Long Range FM Transmitter Circuit (2km)

This long range FM transmitter circuit (2km) works like small radio station in one box.

We can put music or talk in microphone it sends sound far away in air more far than normal bluetooth speaker.

It is good for many thing like small community radio, wireless speaker in big garden or playing music for all people near our home.

But in some place but maybe one may need license to use this.

So better check the rules in their area before using it.

Circuit Working:

Parts List:

This long range FM transmitter have four parts for sending radio signal.

First part is VHF oscillator with transistor BF494 T1.

After that signal goes to small amplifier using BF200 T2, then to driver using 2N2219 T3 and last big amplifier with 2N3866 T4.

Microphone which is small condenser type connect at the start of oscillator.

When we talk in mic it makes FM signal at T1 output.

Signal goes stronger by preamplifier and a driver if required we can also use 2N5109 instead of 2N2219.

Preamplifier work like tuned class A amplifier and driver works as class C type.

Then signal goes to power amplifier class C and send out from antenna 50 ohm dipole or ground plane type.

Transistor 2N3866 need heat sink to not get hot.

We can also use 2N4427 for better result on 12V where it gives 1 watt RF power.

There is small part called VC1 near coil L1 and adjust this to set at FM frequency between 88 to 108 MHz.

Also adjust VC2 to VC7 to get best signal and more range.

Oscillator get stable 9V power from 7809 regulator IC so frequency does not change if voltage goes up or down.

We can run whole circuit using 12V battery.

Make this circuit on general PCB.

Antenna must be fixed well for long range.

Coils L1 to L5 made by 20 SWG copper wire on 8mm air core.

They have 4, 6, 6, 5 and 7 turns.

How to Build:

To build a Long Range FM Transmitter Circuit we need to follow the below mentioned steps:

Circuit Assembly:

• Look at circuit diagram and put all parts on PCB board.
• First solder small parts like resistors, capacitors and other small things.
• Then fix transistors BF494, BF200, 2N2219 and 2N3866 in right place on PCB.
• Put condenser mic at input of oscillator transistor BF494.
• Connect coils L1 to L5 same as given and count number of turns, wire size, etc.

Power Supply and Regulation:

• Give power from 9V or 12V battery.
• Use 78C09 regulator IC to make stable 9V for oscillator.

Testing and Adjustment:

• Slowly adjust VC1 with coil L1 to make frequency between 88 to 108 MHz.
• Also adjust VC2 to VC7 to get strong signal and long range.

Final Step:

• Fix heat sink to transistor 2N3866 or 2N4427 so it does not get hot.
• Install antenna properly and use 50 ohm dipole or ground plane type for more range.

Note:

• This transmitter is only for learning and testing.
• Do not use with outdoor antenna which maybe not legal in some place.
• Be careful while working with this type of circuits.
• Check wire side polarity, no short circuits and follow safety rules when soldering or touching the circuit parts.

Conclusion:

This long range FM transmitter circuit is made to send FM signal very far or even many kilometers.

It give strong power, clear sound, good antenna, stable frequency and follows radio rules.

To make this circuit we need to build carefully and adjust it well so it works good and does not break any law.

References:

Datasheet IC 7809

Datasheet BF494

Datasheet BF200

Datasheet 2N2219

Datasheet 2N3866