Seems kind of bizare that an FM Rig PA would run anything but Class C mode. The PA and related circuit should be all about practical power control or someone didn't do their homework back at square one.
In regards to your the last portion of the below text... pretty much many a PA section can make a nice generator of unwanted products but we shouldn't right away point fingers at an rf power amplifier not running full tilt as being an instant source. Each and every case is or should be considered different. As a general rule... I'm not running most of my class C rf amplifiers full-tilt (power). Everything out of the pipe is as clean as can be expected/desired. As always... your results will probably vary. cheers, skipp > It may come as a surprise to the owners of offshore-produced > mobile radios that you can burn up a PA by running the radio > at low power! A case in point: My radio club has a Yaesu > FT-2500M that is the 2m side of a packet node. Mindful of the > high elevation of the packet site, we had the radio set for the > lowest of its four power levels. After a year of trouble-free > service, it died from a toasted RF module. > > I replaced the module- which cost about $85 as I recall- and > bench-tested the radio afterwards. It seems that the current > draw of the radio varied relatively little as the RF output power > was changed. In fact, the current draw at the lowest output power > of about 6 watts was 85% of the current draw at maximum output > power of about 45 watts! Do the numbers, and you'll see that the > extra power being consumed was going into heating the PA module > instead of being radiated as RF. > At full power, the PA runs Class C and is fairly efficient. But > at reduced drive, the PA is running Class AB or B (gasp!) and > becomes remarkably inefficient. And why is this? Because the > radio designers didn't do all of their homework to ensure that > the PA would maintain its efficiency throughout the power level > adjustment. Thus, an Amateur Radio operator who diligently uses > the minimum power necessary for communication may be toasting > the PA in his or her radio! > > Finally, the power efficiency is only part of the issue here. > The other part, which Jeff mentioned, is that a PA which is > operated at a point below its specified range may become a > prolific generator of spurious signals. > That's not good! >
