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1 watt fm transmitter

1W FM Transmitter Circuit

A very good 1 watt fm transmitter circuit, very easy to build circuit. It has 4 transistors, one is a very stable oscillator, followed by a buffer stage to prevent frequency variation when you adjust the transmitter. Next is a resonance stage and the final stage built with a minimum 1W transistor which must have a heatsink. You must use a LM7805 stabilizer for the oscillator diodes and one LM7809 for powering up the T1 oscillator stage. This will give you a very stable transmitter frequency.

1W FM Transmitter Circuit Diagram

1 watt fm transmitter

First build the oscillator stage and the buffer, power it up and trim the 10k linear potentiometer untill you can here a blank signal on your receiver. If you put a small piece of wire on the T2 emitter you can see that the cover range of the 2 stage transmitter is about 3 meter.

After you are sure that your oscillator+buffer stage are working properly, remove the power supply and continue building the T3 resonance stage. Connect the power supply and if you adjust the trimmer (variable capacitor) from T3 collector you can see how the fm transmitter power can be varied. This stage is very important for proper functionality of the entire 1 watt fm transmitter. You must adjust the trimmer for maximum power.

1 Watt fm transmitter adjustment

The final stage of the 1W fm transmitter is built with 2N4427 (recommended) or the transistors from the list. If you can’t find any, use a BD139 transistor but only for frequencies lower than 90 MHz. The output power will be lower but you get the idea. If you decide to use 2N2219 transistor for the final stage of the transmitter you must know that the output rf power will be 0.4W.

Adjust the last 2 trimmers for maximum output power in the antenna. Initially use 2 x 100 Ω 0.5W resistors in parallel at the RF output. Then connect this rf probe to the output and adjust all the 3 trimmers starting from T3 to output. You must adjust it to obtain the maximum multimeter indication. Then power it off, connect the antenna and make the final adjustments for maximum broadcasting coverage distance.

The oscillator and buffer stage must be enclosed in a 1 mm copper case, then do the same with the T3 and T4 stages.

Use a 12Vdc power supply to power up this fm broadcast circuit. T4 will have a current consumption of around 150 mA at full power output adjustments. The total current consumption of the entire 1 watt transmitter will be around 500 mA.

Components values:
T1 = T2 = T3 = BF199
T4 = 2N4427, BLX65, 2N3866, 2N3553

L1 = rf choke (20 turns, 0.2mm on ferrite core, 0.3mm)
L2 = 4 t, 0.7mm, 4mm
L3 = 6 t, 0.8mm, 6mm, at first turn from T3
L4 = 10 t, 0.2mm over ferrite core
L5 = 7 t, 0.8mm, 6mm
L6 = 4 t, 1mm, 8mm, 10mm long

This 1 watt fm transmitter has been tested and is working as mentioned in this article. If you use replacement components the performances can vary a lot.

53 Comments

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  • Knight

    Sir, I have some problems with this circuit.what is the diameter of ferrite core? What does mean(L=4t,1mm, 8mm, 10mm long). There are some components such as 100, 560. What are they?

  • Knight

    Sir, I have some problems with this circuit.

  • muhamad

    I wont the controler circute 4 repeater and the noise filter thanx

  • Polybus

    Sorry – newby here – I want to make sure I understand the specifications for the inductors properly.

    L4 = 10 t, 0.2mm over ferrite core
    L6 = 4 t, 1mm, 8mm, 10mm long

    L4 = 10 turns of 0.2mm wire over a ferrite core – what is the diameter of the ferrite core? Or doesn’t it matter?

    L6 = 4 turns of 1mm wire into a coil of diameter 8mm – What is 10mm long? Do we stretch out the 4 turns so the whole coil is 10mm long?

    Thanks for the help 🙂

    Polybus

  • Mick

    Come on guys, where can I get a Pcb for this project?

    • Star One

      If you want a PCB, just make one. Download Eagle CAD and re-create the schematic in that software. It will automatically make a board file, you arrange the parts, run the “auto-router” then whatever board house prints the board you need their ‘board design rules for eagle’.

      If you want to learn all this there are great series on Youtube that shows you how to use all this. http://www.youtube.com/watch?v=1AXwjZoyNno

      It’s extremely simple, which from the videos you will see. Have Eagle CAD open, while watching the video, and you’ll get it.

  • Mick

    How can I purchase the pcb?

    I like this article!

  • prashanth suvarna

    This is a good circuit. Points 2note is that u got2 use good hi frequency construction practices like assembling all similiar projects on pcb,s with a earthed back plane (appropriately isolated from the working points of course, short connections between each other components(Of course you have to orient the layout of the circuit so that the input&the Opt r not close2One another. It could send the whole circuit in2 oscillation.) Up2 1Watt u could also try out the very common BD115 quit freely available almost everywhere.(they r used commonly as Horizontal driver transistors in most Tv chassis. BFW16 is also a good choice ideal 4 this app.
    Best of luck…(:-)

  • saman

    1 watt with 9 volt regulate supply for final stage?”????????????????????????? only 9 volt??? I know the best supply for mentioned transistors is above 18 volt and u recommand only 9 volt ….?

  • Rony

    Hi Liren,

    You can use general zener diodes replace of varicap. A 20v zener diode operated at 1v to 13.8v exhibits a capacitance of typically 10pf to 2pf as the voltage is varied. I have even used this effect to make a 5.0MHz – 5.5MHz VCO for this synthesizer, just as an experiment

  • fabiano silva

    hi can someone help me if this transmitter can generate frequency within 88-108 MHz

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