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Thread: homebrew lowpass filter

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  1. #1
    Join Date
    Mar 2007
    Location
    caldwell tx
    Posts
    80

    Default homebrew lowpass filter

    hi all . dose anyone know how to make a home brew lowpass filter?? my rig is in the basement (climent control) and when i run my amp i come over the tv . i put all new coax on the tv and all new jumpers ..dont know if i can rig somethig up to use temp untill i can get a reg lowpass filter... thanks phil.. w1acc.
    philip- kb1oox

  2. #2

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    There are all sorts of low-pass filter designs in the older ARRL Handbooks, especially those from the 1950s and 1960s.

    However, the problem often is not with harmonics generated by your transmitter but from pure overload of the television set. For that you need a high-pass filter installed right at your television set.

    Unfortunately, the Drake TV-300 and TV-75 series filters have not been made for years and these were the best. Radio Shack has a very good 75 ohm filter that screws directly into the coaxial jack on the back of the television set. The part number was 15-579A. Unfortunately, that is no longer a good part number. I am almost certain that Radio Shack still has such a filter. But, what they actually call the filter is nothing like what you would imagine. I tried all sorts of things using the search function on their website and got nothing. If you can find an employee at your local Radio Shack who actually knows anything except for cellular telephones that person probably will locate the filter for you.

    Glen, K9STH

  3. #3

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    Good tips from Glen.

    I might add, "What do you mean you come over the TV set?"

    If you mean you can hear your modulation coming through the TV's speaker, a low pass filter won't do anything do solve that problem. In fact, even if you're interfering with the picture, 99% of the time a low pass filter won't help that, either.

    The reason you don't see many hams using low pass filters nowadays (they've gone the way of dinosaurs) is because with modern electronics they won't do anything to resolve interference. They harken back to the days when hams used transmitters that generated lots of harmonics (this era passed probably 40 years ago) and people using TV sets were receiving them over the air (not via satellite or cable), often using inadequate antennas so signals were weak to begin with and very easily interfered with. All of that, for the most part, has gone by the wayside.

    WB2WIK/6

  4. #4
    Join Date
    Jan 2000
    Location
    Kilowatt Alley
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    9,938

    Default

    However, the problem often is not with harmonics generated by your transmitter but from pure overload of the television set. For that you need a high-pass filter installed right at your television set.
    Sometimes that won't help either but it is a good place to start.
    For what little a low pass filter costs it probably wouldn't be cost effective to build one. See if you can borrow one to see if it helps out or not but I am betting it will make no difference. I had one TV in the radio room that was always a problem. I swapped it for a different one and had no problems after that.
    Last edited by K7MH; 09-21-2008 at 04:24 PM.

  5. #5
    Join Date
    Mar 2007
    Location
    caldwell tx
    Posts
    80

    Default lowpass

    i know what you mean about radioshack the one by my qth when i went in to ask them they said that they dont know what that was and never heard of a ham radio.. (HE ASK ME IF IT WAS A JOKE how can i make a radio out of a ham)i gave up but i did see that ars had highpass filters for around 14$$$ i will go that way.. thanks for everyone ones in put and 73 w1acc...phil
    philip- kb1oox

  6. #6
    Join Date
    Jan 2002
    Location
    Delray Beach, Florida
    Posts
    134

    Default

    With modern TVs and modern ham equipment, it is highly likely your problem is just plain overload.

    Is your tv using an antenna or cable tv or satellite?
    If you are using cable and your neighbor tvs are not bothered, then your probably overloading your tv via the power lines. This can be 'cured' with some ferrites on the power cord, usually.

    Another 'trick' is to get two 75 ohm to 300 ohm transformers.... put them back to back... go from the 75 ohm coax to 300 ohm, the 300 ohm to the next transformer so you go back to 75 ohm coax and then into the tv. This breaks the 'ground loop' and often cures TVI for satellite or cable tvs (and more rarely, for antenna tvs).

    Here is a used Johnson low pass filter for sale and low pass filter by B&W

    I have a Drake high-pass filter for television inputs. I will NEVER part with it. They are the best, they are fantastic, and they work great. At my last QTH I had my ham antennas on the same tower as my tv antenna, about 20 feet apart. I had a Toshiba tv and the only way I could bother that tv was running more than 50 watts on 10 meters or more than 500 on 15 meters (didnt have 6m then). Putting in the Drake high pass filter meant I could run 1 kW while the xyl was watching any tv channel. The VCR always hated 80 meters though.

