[RFI] Which filter to use?

doc at kd4e.com doc at kd4e.com
Mon Apr 4 10:46:09 PDT 2011

This is a commercial Harris amp, built for CH2, which is being
converted to HF using transformers and splitter-combiners from
Communications Concepts.

The FCC requires filters as I seriously doubt it will be harmonic

While those filters were originally designed to prevent TVI in
my app they may also be effective in knocking-down harmonics.

> 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


Thanks! & 73, KD4E
David Colburn http://kd4e.com
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