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.

This the Good Quality FM transmitter for your stereo or any other amplifier gives you a pretty good signal strength up to a range of 500 metres having a power output of about 200 mW. This circuit can be operated with a 9V battery.

The audio-frequency modulation stage is constructed close to transistor BF494 (T1), that is wired as a VHF oscillator and modulates the audio signal present at the base. Working with preset VR1, you’ll be able to alter the audio signal level.

The VHF frequency is decided by coil L1 and variable capacitor VC1. Decrease the value of VR2 to obtain a higher power output.

The next stage is designed close to transistor BC548 (T2), which serves to be a Class-A power amplifier. This stage is inductively coupled towards the audio-frequency modulation stage. The antenna matching network contains variable capacitor VC2 and capacitor C9. Fine-tune VC2 for the optimum transmission of power or signal strength at the receiver.

For frequency stability, use a regulated DC power supply and house the transmitter inside of a metallic cabinet. For better antenna gain, utilize a telescopic antenna in place of the simple wire. Coils L1 and L2 are to be wound over the same air core such that windings for coil L2 start from the end point for coil L1.

Here the coil winding specification:

• L1: 5 turns of 24 SWG wire closely wound over a 5mm diameter air core
• L2: 2 turns of 24 SWG wire closely wound over the 5mm diameter air core
• L3: 7 turns of 24 SWG wire closely wound over a 4mm diameter air core
• L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core

This good quality 500M FM transmitter circuit absolutely works fine since it already tested.

This is the FM transmitter circuit which apply 4 radio frequency stages, that are a VHF oscillator designed around transistor BF494 (T1), a preamplifier designed around transistor BF200 (T2), a driver designed around transistor 2N2219 (T3) and also a power amplifier designed around transistor 2N3866 (T4). A condenser microphone is wired at the input of the oscillator.

The Operational of this FM transmitter circuit is very simple. At the time you speak close to the microphone, frequency-modulated signals are produced at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and also the pre-driver stage. You’ll be able to also use transistor 2N5109 as a placement of 2N2219.

The preamplifier works as a tuned class-A RF amplifier while the driver works as a class-C amplifier. Signals are finally fed towards the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna.

Regulator IC 78C09 gives you stable 9V supply to the oscillator, so variation in the supply voltage is not going to have an impact on the frequency produced. You are able to also use a 12V battery to power the circuit.

Use a heat-sink with transistor 2N3866 for heat dissipation. Alter the trimmer VC1 connected across L1 to produce frequency within 88-108 MHz with care. Also alter trimmers VC2 through VC7 to obtain maximum output at maximum assortment.

Assemble the circuit on a generalpurpose PCB. Set up the antenna correctly for optimum range. Coils L1 through L5 are built with 20 SWG copper-enamelled wire wound over air-cores that have 8mm diameter. They will have 4, 6, 6, 5 and 7 turns of wire, respectively.

This 4 stage FM transmitter tested by Electronics For You mag and should be work. You can download this circuit in PDF version from the following link:

Four Stage FM Transmitter Circuit Project

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The coil L1 was consist of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on this prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work.

Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.

One final point, don’t hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations.