Take care with transmitter circuits. It is illegal in most countries to operate radio transmitters without a license. Although only low power this circuit may be tuned to operate over the range 87-108MHz with a range of 20 or 30 metres.
I have used a pair of BC548 transistors in this circuit. Although not strictly RF transistors, they still give good results. I have used an ECM Mic insert from Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.
One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some first hand experience of this problem.
Using a circuit from the RF schematics section, you can estimate the resonant frequency for this oscillator:
Working out the parallel capacitance, C1 C2, (27*27)/(27+27) = 13.5pF
Assuming that the coil, L has an inductance of 0.2uH then:
= 96.8 MHz
This puts the frequency of oscillation in the middle of the UK FM Band (87.5 - 108 MHz). If an inductor with a core is used, then the oscillator can be tuned by moving the core. If an air spaced core is used, then tuning is achieved by physically squeezing or separating the turns.
I have used a pair of BC548 transistors in this circuit. Although not strictly RF transistors, they still give good results. I have used an ECM Mic insert from Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some first hand experience of this problem.
Using a circuit from the RF schematics section, you can estimate the resonant frequency for this oscillator:
Working out the parallel capacitance, C1 C2, (27*27)/(27+27) = 13.5pF
Assuming that the coil, L has an inductance of 0.2uH then:
= 96.8 MHz
This puts the frequency of oscillation in the middle of the UK FM Band (87.5 - 108 MHz). If an inductor with a core is used, then the oscillator can be tuned by moving the core. If an air spaced core is used, then tuning is achieved by physically squeezing or separating the turns.
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