Ground Loop "Hum" Using BT Modulator

[zenith2134]

Regarding RF reflection back into the transmitter: When using an impedance-matching balun( which is essentially a transmission-line transformer), not all emitted energy gets relayed to the antenna. Some gets sent backward into the final stage. This causes less output power and heats up the rf amp more. That is my understanding of it, anyway.
Remember, these agile modulators, as originally used, fed coax cable distribution networks which were a solid 75-z load.