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
cables.
> 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.
http://audiosystemsgroup.com/RFI-Ham.pdf
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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