Overview
The circuit was designed for the purpose of increasing an input signal of 4 Watts to 6 Watts operating on the VHF radio frequency band specifically the FM band with 88 to 108 MHz frequency.
Terminology
- Very High frequency (VHF) – the radio frequency range from 30 MHz to 300 MHz
- 2SC1946A – a silicon NPN epitaxial planar type transistor designed for RF power amplifiers on VHF band mobile radio applications; has equivalent input/output impedance at rated operating conditions, has the ability of withstanding more than 20:1 load, has low thermal resistance ceramic package with flange, high power gain, and emitter ballasted construction and gold metallization for high reliability and good performances
Circuit Explanation
The essence of having a Butterworth low pass filter in the circuit is to provide a flat frequency response to produce a clean output and make the circuit stable at any location. To obtain high-quality product, the circuit utilizes the functions of 2SC1946A VHF RF power transistor, which can operate up to 175 MHz. to prevent undesired signals in the power supply system, well decoupling of the power line is established and over 5 A current can be achieved by the amplifier.
The inductors play a main role in stabilizing the circuit. They are made of standard wire gauge of 16, with silver copper lamination on the wire. The radio frequency control can be made of a toroidal core that operates on high frequency. it consists of a coil wound around as in the shape of a doughnut where all the magnetic flux is held inside the core by turning the magnetic field around the loop. The advantage of using a toroid is that it gives more inductance than a solenoid.
To suppress any transient or rapid rise of voltage in the circuit, the capacitor C3 and resistor R1 forms a snubber circuit. Self oscillation normally appears when the filter circuit produces a tone by itself after experiencing a sudden drive because of the high resonance. Hence, the combination of capacitor C6 and resistor R2 is used to avoid this incident.
Using multiple power design, by combining two power transistors, can lead to more responsibility compared to a single power, especially in the UHF and VHF bands. It may result to great interference because of the out of phase operation of the two amplifiers. To amplify the output at its operating frequency, it simply involves connecting the output to a good antenna with a 50 ohms transmission line. With this, the amplifier can be adjusted easily.
Application
There are several services where VHF radio band is being allocated such as amateur radio or ham radio, which are utilized for self-training, recreation or public service; in FM radio broadcast where it uses 87.5 MHz to 108 MHz; in marine communications operating at 156 to 174 MHz, where they play vital roles in rescue operations and providing transmission with other harbors and marinas; in television broadcast; in air traffic control communications where ground control used VHF radio to get in touch with an aircraft, vehicle or a person for clearance in the area; in land mobile stations that provide military, business or emergency operations; and in air navigation systems being used Instrument Landing System (ILS), Distance Measuring Equipment (DME), and VHF Omni-directional Radio (VOR).
This VHF amplifier may be used in wireless infrastructure preamplifiers, in ISM band application, hands-free sets, professional phones, in military medical like nuclear magnetic resonance, and radar applications.
Schematic
Important Note
Operating unlicensed transmitters is illegal in some countries, including the UK. The circuit presented here is for educational purposes only.
Source:www.zen22142.zen.co.uk/Circuits/rf/30wvhf.htm
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