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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. |
