|
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.
|
