THE GACW40T

A CW TRANSMITTER FOR THE BEGINER

Guillermo H. Necco LW3DYL

lw3dyl@yahoo.com.ar

This simple CW transmitter works on the 40 mts. (7MHz) band with an output power of 2 Watts ( more than enough for local contacts and propagation allowing some DX ). The assembling may be done using copper clad cut with a hobby knife or etched with the usual products.

It consist of a crystal oscillator at 10 MHz, connected by a BD558 PNP transistor to a telegraph key. When we act the key the base of the transistor is put at ground potential and the transistor conducts. This makes the crystal oscillator start generating a 10 MHz signal at the rhythm of the manipulation and free of irregularities because the crystal oscillator starts immediately and is very stable thus avoiding "clicks" and "chirps" in the output signal.

The signal of the oscillator is connected to Q2 which is a BC548 transistor configured as phase inverter that feeds a simple diode mixer, this mixes the signal from the crystal with the one from the VFO that works in the range of 2.998 MHz to 2.960. In this case we use the subtraction, so 10 - 2.96 = 7.040 MHz and 10 - 2.998 = 7.002 MHz.

By using the phase inverter we avoid the use of a toroid for the coupling between the crystal oscillator and the mixer. The output of the mixer is sent to an amplifier tuned at 7 MHz, based on a transistor BC548, that cleans out the signal of the undesired products of the mix. Then the signal is sent to another amplifier to give it enough amplitude and make it as pure as possible. The output of this last amplifier is coupled to an emitter follower that provides the low impedance required to feed the output stage.

The power stage was designed as simple as possible to avoid complications and unnecessary adjustments. At the input there is a transistor BD139 working in class AB1; that is, with a little input polarization to make it more sensible and avoid distortions. Its output is coupled through a LC network to the bases of another two BD139 working in class C, this means that they are not on all the time there is signal and this is reconstructed at the output filter. The classic explanation of the class C is as follows: Lets suppose a swing. We don't have to push and hold it all the way to get it moving. Its only necessary to give it a push and it goes forth and back and when it arrives we give it another push an so on. We can say that the output transistors receive little pulses at a "rhythm" of 7 MHz and they are amplified at its collector, then by the smoothing effect of the output filter the signal is made again sinusoidal to be send to the antenna.

This kind of configuration makes that the yield is maximized while the dissipation of the transistors is at minimum. We also get a collector impedance very near to 50 ohms, which allow coupling the antenna with a simple LC filter.

The VFO is identical to the one at the receiver and the coils are similar.

14 uHy = 65 turns of 0.5 mm2 wire (AWG 20) over a 2.5 cm3 syringe (diameter 1.25 cm, 0.5 inch), tap at turn nbr. 10 from the ground.

12.5 uHy = 60 turns of the same wire over the same form.

1.1 uHy = 13 turns of 0.70 mm2 (AWG 19) wire air-wound using as form a 10mm (25/64") drill bit.

Reference:

With T37-61

14.2 uHy = 16 turns AWG 24

12.0 uHy = 15 turns AWG24

With T37-41

14.2 = 22 turns AWG24

12 uHy = 20 turns AWG24

English translation made by Juan Ferrari KG4FSN GACW # 599