This is a very simple 5 watt CW Transmitter based upon a TTL logic chip. There is just one "tricky" component and this is Cx. This component should have an impedance of about 10 - 50 ohms at the frequency of interest. If you wish to reduce the transmitter power, increase the value of Cx. It is Cx which causes the square wave from the output transistor to approximate a sine waveform. The value of Cx is the price of simplicity in this TX. This is the figure of the circuit.
It is far better to use too high a value for Cx initially then reduce it to achieve the correct RF output power. The value of Cx will depend upon your choice of TR1. Virtually any RF power transistor will work well in this application, as long as it will handle 800mA continuously. I have even used a BC108 in this application but the RF power was restricted to about 150mW. Cx was about 5x the value quoted above.
Part:
[Schematic diagram source: Harry Lythall - SM0VPO]
It is far better to use too high a value for Cx initially then reduce it to achieve the correct RF output power. The value of Cx will depend upon your choice of TR1. Virtually any RF power transistor will work well in this application, as long as it will handle 800mA continuously. I have even used a BC108 in this application but the RF power was restricted to about 150mW. Cx was about 5x the value quoted above.
Part:
- 1.8 MHz = 4.7nf
- 3.5 MHz = 2.2nf
- 7.0 MHz = 1.2nf
- 10 MHz = 820pf
- 14 MHz = 560pf
- 18 MHz = 470pf
- 24 MHz = 390pf
- 28 MHz = 330pf
[Schematic diagram source: Harry Lythall - SM0VPO]
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