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    The wireless fm transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this wireless fm transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.

    The wireless transmitter circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the fm 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 generated at the collector of transistor T2.

    FM wireless transmitter circuit diagram

    fm transmitter circuit diagram

    For better results, assemble 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.
    Coil winding details are given below:

    • 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 4mm diameter air core.
    • L4 – 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

    Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the wireless fm 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 fundamental frequency near 100 MHz.

    This fm wireless transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a licence is illegal in most countries.

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    4 Responses to "Wireless FM Transmitter Circuit"

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    1. jaspartap singh says: on February 5, 2010 at 4:25 pm

      the best site ,i had ever seen for electronics students

      Like or Dislike: Thumb up 0 Thumb down 0

    2. Sir, is this circuit contain 2 modulation?

      The 1st stage modulation output from Transistor T1 seems does not link to T2. What is the purpose the circuit design to have L1 and L2 separately? Step up/step down?

      Like or Dislike: Thumb up 0 Thumb down 0

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    4. Sibaram Datta says: on November 4, 2012 at 6:38 pm

      Where can I purchase two 50pf trimmer that are used in output stage & what is the result if I use 7turns of 24swg enamelled wire without a ferrite bead. How can I understand that the output stage is functioning well & what is the voltage to be measured in antenna output terminal or 2n3866 transistors collector, base & emitter voltage. Please answer my query.

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