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5W 7MHz (40m) AM Transmitter

     Here is a simple schematic of a 5W transmitter in the short wave band 7MHz (40m). The circuit includes four transistors. The first transistor operates as an oscillator controlled by the crystal. The frequency can be adjusted by changing the crystal, change is possible in the range of from about 3.5 to 14 MHz. The second stage of the amplifier. It still operates with low power and therefore there's only a resistor in the collector for simplicity. The third stage has the LC circuit in collector. The transistor has a small heatsink (10cm2), its output already has almost 1W output power. At this stage it is also introduced the AM modulation. Modulation choke has high inductance. You can use a small transformer secondary. I used a 9V winding of the 220/9V 1.6 VA transformer. Double choke in the oscillator has 2x 6 turns of wire 0.5 mm on the ferrite rod, 5mm diameter. Choke of third stage is without core and has 24 turns coil wire diameter of 0.5 mm wound on 10 mm diameter. Choke of output stage is without core and has 20 turns coil wire diameter of 0.8 mm wound on 15 mm diameter. The modulated signal enters the final transistor that works as an Class A amplifier. This transistor has a sufficiently large heat sink (50cm2). In the collector there is a circuit for reducing the harmonic frequencies. The tuning capacitor adjusts the antenna. Lamp at output is used only as a check and it is possible to replace it by a short.
     When putting the transmitter into operation, just connect the antenna and the source audio signal. The circuit is supplied by 12 and draws about 800mA. Then just set the tuning capacitor. Caution - when you turn the transmitter on without an antenna, it may cause damage (especially to the power transistor).

     Warning! Operating this transmitter without permission is illegal.

Schematic of the 5W 7MHz (40m) AM Transmitter.

This is how my transmitter looks like. I also put the volume knob and morse code button into the box. I used a box from some data switch.

Note for lamas:
I do not response the questions about what is the range of this short wave transmitter.

Added: 2002