[CQ-Contest] Output pad for running QRP?

Tom Rauch W8JI at contesting.com
Fri Sep 28 15:33:06 EDT 2001

> What is the best way to attenuate the output of a 100w transceiver to
> achieve 5w output?  My current radio puts out around 10 watts when the
> level is at the minimum.

Since I use radios to measure IMD of amplifiers on a regular basis, 
I can offer an opinion based on measured data.

I'd ALWAYS use an attenuator pad, and never rely on anything 
else, unless I cared less about IMD performance. The only 
exception would be if I checked the system and knew it was OK.

Many radios have notoriously slow ALC systems, and virtually all 
solid state radios go through a minimum in net transmitter IMD 
when operated at the rated power. Even if that isn't a factor, every 
radio I know of biases a stage closer towards cutoff to reduce drive. 
That works OK if the gain reduction is slight, but it is a big time 
problem if you are reducing gain more than the factory planned on 
for normal operation. What you are doing is moving a very early 
class A amplifier towards class C (cut-off) to reduce gain! 

For example, my stock IC751A's have the best IMD performance at 
about 50 watts PEP output with two steady equal tones. I want to 
keep the peak power mostly around that point, which means when 
I get frequent peaks at 100 watts a lot of the TIME of the envelope 
is spent around the optimum power value. If I run less power, down 
around 5 watts, IMD is actually about 5 dB worse. At about 2 
watts,obtained through ALC reduction, IMD skyrockets.

I've seen some systems change even more than that, when stages 
are not biased properly. If standing current is too low, IMD 
performance improves with more drive! So it is more than an ALC 

IMD power almost always gets worse at low power levels in solid 
state radios, and virtually always much worse at higher than rated 
power levels, when compared to the main signal power at that level. 

I've noticed some radios even have internal adjustments moved to 
allow more power than the factory uses. That's bad news just like 
excessive power reduction is (or worse).

73, Tom W8JI
W8JI at contesting.com 

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