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for circuit diagram click here
The circuit for a powerful AM transmitter using ceramic resonator/filter of 3.587 MHz is presented here. Resonators/filters
of other frequencies such as 5.5 MHz, 7 MHz and 10.7 MHz may also be used. Use of different frequency filters/resonators will
involve corresponding variation in the value of inductor used in the tank circuit of oscillator connected at the collector
of transistor T1.
The AF input for modulation is inserted in series with emitter of transistor T1 (and resistor R4) using a transistor radio
type audio driver transformer as shown in the circuit. Modulated RF output is developed across the tank circuit which can
be tuned to resonance frequency of the filter/resonator with the help of gang condenser C7. The next two stages formed using
low-noise RF transistors BF495 are, in fact, connected in parallel for amplification of modulated signal coupled from collector
of transistor T1 to bases of transistors T2 and T3. The combined output from collectors of T2 and T3 is fed to antenna via
100pF capacitor C4.
The circuit can be easily assembled on a general-purpose PCB. The range of the transmitter is expected to be one to two
kilometers. The circuit requires regulated 9-volt power supply for its operation. Note: Dotted lined indicates additional
connection if a 3-pin filter is used in place.
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