[RFI] Which filter to use?

Jim Brown jim at audiosystemsgroup.com
Mon Apr 4 10:43:04 PDT 2011

On 4/4/2011 9:35 AM, doc at kd4e.com wrote:
> Or, are there specific alternative products you can recommend
> for this application, please?

It's been a long time since low pass filters were actually needed on the 
output of power amplifiers. There are several reasons. First, a GOOD 
power amp should not need one -- it has that filtering built in. Second, 
most RFI and TVI is caused by Pin One problems in equipment, not by 
harmonics of transmitters. Third, harmonics and intermod (splatter) in 
power amps is most often caused by mis-tuning the amp, or by depending 
on ALC to set the output power. Fourth, TVI is far less common today 
because of the shift to reception via cable, and the change to DTV.  And 
in the US, TV channels 2-6 are used in only a very few cities -- nearly 
all current stations have been moved to channels 7 - 59. Ch 7 begins at 
174 MHz, so this places low order harmonics of our transmitters further 
from a roof-top antenna that might detect it.

Far better to simply use a decent quality amp and make sure that you're 
using it properly -- set the output power by reducing the drive power 
from your transceiver, and carefully tune the amplifier for maximum 
output power. IF you connect ALC, use it ONLY to protect against 
failures in the antenna system that might otherwise damage the 
amplifier. If it's an untuned amp, you probably need an outboard antenna 
tuner to match some of your antennas, and antenna tuners also act as low 
pass filters.

73, Jim K9YC

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