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7MHz
CW/AM QRP transmitter
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circuit of a 7MHz CW/AM QRP transmitter described here
can be used to transmit either CW or audio frequency modulated
signal over a 7MHz carrier. The carrier frequency oscillator
is crystal controlled using 7MHz crystal in its fundamental
mode. The tank circuit comprises a shortwave oscillator
coil which can be tuned to 7MHz frequency with the help
of ½J gang capacitor VC1. Transistor T2 (with identical
tank circuit connected at its collector as in case of
transistor T1) serves as a power amplifier. The RF output
from oscillator stage is inductively coupled to the power
amplifier stage. The output from power amplifier is routed
via capacitor C3 and inductor L3 to a half-wave dipole
using a 75-ohm coaxial cable. ½J gang capacitor VC3 along
with inductor L3 forms an antenna tuning and matching
network between the output of power amplifier stage and
coaxial transmission line for maximum power transfer.
Suitable heatsink should be used for transistor T2. Tuning
adjustments may be accomplished using a 6-volt torch bulb.
Connect the bulb to the collector of transistor T1 first
through a coupling capacitor and tune ½J gang VC1 for
maximum brilliance. (Note: the bulb would light according
to intensity of RF energy.) Same procedure may be repeated
for power amplifier stage and antenna tuning network for
ensuring maximum power transfer.For CW operation, switch
S1 is to be kept on for bypassing the audio driver transformer
and morse key is used for on/off-type modulation. CW would
be generated during key depressions. For AF modulation,
morse key points should be closed and switch S1 should
be flipped to ‘off’ position. Any suitable mic. amplifier
may be used to feed audio input to the audio driver transformer
XI. (For transformer X1 you may use the transistor-radio
type AF driver transformer.) |
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