In the last article I told about the VHF radio microphone, which I ordered on Ali for 100 rubles. I tested this radio microphone for range, it is a little more than 150 meters with direct visibility. But if you go behind the house, the signal disappears. I decided to make a power amplifier for this low-power transmitter. Now the signal of the radio microphone can be received kilometers away. But when testing a powerful transmitter, you must not interfere with the reception of commercial FM radio stations, otherwise this is a violation of the law with possible consequences. The amplifier is assembled on two transistors: preamplification on 2SC3355 and output stage on field-effect transistor RD15HVF1. When powered by 13 volts, the output power is 7.3 watts. This transistor can produce 15 watts of power at a frequency of 175 MHz when powered by 12.5 volts, but I have a low-power preamplifier and therefore it is not possible to pump up the output stage to such power. I also bought the transistor on Ali, they are sold individually or in fives and they are used. These transistors will work perfectly since they are original and were probably soldered from radio stations. Those that are new for sale, their operation and quality are unknown to me.
First, check the output power of the radio microphone. As you can see, the power shows zero, it does not reach 100 mW and the GY561 device will not show anything
The output power of the radio microphone is probably several tens of milliwatts. This small power cannot drive the RD transistor, since it is not enough to drive it. Therefore, a preamplifier on the 2SC3355 transistor is needed.
Winding data. Coil L1 contains 4 turns of 0.7 mm wire on a mandrel with a diameter of 5 mm. L2 contains 7 turns of 0.7 mm wire and is also wound on a mandrel with a diameter of 5 mm. Coils L3 and L4 have 15 turns of 0.6 mm wire on a mandrel with a diameter of 3 mm. L5 - 4 turns of 1 mm wire on a mandrel with a diameter of 5 mm. Resistor R3 is used to set the permissible base current, at the beginning it should be at maximum resistance, otherwise the transistor may fail. Resistor R5 is used to set the drain current, I set it to about 600 mA. Trimmer capacitors are used to adjust the signal power. The field-effect transistor is mounted on a radiator.
Next, I check the operation of the preamplifier. Its output power at 9 V is 500 mW. The transistor will heat up, but only slightly, and this is normal. It is also necessary to adjust the ferrite core of the L1 coil of the radio microphone to the maximum output power of the signal
Now the setup together with the output stage. The GY561 is connected to the output stage through a 47 pF capacitor. By adjusting the capacitors, you can squeeze out power into a load with a resistance of 50 Ohms of approximately 7.3 W with a power supply of 13 Volts
The antenna used was homemade. It is a dipole with two 73 cm arms, made of aluminum wire with a diameter of 4 mm. The cable length is 1 meter, REXANT RG-58A/U 50 Ohm. The characteristics of such an antenna: SWR - 1.8, resistance 46 Ohm at a frequency of 85.2 MHz.The antenna was tuned using the NanoVNA device.
I walked with the receiver in a radius of 450-500 meters, broadcast from the 10th floor, the antenna in the room, the signal was excellent everywhere, and the antenna in the receiver was not extended. The signal with such an amplifier can be received for kilometers. By the way, the transistor is "oak", with different settings it did not fail
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