4 Watt FM transmitter

Posted on Sep 14, 2012

This is a small but quite powerful FM transmitter having three RF stages incorporating an audio preamplifier for better modulation. t has an output power of 4 Watts and works off 12-18 VDC which makes it easily portable. It is the ideal project for the beginner who wishes to get started in the fascinating world of FM broadcasting and wants a good basic circuit to experiment with. The circuit of the preamplifier is very simple and is built around TR4. The input sensitivity of the stage is adjustable in order to make it possible to use the transmitter with different input signals and depends upon the setting of VR1. As it is the transmitter can be modulated directly with a piezoelectric microphone, a small cassette recorder etc. It is of course possible to use an audio mixer in the input for more professional results.

4 Watt FM transmitter
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4 Watt FM transmitter - image 1
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As it has already been mentioned the transmitted signal is Frequency Modulated (FM) which means that the carrier?s amplitude stays constant and its frequency varies according to the amplitude variations of the audio signal. When the input signal?s amplitude increases (i.e. during the positive half cycles) the frequency of the carrier increases too, on the other hand when the input signal decreases in amplitude (negative half-cycle or no signal) the carrier frequency decreases accordingly. In figure 1 you can see a graphic representation of Frequency Modulation as it would appear on an oscilloscope screen, together with the modulating AF signal. The output frequency the transmitter is adjustable from 88 to 108 MHz which is the FM band that is used for radio broadcasting. The circuit as we have already mentioned consists of four stages. Three RF stages and one audio preamplifier for the modulation. The first RF stage is an oscillator and is built around TR1. The frequency of the oscillator is controlled by the LC network L1-C15. C7 is there to ensure that the circuit continues oscillating and C8 adjusts the coupling between the oscillator and the next RF stage which is an amplifier. This is built around TR2 which operates in class C and is tuned by means of L2 and C9. The last RF stage is also an amplifier built around TR3 which operates in class C the input of which is tuned by means of C10 and L4. From the output of this...

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