Easy FM transmitter 0,4 W

Technical specifications

Power supply: 12-14 V stab., 100 mA
RF power: 400 mW
Impedance: 50-75 ohm
Frequency range: 87,5-108 MHz
Modulation: wideband FM

Schematic diagram

Parts list

Capacitors:
C1, C2, C12 - 100 pF (ceramic)
C3, C5 - 0,22 uF (electrolytic)
C4 - 1,8 nF (plastic)
C13, C16, C17, C19 - 1 nF (ceramic)
C8, C9 - 10 pF (ceramic)
C10 - 47 pF (ceramic)
C11 - 8,2 pF (ceramic)
C14 - 60 pF trimmer
C15 - 35 pF trimmer
C18, C7 - 100 nF (ceramic)
C20 - 470 uF (electrolytic, 16 V)

Coils:
(All coils are free-standing air-core types, wound of 0,7 mm Cu wire, 6 mm internal diameter.)
L1 - 4,5 coils
L2 - 9,5 coils
L3 - 4,5 coils

Resistors:
R1, R2 - 10 k pot.
R3 - 33 k
R4, R7, R12 - 10 k
R5, R11 - 470
R6 - 27 k
R8 - 22 k
R9 - 270
R10 - 100

Misc.:
D1 - BB409 (BB109G, BBY31) varicap
T1 - BC547C (BC548C, BC547B)
T2 - BFR91A (BFR96)
T3 - BFR96

Antenna
Power supply
3,5 mm jack
CD player, computer
6 V / 0,1 A bulb

Assembly and PCB layout

The PCB is designed for assembling all the parts on the Cu circuit side, without drilling holes.

Tuning

Connect the 6 V / 0,1 A bulb to the output and use R1 to tune the right frequency. Maybe you might stretch coils of the L1. Then use C14 and C15 to adjust the highest power (the highest light of the bulb). Then you can connect antenna and audio signal. Adjust R2 until the audio sounds as loud as the other stations.

With good antenna (dipole placed outdoor and high) the transmitter has very good coverage range about 500 meters, the maximal coverage range is up to 4 km.

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