[RFI] Is a low pass filter necessary?

Roger (K8RI) k8ri at rogerhalstead.com
Fri May 11 11:18:24 PDT 2012

On 5/11/2012 12:20 PM, Jim Brown wrote:
> 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,

Low cost?  The real culprits I've seen, seem to be some very expensive, 
top end rigs when run at partial power.


Roger (K8RI)

>   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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