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Low Pass Filter installation HF

Sep 26th 2011, 13:13


Joined: Apr 4th 1998, 00:00
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Where is the proper or best location to install a low pass filter? I run a Pro III to a PW1-amp. Between Pro III and amp or after the amp. How short to keep the coax as well. thanks
Sep 27th 2011, 00:54


Joined: Nov 28th 2000, 00:00
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A low pass filter is almost a thing of the past. They had value with old tube radios, but most solid state stuff is very well filtered and shielded.

Also, very few TV systems are on antennas and none of them have low TV channels any longer.

I can't imagine why you would want an LPF, but if you do it should go right on the output port of the PW1.
Sep 27th 2011, 12:42


Joined: Jul 25th 2011, 14:25
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Although modern transmitters perform better than they did in the heyday of antenna-connected television, the FCC Part 97 limits on harmonics and other spurious emissions generally do not offer sufficient protection for an immediate neighbor with an antenna-connected television. In my own neighborhood, one neighbor chooses not to have cable, but watches the two or three local stations that make it into this RF hole I call home. :-)

If I were running high power, it is quite likely that I would need a low-pass filter for my transmitter.

Most interference, even to televisions, is not caused by spurious emissions from a transmitter, but is instead caused by overload of the affected equipment by the fundamental-frequency signal from the transmitter.

This can be somewhat difficult to explain to neighbors. In that case, installing a low-pass filter, whether you need one or not, may help with your conversation with a neighbor, as you can point to the filters on your transmitter, demonstrating that your station is already well filtered, and then suggesting that similar filters be applied to his or her equipment. In this case, the real value of the low-pass filter is social.

If used for any reason, the filter should go directly at the output of the amplifier, in between the amplifier and the feed line to the antenna, or in between the amplifier and any antenna tuner, if used. If you are using a tuner, putting the filter between the amp and the tuner will ensure that the filter sees a 50-ohm load on the fundamental frequency of the amplifier. This is important because if the filter sees a high SWR, the rating of its components may be exceeded.

Ed Hare, W1RFI
Technical forums moderator
Sep 29th 2011, 16:00


Joined: Apr 4th 1998, 00:00
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Total Posts: 0
Thanks for both of your thoughts. Both of you confirmed my thinking as to the use and location. Been at my location for over 35 years without any issues but a new neighbor just moved in and has been asking a lot of questions regarding my antenna farm which is not huge and barely visable from the street. So I am just getting my ducks in order, as they say, just in case I need to calm this guy down in the future. No complaints so far so will see how it goes. Thanks again for your replys. K1SND

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