Here the simple and low cost FM transmitter circuit. The frequency range of this FM transmitter should be about 89MHz ? 109MHz. Output power is about 9mW at 9V.

Components List:

Resistors:
R1, R6        : 12K
R2, R4        : 100K
R3            : 22K
R5            : 1K
R7            : 100R

Capacitors:
C1            : 10uF/25V
C2            : 22n
C3            : 10n
C4            : 3n3
C5            : 2p7
C6            : 22p 
C7            : 100uF/16V
Trimmer       : 0-30pF tuning capacitor

Mic           : ECM-60P

Transistors:
Q1            : BC338
Q2            : BC548B

The battery supply rails have been well tied together with respect to radio frequencies (C1, C2 and C7.) The tracks are also thicker. This makes the circuit a single “solid” block eliminating RF currents in different parts of the circuit. This also means the battery no longer has RF on it which makes the whole unit a lot more frequency stable. A battery is perfect choice.

Download the complete explanation of this FM Transmitter (include circuit diagram, component list, the circuit’s work) in PDF format:
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Source: kitrus.com

2SC2539 is a silicon NPN epitaxial planar type transistor designed for RF power amplifiers in VHF band mobile radio applications.

FM RF Amplifier Circuit Diagram:

Coil Specifications:

L1: 2T Coil diameter:4mm
L2: in series 6pcs Toroidal ring
L3: 4T Coil diameter:10mm
L1: 3T Coil diameter:10mm

FEATURES

• High power gain: Gpe >= 14.5 dB at Vcc=13.5V, Pout = 14W and f=175MHz
• Emitter ballasted construction and gold metallization for high reliability and good performances
• Low thermal resistance ceramic package with flange
• Ability of withstanding more than 20:1 load VSWR when operated at Vcc=15.2V, Pout=18W, f=175MHz, Tc=25⁰C

Download the datasheet of RF Power Transistor 2SC2539 from te following link:
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Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations

Transistor is not a critical part of the circuit, but selecting a high frequency / low noise one contributes the sound quality and range of the transmitter. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor.

Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Then note the value of the variable resistor and replace it with a fixed one.

The critical part is the inductance L which should be handmade. Get an enamelled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. Wire size may vary as well. Rest of the work is much dependent on your level of knowledge and experience on inductances: Have an FM radio near the circuit and set frequency where is no reception. Apply power to the circuit and put a iron rod into the inductance loops to chance it”s value. When you find the right point, adjust inductance”s looseness and, if required, number of turns. Once it”s OK, you may use trimmer capacitor to make further frequency adjustments. You may get help of a experienced person on this point. Do not forget to fix inductance by pouring some glue onto it against external forces. If the reception on the radio lost in a few meters range, than it”s probably caused by a wrong coil adjustment and you are in fact listening to a harmonic of the transmitter instead of the centre frequency. Place radio far away from the circuit and re-adjust. An oscilloscope would make it easier, if you know how to use it in this case.

Every part should fit on the following PCB easily. Pay attention to the transistor”s leads which should be connected right. Also try to connect trimmer capacitor”s moving part to the + side, which may help unwanted frequency shift while adjusting. PCB drawing should be printed at 300DPI, here is a TIFF file already set.

Mini FM Transmitter PCB Layout:

Technical data:
Supply voltage : 1.1 - 3 Volts
Power consumption : 1.8 mA at 1.5 Volts
Range : 30 meters max. at 1.5 Volts

The circuit is in two halfs, an audio amplifier and an RF oscillator. The oscillator is built around Q1 and associated components. The tank circuit L1 and VC1 is tunable from about 500kHz to 1600KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite ends of the tank circuit.

The 1nF capacitor C7, couples signals from the base to the top of L1, and C2, 100pF ensures that the oscillation is passed from collector, to the emitter, and via the internal base emitter resistance of the transistor, back to the base again. Resistor R2 has an important role in this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal emitter resistance, re of Q1, and also increases the input impedance so that the modulation signal will not be shunted. Oscillation frequency is adjusted with VC1.

Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor and realising full gain of this stage. The microphone is an electret condenser mic and the amount of AM modulation is adjusted with the 4.7k preset resistor P1. An antenna is not needed, but 30cm of wire may be used at the collector to increase transmitter range.