[RFI] Is a low pass filter necessary?
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.
> 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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