The document describes a simple circuit for a multiband CW transmitter that can transmit Morse code signals in the shortwave band from 5.2 MHz to 15 MHz. The circuit uses a variable frequency oscillator with a coil and gang capacitor tank circuit to generate the radio frequency oscillations. Tappings on the coil allow selection of different frequencies by varying the inductance. Transistor T1 provides positive feedback for oscillation, while transistor T2 functions as an emitter follower to output the signal from a telescopic antenna for short distance transmission.

1. CIRCUIT IDEAS
MULTIBAND CW TRANSMITTER SAN
I THE
O
REJIMON G. VU2RGQ ceiver. The circuit works off a 9V the required positive feedback for
battery. oscillation and transistor T2 (BC547)
A
radio frequency oscillator is at Connect the Morse key (S1) across functions as the emitter follower.
the heart of all radio transmitters capacitor C5 as shown in the figure. At- The output is taken from the emitter of
and receivers. It generates high- tach a telescopic antenna (capable of T2.
transmitting
over a short
distance) at
the output
terminal.
The coil and
gang capaci-
tor C2 form
the tank cir-
cuit. The
coil (L) has
a total of 60
t u r n s .
Winding de-
tails are
given in Fig.
2. Tappings
on the coil
allow selec-
Fig. 1: Circuit of multiband CW transmitter tion of the
required
frequency oscillations, which are known band. The frequency can be varied using
as carrier waves. Here’s a continuous- C2 (main tuning).
wave (CW) transmitter for transmitting On reducing turns of the coil (using
Fig. 2: Details of the inductor
Morse code signals in the shortwave band selector switch S2), the oscillator’s
(see Fig. 1). It is basically a variable frequency increases because frequency is For stable oscillations, use a polysty-
frequency oscillator (VFO) whose fre- inversely proportional to inductance. rene capacitor as C1. All other capacitors
quency can be varied from 5.2 MHz to Capacitor C1 couples the signal from may be ceramic disk type. Enclose the
15 MHz. The signal can be received in the tank circuit to the base of transistor circuit in a metal box for better
the shortwave band by any radio re- T1 (2N2222). Transistor T1 provides shielding.
124 ELECTRONICS FOR YOU JANUARY 2004