Very high efficiency, high power VLF/LF amplifier

VLF/LF airborne communications transmitters require maximum overall efficiency and minimum weight to conserve prime power and fuel. The transmitters operate with Frequency Shift Keying into a high Q antenna. Therefore, when operating during high baud rate modulation, capacitive or inductive load phase angles of at least 20 degrees are presented to the power amplifier at the sidebands. In ordinary amplifiers this causes considerable dissipation in the active devices. However, it is desirable to maintain efficient operation for all load conditions. By the use of clever techniques the desired high efficiency can be maintained for these conditions. The heart of the transmitter is the power amplifier which has most impact on efficiency and power supply complexity. By using a very high efficiency voltage fed bridge inverter circuit which operates from an unregulated power supply direct rectified from the 3-phase, 400-Hertz source, power transformers are eliminated and power supply complexity is minimized. Isolation from the power lines and impedance matching can be performed at the power amplifier RF output where the transformers are much smaller, lighter, and more efficient.