High-Power Push-Pull FM Transmitter (TEL007E)

Features

Power supply voltage: 6 to 12 Vdc

Frequency range: 88 to 108 MHz

Number of transistors: 4

Range: several Miles

A high-power push-pull output stage using two transistors gives this project a performance range of up to several miles when used with an external antenna.

The output power also depends on the power supply voltage.

The reader can use supplies ranging from 6 to 12 V with this transmitter.

If dry cells are used for the 6 V version, they must be D or C types, as the current drain is high.

The circuit needs three adjustments to be made using three trimmer capacitors.

Procedures for making these adjustments can be found in the descriptions of other projects in this section.

The external modulation can come from various sources such as mixers, a high-impedance microphone, CD players, or others.

A mixer can be used to plug several signals into the transmitter if it is used in an experimental radio station.

The operational frequency is determined by the coils, and the reader has some options as shown below:

50 to 80 MHz

L1 6 turns

L2 4 + 4 turns enlaced with L1

L3 6 + 6 turns

L4 5 turns enlaced with L3

80 to 120 MHz

L1 4 turns

L2 3 + 3 turns enlaced with L1

L3 4 + 4 turns

L4 4 turns enlaced with L3

All the coils are wound around a pencil as reference.

The wire can be the AWG 20 to 24 (enameled or plastic covered).

Figure 1 – Schematic diagram of the transmitter.

Figure 1 shows the schematic diagram of the transmitter.

All the components are placed on a printed circuit board as shown in Fig. 2.

Figure 2 – PCB for the project

Pay special attention to the placement of the coils.

The transistors can be replaced by equivalents.

Q3 and Q4 must be mounted on small heat sinks if the circuit is powered from a 12 V supply.

The recommended trimmers capacitors can be porcelain or plastic types with capacitance ranges from 2-20 to 4-40 pF.

The small capacitors must be ceramic types.

The electrolytic capacitors are rated to voltages up to 16 V or as specified in the parts list.

If an external antenna is used, it must be wired to the circuit using a coaxial cable.

The trimmer potentiometer is used to adjust the modulation according to the audio signal source.