0,4 Watt FM transmitter


Posted on May 21, 2012

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. 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.


0,4 Watt FM transmitter
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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




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