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Re: [TowerTalk] Low Pass Filters: Are They Of Any Use Today ?

Subject: Re: [TowerTalk] Low Pass Filters: Are They Of Any Use Today ?
From: Jim Brown <>
Date: Tue, 17 Apr 2012 16:38:21 -0700
List-post: <">>
On 4/17/2012 2:50 PM, David Thompson wrote:
> This topic is far a field from towers and antennas
> except a low pass filter still controls fundamental overload of TV sets on
> an antenna/digital converter.
That is absolutely false.  A low pass filter ONLY suppresses harmonics 
above its cutoff frequency, usually 30 MHz for traditional products.  
ARRL's definition of "fundamental overload" is that the fundamental 
transmitted frequency is overloading the victim.  A low pass filter does 
NOTHING to help that.  The only thing a low pass filter does is kill any 
trash that a transmitter is putting out on the TV channel.

Interference to TV receivers from HF transmitters is generally best 
cured with common mode chokes on antenna leads and other interconnecting 

> I plan to add another low pass to the back of my new Ameritron amp, too.

Your hard earned money would generally be better spent on suitable 
ferrite cores, and winding multiple turns through them to form common 
mode chokes on wiring connected to victim equipment.  Coax ferrite 
chokes on TX feedlines can also be quite helpful. See my RFI tutorial 
for detailed advice.

FWIW, beginning about three years ago, I put a DTV converter on a 25 
year old Panasonic CRT set that sits in the shack.  The feedpoint of my 
160M vertical is 25 ft away, there's a 20/15/10 fan dipole almost 80 ft 
directly above it, there are dipoles for 80 and 40 about 110 ft more or 
less directly above it, and a 2M vertical about 10 ft away.  I run 1.5kW 
to all of those antennas, CW, SSB, RTTY, JT65, except for 2M, where I 
run a 170W brick amp. I've never seen a single instance of TVI.  The TV 
antenna is an 8-bow-tie with a reflector up about 20 ft, only 15 ft from 
the 160M vertical, it's preamped, and the stations are roughly 40 miles 
away.. I have a common mode choke on the feedline, but no filters.  One 
station is on Ch 8, another on 12 or 13, the rest on UHF.  The converted 
output to the TV is on either 3 or 4 -- I don't remember.  I never 
watched it much, it was only to prove I was clean, and I've since taken 
down the antenna because I needed the space for another one. :)

I have had some RFI issues with stereo gear, all caused by Pin One 
Problems, all fixed with multi-turn ferrite common mode chokes on wiring 
connected to the victim.

73, Jim K9YC

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