[1000mp] Amps & IMD Low power

[Tom Rauch]

> Hi all
> The IMD distortion on 12 volt final radio's goes UP as power goes
> down. 

That isn't what I have seen. 

I've found most radios have the best IM performance at about half 
factory rated power. You don't want to go too low, you certainly 
never want to go too high.

At low power, cross-over distortion is a problem. At high power, the 
driver and output devices get nasty. The problem is often not even in 
the PA transistors. My IC751A's all improved greatly when I replaced 
the driver transistors with higher power devices. 

I've seen some radios where contesting people crank the internal 
power limit pots up to get more power. That is just begging for IM 
problems on the TX, and may explain why some contesting stations have 
such wide SSB signals. Anyone who cranks the factory internal power 
limit controls up thinking the radio will work OK is making a 
mistake.

Transistors, unlike tubes, need lots of headroom. They can never be 
operated cleanly when power approaches the saturated power. About 50-
75% of saturated power seems to be the point where they begin to get 
really nasty.

73, Tom W8JI
W8JI@contesting.com