This long-range FM radio transmitter has an additional RF power amplifier stage, after the oscillator stage. The extra RF power amplifier used to strenght up the power output to become 200-250 milliwatts. With a great matching multi-element Yagi antenna or 50-ohm ground?plane antenna, this transmitter can give great good signal strength up to a distance of about 2 kilometres.

The circuit designed using transistor T1 (BF494) as a basic low-power variable frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals” power four to five times. Thus, 200-250 milliwatts of power is produced at the collector of transistor T2.

Here the coil winding specifications:
L1 ? 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
L2 ? 2 turns of 24 SWG wire near top end of L1. (Note: No core (i.e. air core) is used for the above coils)
L3 ? 7 turns of 24 SWG wire close wound with 3mm diameter air core.
L4 ? 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is utilized to set the centre frequency whereas potentiometer VR2 is used for power control. For humfree operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the centre frequency near 100 MHz.

For better comes about, build the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet.

PCB Design:

Easy Build FM Transmitter Circuit Notes

• L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it.
• C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
• You may want to bypass the battery with a .01uF capacitor.
• An antenna may not be required for operation.

Easy build FM transmitter circuit source: http://www.aaroncake.net/circuits/fmtrans.asp

Components List:

FM Transmitter Circuit Notes:
This circuit delivers an FM modulated signal having an output power of about 500mW. The input microphone pre-amplifier is designed close to a pair of 2N3904 transistors (Q1/Q2), and audio gain is restricted by the 5k preset trimmer potensiometer (trimpot).

The oscillator is actually a Colpitts stage, frequency of oscillation governed by the tank circuit built from two 5pF ceramic capacitors along with the L2 inductor. Frequency is about 100Mhz with values shown.

Audio modulation is fed in to the tank circuit via the 5p capacitor, the 10k resistor and 1N4002 controlling the quantity of modulation. The oscillator output is fed in to the 3.9uH inductor (L1) which will have a high impedance at RF frequencies.

The output stage operates as a ‘Class D’ amplifier, no direct bias is applied but the RF signal developed across the 3.9uH inductor is enough to drive this stage. The emitter resistor and 1k base resistor avoid instability and thermal runaway in this stage.

Some substitutes for the 2N3904 are: NTE123AP, 2N4401, BC547 (watch lead orientation), and so on. The 7001 (Q2/Q3) oddly enough also reference towards the NTE123AP. Even though the 2N3904 is really common and effortlessly available.