1W AM radio transmitter at 14 MHz with a BS170 transistor

A simple 14 MHz amplitude-modulated radio transmitter. Containing only ten parts and built on a single BS170 transistor, it's easy to assemble and configure.The output power is 1W (load 50 Ohm).

First, assemble a transmitter using a simple circuit and test its operation. Coil L2 contains 18 turns of 0.3mm diameter wire wound on a 20mm diameter frame. The power supply is 9 volts, and the current consumption will be 140 mA. But first, apply a 5 volt supply and use a voltmeter and a variable resistor to set the gate-to-source voltage to around 2.3 volts. Instead of an antenna, connect a 50 ohm resistor, this will serve as an antenna equivalent. Connect an oscilloscope to this resistor and observe the signal. Use a variable capacitor to tune the coil to resonance. Next, increase the voltage to 9 volts and observe the gate-to-source voltage at 2.3 volts.

If the signal amplitude is 20 volts full-scale, then the output power is 1 watt. The BS170 transistor will be warm, but not very hot.

The result is a simple telegraph radio transmitter that can be used for radio communications. A power of 1 watt is sufficient for radio communication over hundreds of kilometers, provided the antenna is tuned.

Now connect the modulator on the 2sk3919 transistor. Increase the power supply to 11 volts and adjust the variable resistor again until the signal amplitude reaches 20 volts on the oscilloscope. The current consumption will be 150 mA.

I tested this transmitter with the 50-ohm resistor disconnected, simulating a disconnected antenna. The transistor did not fail, and the current consumption was significantly reduced. Therefore, this transmitter is quite reliable and easy to set up.

To minimize the influence of your hands on the oscillator's operation, you can wind the coil on a T50-2 ferrite ring measuring 13x7.5x5 mm. I also tested the generator's operation by replacing the BS170 transistor with a 2n7002 transistor in an SMD package. This transistor also works, but its drain current is 200 mA, so it can only be used in the oscillator with an output power of up to approximately 200 mW,otherwise, this transistor will overheat and fail.

I also tested the APM3055L transistor, and it can work in this oscillator, but it's less efficient than the BS170. The 06N03LA also works, but it's also less efficient.

The following transistors don't work in this oscillator: K3467, FDB6035AL, 15N03H, K3918, P75N03LDG, and A09N03