Have had for years. With the Analog TV it was only visual. With the Digital Ready TV it is both visual and audio. Really loud crackling. I can sometimes hear/understand the radio voices on local Analog Channel 38. I believe it is also partially blanketing the digital signal some days but not others.

We are in a relatively weak signal area except for one Digital station that comes in strong enough that no interference is seen/heard.

I'm thinking of trying an FM Band Rejection Filter in the antenna curcuit. Anybody tried these under this condition? Would it further reduce our already weak TV signal? Or does the TV signal pass thru untouched?

If I installed a TV signal booster that would almost certainly boost the FM signal too; therefore requiring the Rejection Filter. Right?


Follow-Up Postings:

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RE: FM Band Rejection Filter

Gee I thought digital was supposed to solve all that ;)
The filter shouldn't affect the signal level so much - I would try it first, then look at other options if that doesn't work. You could also look at the condition of your aerial and even consider if a different orientation would help.


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RE: FM Band Rejection Filter

Yes, Digital TV reacts less to interference, but it is not impervious. Remember, we are still talking about RF signals. Check out the link I provided, it is a document by the FCC in determining any interference problems. If that doesn't work you will need a Notch Filter. The notch is for the frequency that it filters out. So in your case the notch filter would be tuned to 104.3 MHz. Make sure that the roof top antenna is properly grounded and all connections are tight. Good luck!

Here is a link that might be useful: FCC InterferenceTroubleshooting Document


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RE: FM Band Rejection Filter

I was aware of notch filthers but had no idea what they did. I had previously read the link several times and tried all their suggestions for nailing down the cause- nuthin. It wasn't till I heard the radio voices that it became clear what was happening.

I just now re-read the filter section (thanx) and will continue attacking the problem with those recommendations.

Will look into the notch filter too. My next door neighbor about 70 feet away has the exact same problem - his antenna is roof top, mine is inside the garage attic. His rotates mine is fixt - same problem.


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RE: FM Band Rejection Filter

Single frequency notches can easily be created by placing a T connector in the cable and an open stub of coax on the branch of the T.
Start trimming the length of the coax to tune the notch.


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RE: FM Band Rejection Filter

That is a cool trick I did not know about. I knew antenna leads/coax are supposed to be trimmed (at least in cb-land) to certain multiples of the wavelength - 6' was it? But I did not know that.

I'll have to file that one away.


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how long?

As a rule of thumb, how long a 'stub' are we talking about?


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RE: FM Band Rejection Filter

I got a new antenna which sits on a board in the garage attic and pulls in the digital signals better. I built an antenna as per this site http://uhfhdtvantenna.blogspot.com/ which cost me exactly $ Zero and took 1-1/2 hours to build (including digging thru the shed to find stuff) but it would have been tough to fit it into my garage attic. Maybe I just got too lazy to shorten it (cut several inches off the 2X4), age takes its toll, it does, however, work.

Anyway with the new antenna the digital signal is recieved strong enough [ie signal strength 70-90] that interference isn't a problem. The 3 remaining analog stations come in unbelievably super with the home made. The FM 104.3 interference is thereby minimized and mostly eliminated. So I just switch leads at the TV (push on adaptors) and since the analog will end in June - no big deal. Actually by rotating the homemade I can even eliminate interference from the microwave oven.

The homemade stands in the living room. I put a handle on the top so its easy to rotate. I think its neat. I spect some would die of shame to have something like that in the house; reverse status symbol and all that. We watch very little TV.

The new antenna is highly directional and one digital channel comes in 0-40%. The homemade in the living room brings it in at 99%. We seldom watch that channel anyhow. Perhap the new antenna mounted outdoors and higher would also bring that channel in.


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