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POWERFUL
AM RADIO TRANSMITTER
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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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