12 Watt FM transmitter

FM transmitters are discussed in this article is a modification of a series of FM Transmitters on the market (type S-083 from Saturn). S-083 series power only produces less than 1 Watt. With a little modification, simplification and addition of a booster will get the final power 12 Watt. S-083 series consists of 3 parts, yaknik the oscillator, the first level Buffer (Buffer 1) and second-level buffer (buffer 2), see in Figure 4 (Components marked with * are part of the modified)

Once tested, S-083 oscillator results are quite satisfactory, in addition to stable oscillator generates a strong signal. Therefore the oscillator is used without modification. The transistors in the first buffer (Buffer 1) the original use of C2053, C930 replaced by transistors, type at a price much cheaper and readily available in the market. For this purpose the value of R6 was changed to 10K, to provide an appropriate bias for the transistor C930.

33pF capacitor on the collector buffer is replaced with the trimmer is worth the 5-60pF C8 for easy tuning. The transistors in the level of the second buffer (Buffer 2) which was originally also replaced C710 with C930, and kapastor the collector is also replaced with the value 5-60 pF trimmer C11. At the second level given additional output inductor and capacitor that serves as an impedance adjustment, so that the output impedance of the buffer level more or less the end of 380 Ohm to 50 Ohm changed.

When assembling should not rush to work directly as a whole, but do each part so that any errors can be caught early.

The first part is done is the oscillator, after completion of assembly can be directly tested, by turning on the FM radio wave of the empty and set the radio volume so the sound is clearly audible hiss (it's easier if you have used radio tuning indicator). Rotate the core of the Koker (L1) to right up to the maximum. (By turning to right Koker oscillator frequency generated lower.) Turn on the FM transmitter, turn left up the core Koker hiss on FM radio tuning is lost or until the indicator lights. If the obtained signal is strong and stable, the oscillator of the transmitter has been working fine.

The next section can begin to be assembled, after assembled, connect the exciter circuit (Figure 4) as the diagram Figure 5. Turn on the power supply and a second rotary trimmer (C8 and C11) in the buffer alternately to obtain the greatest power and lowest SWR. If the exciter circuit works well, will get less than 0.25 Watt power.

Testing the block diagram of the exciter

Until this stage the exciter is ready to use. To get an even greater power can be added to the booster circuit 12 Watt, so it will increase the emission range up to 7-fold.

12 Watt booster circuit in Figure 6, consists of two levels of transistor amplifiers, each working on a class C, masomg respective input and output transistor amplifier circuit is given impedance adjustment.

Strengthening of the first level using transistor C1970. Strengthening the circuit has a 9.2 dB power gain (8 times), so from exciter power should be generated 0.25 W power 2 W. In fact the output of this first level of reinforcement produces only 1.75 Watt power, this is due to the loss of matching network circuit.

Strengthening of the second level using transistor C1971. This amplifier circuit has a 10dB power gain (10 times). So that the power of the first level of 1.75 W can be strengthened to 17.5 W. In fact, strengthening the power of the second level only reached 12.5 Watt. This is due to the loss of the matching circuit network and the limitations of the transistors C1971.

Because the heat generated both transistors is large enough then do not forget to attach sufficient cooling.

Once the booster is complete then assembled and tuned the booster can be tried, by assembling the exciter, booster, SWR & Power Meter and Dummy Load as Figure 7. Before the power supply is turned on, all the booster trimmer played on the middle position. Make sure the power supply is used to deliver currents of more than 3 Amperes. Observe the power meter. Power meter should show a few watts of power. Turn the trimmer on the booster starts from the input to obtain the greatest power. Repeat several times. Should be obtained up to 12W power.

Block diagram of the test booster

Measurements obtained from the current requirement is 2.2 amperes and the maximum power that can be achieved is 12.5 Watts. Power is too great of course will shorten the life of the final transistor level. It is recommended to lower the power output by lowering the supply voltage to 12 volts.