Demonstration available at [ Ссылка ]
Audio input is isolated using a small audio transformer which is then amplified by the D667 or any GP NPN.
The oscillator is made using a 2N5551 but any HF NPN will do.
The RF transformer was made from 5 turns of 18awg enamel wire wound on a 5.5mm drill bit with a polyvaricon variable cap. This is coupled to 2 turns of slightly thicker 15 awg enamel wire in parallel with a small trimmer and feeds the base of the RF pre-amplifier. I used the ZTX653 but any medium power RF NPN will do. A general purpose NPN will not work.
This pre-amp feeds its 1W output into the gate of the MRF101 power RF mosfet. The gate bias network consists of a 10k R and a 10k / 10 turn pot with a signal diode to ground. This signal diode is thermally coupled to the mosfet. I also added a 10V ESD / TVS diode on the gate to clamp the input voltage.
The LC low pass filter consists of two air coils of 7 turns each of 18awg enamel wire wound on a 5.5mm drill bit with 7-120pF high voltage trimmer caps. The final 104 (100nF) output caps must be rated for high voltage.
All the 6.8uH and 10uH inductors are fixed RFC chokes, except for the choke on the mosfet drain which is hand wound from 10 turns of 15 awg on an old piece of ferrite rod. I used another hand wound choke at the power supply input (not shown) with several 1000uF / 50V and 100nF / 1kV caps evenly distributed around the main board for decoupling. Test each choke using a 100MHz signal generator to see if it can block the signal properly.
The MRF101 outputs 100W and requires a heat sink. I also added a small fan attached directly to the sink.
The transmitter will run on 20-40 volts @ 500mA. I used a 24V power adapter from an old printer.
Do not test the transmitter without a suitable 50 Ohm dummy load or antenna or you will destroy the mosfet. Set the initial gate bias voltage near zero and slowly adjust the ten-turn potentiometer until the mosfet draws about 500mA.

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