    A nyway, do a few experiments and see if your problem is getting into the tv by power line or by the rf input. If its not very bad now, just trash the tv and go ahead and go digital. You might find that you don't have a problem.... especially since VHF tv will be very very very very very very rare after February 2009. (Most ATSC is going to be UHF.)
    [URL="http://www.wb9jtk.com"]WB9JTK.com[/URL]

  7. #7

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    The Drake TV-1000LP is about the finest low pass filter ever made. It will take 1000 watts of AM (4000 watts peak) without any problems. Unfortunately, they were discontinued a number of years ago just like the high pass filters. I do use the Drake low pass filters on all of my "boat anchor" equipment. Before cable television I could run full legal power and not have any interference on my own sets and my neighbors' sets. Now I had a complaint of interference with a high fi system down the alley. That was cleared up with a handful of 0.001 mfd disc ceramic capacitors from each side of the speaker leads to ground.

    Then one evening I got a call from a neighbor 2 houses up the hill asking me if I could stay off the air for an hour. I asked why and was told that I was getting into their portable tape recorder and they were going to tape a particular radio broadcast that evening. Now they were moving the next day and had been good neighbors. I then asked them how long they had been having interference to the tape recorder and they said ever since I moved in. So I then asked why they had never complained and they replied that they discovered if they ran the tape recorder from batteries they didn't have any problems. It was just when they used their "wall wort" that I interfered with the recorder but their batteries had run down and they didn't have time to go get some more batteries before the show started. Of course I stayed off the air.

    My next door neighbor is a computer consultant and he has about 10 different computers operating in his house. Each of these computers has at least a pair of speakers hooked up and several have 4, or more, speakers. He has 1 speaker that I get into sometimes. Not all the time but occasionally. I have offered him ferrite filters for the speaker but he refuses to take them. If he just happens to get interference he just unplugs that speaker. He says that unplugging the speaker is easier than adding the ferrite!

    Glen, K9STH

  8. #8
    Join Date
    Jul 2001
    Posts
    21,474

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    Glen, your neighbors are far more tolerant than one of mine ever was!

    I had a neighbor who swore that I blew up her television set! I was using Collins equipment at the time, ( KWM-2 and 30L-1 ) and didn't see a flicker of RF on our own TVs.

    One day, her television set went up in smoke, and she was just about ready to attack me with the pitchfork! She swore to the day she left the neighborhood that "my radio junk" had blown up her set and she never even LOOKED straight into my face, much less talked to me. But the thing she didn't care to consider was that the day her set died, I was about 3 miles away from the house, in Church ! She came after me as I drove up to the house, following the Sunday Morning Service.

    Jim

  9. #9

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    Quote Originally Posted by AG3Y View Post
    Glen, your neighbors are far more tolerant than one of mine ever was!

    I had a neighbor who swore that I blew up her television set! I was using Collins equipment at the time, ( KWM-2 and 30L-1 ) and didn't see a flicker of RF on our own TVs.

    One day, her television set went up in smoke, and she was just about ready to attack me with the pitchfork! She swore to the day she left the neighborhood that "my radio junk" had blown up her set and she never even LOOKED straight into my face, much less talked to me. But the thing she didn't care to consider was that the day her set died, I was about 3 miles away from the house, in Church ! She came after me as I drove up to the house, following the Sunday Morning Service.

    Jim
    ::Heck, you haven't lived until you've had a neighbor show up at your front door with a loaded shotgun (a TVI complaint). As I've posted several times before, this happened to me in 1974 (pre-cable, pre-satellite) when I had an idiot neighbor trying to watch television on Sunday morning at 6 AM while I was running a kilowatt on six meters working meteor scatter.

    Back in those days, nothing was on TV at 6 AM on Sunday except cartoons, this is even pre-"infomercial" days.

    Guy shows up at my door (rang the bell and banged on the door), I answered it, spotted the shotgun. He threatened that if I didn't get off the air he was going to shoot me. Luckily his reflexes weren't as fast as his mouth and I took the rifle away from him and tossed it into my living room behind me. Then, I slammed the door in his face and called the police.

    The cops came and arrested him. That was a happy ending to that story, but I must admit we were shaken by the incident and moved only a few months later. Which was a good thing, because the next QTH was a better location and I didn't have to deal with gun-toting morons.

    Who the heck is watching TV at 6 AM on Sundays?

    Of course, cable and satellite has pretty much eliminated all of that.

    WB2WIK/6

  10. #10

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    The late Harold Brooks, W9VW (that was an original call and not a vanity), went out to get his mail (roadside mail box) and had the mailbox shattered by a shotgun blast. His next door neighbor who lived about 200 yards away, blamed him for every line and any interference on his television set. Now this was in a fringe area for the Chicago stations and it took some pretty good antennas to get a 100 percent signal all of the time and the neighbor definitely did not have a quality antenna.

    The neighbor was one of the county commissioners and also was a good friend of the county sheriff. Therefore, Brooks was not even able to swear out a complaint. Eventually he gave up operating from home and either went over to W9IVZ's station (who lived a couple miles away) or over to South Bend to operate W9IOP (who had one of the best contest stations in the mid west).

    Glen, K9STH

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