AM transmitter at 27 MHz.Output power more than 5W.Power amplifier on IRF610

I built this 27 MHz radio transmitter to show beginners how to tune radio transmitters that have a power amplifier. Beginners usually make a transmitter on one transistor, it is easy to tune, but its range is short, only a few tens to hundreds of meters. To achieve a range of several kilometers, you need a power amplifier, and beginners will not be able to tune it because of the complexity. Here I will show how to roughly tune powerful transmitters.

The transmitter with amplitude modulation operates at a fixed frequency of 27 MHz and its output power is about 5 W. The master oscillator is made on the KN2222 transistor. The preamplifier is made on the BD139 transistor and the power amplifier on the IRF610. The modulator is on the 2SK3570 transistor.

First, we will assemble the master oscillator on the KN2222. Its coil L1 contains 15 turns of wire with a diameter of 0.3 mm, wound on a frame with a diameter of 5 mm. Between the capacitor C4 and the ground, we will connect a resistor with a resistance of 50 Ohm and a power of 0.25 W. By connecting the oscilloscope probe to this resistor, we will get a sinusoid with a swing of 1.14 V. This means that the generator is working, and we need to move on.

Now we connect the preamplifier on the BD139 transistor to the master oscillator via the 2sk3570 modulator transistor. This amplifier is needed to amplify the weak signal of the master oscillator. This weak signal cannot be fed to the power amplifier, it will not "drive" the IRF610 transistor. Again, we connect a 50 Ohm resistor to the output of the preamplifier between the C6 capacitor and ground, but its power must be at least 500 mW.

The preamplifier amplified the signal to 7.5V (swing). It was 1.14V and became 7.5V. By the way, I may have used the wrong 50 Ohm resistor. In the photo you see a resistor that may have inductance, since it contains a wire spiral, and to set up the transmitters you need a non-inductive resistor. This should be kept in mind, otherwise the power readings will be incorrect. Oh well. Yes, coil L2 is a choke with an inductance of 10 μH and the wire should be thicker.

Now let's move on to setting up the power amplifier. Coil L3 contains 15 turns of 0.5 mm wire wound on 6 mm ferrite. Coil L4 contains 13 turns of 0.7 mm wire wound on a 6 mm frame. First, you need to make sure that the power amplifier works at all, does it amplify the signal? To do this, connect a 10 W incandescent lamp with a voltage of 12 volts in parallel with the variable capacitor C8. When the rotor of the capacitor rotates, the lamp should glow at half the power or even brighter. This means that there is a high-frequency signal on capacitor C8 and the power amplifier is working.But don’t expect the lamp to show the output power with its brightness, this is only a test of the output stage, the real power will be several times greater than the brightness of the lamp. But before checking the amplifier, set the resistance of resistor R6 to the middle position, and when setting up with a lamp, also set the resistance so that there is maximum lamp brightness and minimum current consumption. When setting up, try changing the turns of coil L4

Now, instead of the lamp, connect a 50 Ohm load resistor and look at the signal on it. Set the oscilloscope probe to the 10 Ohm position. Rotate the variable capacitor rotor until the signal amplitude reaches its maximum. This will give you the transmitter output power. The video shows that the signal amplitude is 9.12 V (or multiply by 10 to get 91 V peak-to-peak), and the current consumption is almost 1.9 Amps with a supply voltage of 13.5 V. Thus, the output power of this radio transmitter exceeds 10 W, but I probably chose the wrong load resistor.I already wrote about how to measure the output power of transmitters using an oscilloscope in one of my articles.

Well, now it remains to make an antenna for 27 MHz and with its help to finally tune the radio transmitter. The antenna is a dipole for 27 MHz and consists of two arms, each 2 meters 55 centimeters long. The arm is a wire with a diameter of approximately 1.3-1.5 mm. A coaxial cable with a wave resistance of 50 Ohm is connected to the arms. The distance between the arms is about 4 centimeters. I placed the antenna on the balcony at a height of 10 floors. It is also necessary to make a high-frequency probe on two diodes and a light-emitting diode.

Here is the antenna. Note that I placed it vertically, because my balcony curb is made of a metal corner and it is located horizontally. If I also turn the antenna horizontally, then part of the power will be directed to the metal corner and in general, this corner will affect the antenna, its SWR.

After connecting the 50 Ohm cable to the transmitter, apply power and tune it to resonance again. To do this, install the RF probe near the antenna and rotate the rotor of the capacitor C8 until the probe LED glows to its maximum. Also adjust the resistor R6 for maximum power

The HF probe should glow in your hands at a distance of 1 meter from the antenna.

I took the radio receiver and walked away from the radio transmitter by 500 meters. The signal was so strong that I did not have to move the antenna out of the receiver. This is a really powerful transmitter and its signal will hit for kilometers.