15w RF power amplifier

15w RF power amplifier

This power amplifier can expand the power of 1-2W, 88-108MHZ FM transmitter to 10-15W. It adopts single-tube Class C amplifier and multi-stage low-pass filter, with high conversion efficiency and strong interference suppression ability.

The circuit is shown in the attached figure. It uses a high-power transmitting tube C1972, and its parameters are as follows: 175MHZ, 4A, 25W, power gain ≥8.5db. According to the parameters shown in the figure, the circuit works at a center frequency of about 98MHZ. When the input radio frequency power is about 2W, the rated output can reach 15W. In order to ensure that the output reaches the rated value at any frequency point within 88~108MHZ, some components can be appropriately adjusted according to the center frequency of the previous stage. If necessary, the number of low-pass filter stages can be reduced to increase the output power. The expanded power signal is filtered out by a three-stage low-pass filter to remove the high-order wave components and fed into the transmitting antenna.
Component selection: Except for electrolytic capacitors, other high-frequency ceramic capacitors are used. C11, C12, and C14 use adjustable capacitors with good high-frequency characteristics and stable performance. Choke inductors RFC1 and RFC2 use finished inductors. Attention must be paid to the current carrying capacity of RFC2. Inductors with thicker wire diameters and magnetic cores should be selected. L1-L6 can be made of 0.8mm high-strength enameled wire with a diameter of about 5MM. The number of turns is indicated in "T" in the figure. Q1 uses a common Q9 socket and is used in conjunction with a plug. Q2 uses a dedicated 50Ω RF output connector with a smaller connection resistance, which is more conducive to impedance matching. The power amplifier uses the more common C1972 dedicated transmitter tube. Of course, if you have enough money, you can buy a high-gain tube such as C2538, and the power will be greater.

When debugging the circuit, be sure to pay attention to the fact that the circuit has high power, so you must connect a dummy load (I use 30 1W, 1500Ω high-precision metal film resistors in parallel), and there must be enough heat dissipation devices. When working normally, the power supply power is not less than 2.5A, and the antenna impedance is strictly equal to 50Ω. Do not use a short rod antenna, otherwise the strong RF feedback current will cause the circuit to cause self-interference, and most of the RF energy cannot be radiated into space and is consumed on the power tube, causing it to overheat and damage; it must be led to the outdoor antenna for transmission through a 50Ω transmission-specific coaxial cable. Whether the circuit can work normally depends on the debugging of the circuit, and the whole process must be very careful. During debugging, only a small excitation power is input, and the power supply voltage drops to 9V. Use a high-frequency voltmeter (do not use an ordinary multimeter) to monitor the high-frequency voltage value at both ends of the dummy load, adjust C12, C14, L3, L4, L5, L6, so that the voltage amplitude reaches about 15-20V, and then adjust C11 and L1 to maximize the voltage. Then gradually increase the voltage, and adjust C12, C14, C11, and L1 repeatedly each time the voltage is increased to make the output voltage the highest. Note that the voltage should increase synchronously with the RF input excitation power to ensure the accuracy of the debugging results. When the rated value is reached, the operating current is about 2A at a power supply voltage of 13.8V, the voltage across the 50Ω pure resistance dummy load is ≥40V, and the RF output power reaches 15W.

Use this RF power amplifier with a 50Ω umbrella-shaped omnidirectional vertical transmitting antenna (gain about 2dB) and test it with a common FM radio. The transmission distance coverage is no less than 15K.