20M, 4W QRP TRANSMITTER

Using this small yet powerful 4W transmitter Hams (licenced amateur radio operators) can transmit Morse code signals to long distances in 20-metre band. Morse code communication with European and neighbouring countries is possible. The transmitter (refer Fig. 1) comprises an oscillator, driver, and power amplifier. The oscillator is crystal-controlled. For this, an inexpensive and readily available crystal with fundamental frequency of 14.314 MHz is used with the horizontal dipole antenna oscillator. The oscillator delivers power of about 200 milliwatts. The next stage is a class-C driver that delivers power of nearly 1 watt. The final stage is a class-C power amplifier wired around transistor BD139, which delivers power of 4 to 5 watts.

After assembling the circuit, connect a 12V, 5W bulb across the antenna’s terminals. Apply regulated 12-15V DC to the circuit. Adjust gang condenser’s knob until the bulb glows, which indicates that the

Fig. 2: Connection of the transmitter to a horizontal dipole antenna

transmitter is okay. (Caution. Don’t switch on the transmitter without an output load.) After checking with bulb as load, the transmitter can be connected to a horizontal dipole antenna via 75-ohm coaxial cable as shown in Fig. 2. Each arm of the dipole antenna is about 5 metre long. The correct length L (in metres) of single pole of the antenna can be calculated using the following relationship: L = 71.6/ frequency (MHz) Winding details of coils are as follows: L1: 9 turns of 24 SWG insulated copper wire over 8mm dia. oscillator coil former with ferrite bead L2: 3 turns of 24 SWG insulated copper wire over coil L1 L3: 9 turns of 24 SWG insulated copper wire over 8mm dia. former with movable ferrite bead L4: 14 turns of 20 SWG insulated copper wire over 1.5cm dia. PVC former. (The coil is to be tapped from 9th to 10th turn from the top end.) RFC: 12 turns of 36 SWG insulated copper wire using TV balun core Warning. Use of this transmitter without Ham licence is illegal, hence only Ham licence holders should assemble this project.

MULTIBAND CW TRANSMITTER

A radio frequency oscillator is at the heart of all radio transmitters and receivers. It generates high frequency oscillations, which are known as carrier waves. Here’s a continuous wave (CW) transmitter for transmitting Morse code signals in the shortwave band (see Fig. 1). It is basically a variable frequency oscillator (VFO) whose frequency can be varied from 5.2 MHz to 15 MHz. The signal can be received in the shortwave band by any radio receiver. The circuit works off a 9V battery. Connect the Morse key (S1) across capacitor C5 as shown in the figure. Attach a telescopic antenna (capable of transmitting over a short distance) at the output t e r m i n a l .

The coil and gang capacitor C2 form the tank circuit. The coil (L) has a total of 60 t u r n s . Winding details are given in Fig. 2. Tappings on the coil allow selection of the r e q u i r e d band. The frequency can be varied using C2 (main tuning). On reducing turns of the coil (using selector switch S2), the oscillator’s frequency increases because frequency is inversely proportional to inductance. Capacitor C1 couples the signal from the tank circuit to the base of transistor

T1 (2N2222). Transistor T1 provides the required positive feedback for oscillation and transistor T2 (BC547) functions as the emitter follower. The output is taken from the emitter of T2. For stable oscillations, use a polystyrene capacitor as C1. All other capacitors may be ceramic disk type. Enclose the circuit in a metal box for better shielding.