The high-frequency oscillator, covering the VHF and UHF band described in this article, can be used in equipment testing as a signal source or even for demonstrations.
Its signals, even without an antenna, can reach several tens of meters which allows its use as a signal transmitter. An electret or piezoelectric microphone can be connected in parallel with R2 for modulation.
The frequency will depend on the used coil and also on the value of capacitor C2, according to the following approximate table.
Frequency Range /L1 C2
80 to 100 MHz/ 4 turns 4.7 pF
100 to 150 MHz/ 2 turns 2.2 pF
150 to 300 MHz/ 1 loop 1.5 pF
300 to 450 MHz ½ turn 1 pF
The capacitors must be ceramic. For C2 one can obtain a low capacitance (1 pF, for example) using a pair of common twisted wires, as shown in figure 1.
The circuit can be powered by voltages between 3 and 6 V and the trimmer should be 10 pF of maximum capacitance. Trimmers with higher capacitances reduce the frequency range generated, in which case the desired values ??may not be reached.
In figure 2 we have the complete circuit of the UHF oscillator.
A printed circuit board for this assembly is shown in Figure 3.
An interesting possibility for higher frequencies is to use a printed coil, as shown in figure 4.
To test the circuit, simply power it and connect it in the vicinity of a VHF or UHF receiver that tunes to the generated frequency. The signal must be picked up with a hiss if the receiver is of the type for frequency or amplitude modulation.
Q1 - BF967 - UHF NPN Transistor
L1 - Coil - see table
CV - trimmer - 1,5 - 10 pF - see text
S1 - Single switch
B1 - 3 to 6 V - 2 or 4 batteries
C1 - 1 nF - ceramic capacitor
C2 - See table - ceramic capacitor or twisted wire
C3 - 10 nF - ceramic capacitor
R1 - 10 k ohm x 1/8 W - resistor - brown, black, orange
R2 - 5.6 k ohm x 1/8 W - resistor - green, blue, red
R3 - 100 ohm x 1/8 W - resistor - brown, black, brown
Others:
Printed circuit board, battery holder, mounting box, wires, solder, etc.
