SWRs increase as RF power turned down

[BC Coyote]

I'm a tech for an FM broadcast station and I have seen that behavior in RF amps before. As simply as I can explain..... It is caused by an increased level of harmonic content at low power levels. RF amp transistors are designed to be most efficient at a certain voltage and drive level. Operating them at a very small percentage of that voltage/drive level pushes the harmonic content up relative to the main signal, resulting in higher SWR.

 

[nomadradio]

In a properly-linear amplifier this suggests an issue with negative feedback not doing what it's supposed to, maybe?

In a Class-C amplifier that uses the radio's AM carrier to turn on the transistors, this is just unfiltered harmonic energy. Good chance that the amplifiers you mention are all lacking any harmonic filtering. As the drive is reduced, the transistor's collector current is cut off completely for part of the RF cycle. Gets you harmonics.

Adding fixed bias to maintain a steady "idle" current with no RF drive is a big deal if you want to use a linear for sideband, but this feature gets left off in a lot of amplifiers. Never mind the front-panel switch marked "SSB". That feature requires only a switch and a capacitor to stop relay chatter. Doesn't make the amplifier suitable to amplify a SSB signal properly.

73