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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