[RFI] Is a low pass filter necessary?

Jim Brown jim at audiosystemsgroup.com
Fri May 11 09:20:26 PDT 2012

On 5/11/2012 7:02 AM, John G. wrote:
> I am using one of the Drake TV-1000LP right now.

Remember that the original Drake filters were designed and built when 
FCC Rules restricted us to 1,000 watts PLATE INPUT power, which, for all 
practical purposes, limited us to 600-700W output power, depending on 
the efficiency of the output stage.  I don't remember when that Rule was 
changed to allow 1.5kW OUTPUT, and I don't know if the Drake filters 
were later redesigned to handle 1.5kW.

Following up on Dale's suggestion that tube amps are more likely to need 
an HPF than a solid state amp -- I would disagree to the extent that it 
depends on the topology of the output network.  My 30 year old Ten Tec 
Titans use a manually tuned Pi-L output network, and their harmonic 
suppression seems to be a bit better than my new Elecraft KPA500.  IM 
seems to also be quite low when I tune them carefully.

It is also well known that using ALC between a power amp and the driving 
rig to set TX power level is a recipe for massive IMD. I suspect that it 
would also increase harmonics.  It's OK to use ALC to prevent damage to 
the amp when something breaks in the antenna system, but NEVER to set 
the power level.  Power output should ALWAYS be set by controlling the 
drive level.

HOWEVER -- W8JI, in  an excellent applications note on his website, has 
observed that MANY rigs, especially low cost rigs, put out an initial 
spike of full power at the beginning of each transmission when adjusted 
for less than full output. This spike WILL cause distortion -- both 
harmonics and splatter. Tom recommends using a simple passive attenuator 
(resistive voltage divider) between the rig and the power amp.

73, Jim Brown K9YC

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