Chapter 26 - QRP Module Lay out

 

Amateur Radio Home brewers have successfully used many transistors in RF stages of low power transmitters operation on 40 meters and frequencies below to it - most of them manufactured for else purposes. Most popular of all such RF power amplifier transistors was BD 139 introduced by late OM Vasanth 9VU2VWN). What I used in my 12 V 5 W 40 meter QRP final was D882. 5 watt was enough for me for an AM contact to almost all south Indian stations and Sri Lanka. Even though any QRP circuitry might look simple and easy o assemble, I earnestly advise that it is not easy to assemble and operate one without the help of an experienced home brewer in some critical points - especially tuning the final stage to an antenna.  In C-26/1 there is the circuit diagram of a QRP RF (final) which will give you 5 W at 12 Volts.

Before connecting the VFO to this RF amplifier, set VR1 at low positions and and set the idle current at TR2 at 100 mA. Also remember that TR2 requires a good heatsink. To extract maximum power at minimum expense, active components are used in parallel. This is most scene in valve equipment. In the case of transistors, at parallel configurations, the changing input out put impedances are to be checked along with setting the idle current to an optimum point. When used in parallel, solid components should necessarily be alike in all the features. In many Dx circuits I have seen transistors used at absolute parallel configuration. Push-pull configuration is another technique to increase the final power. 

The value of the resistors used at the base of the final transistors is to be fixed to make a quiescent current is between 5-10% of the collector current as base current at no input signal situation. Transformers in the tuned circuits are to be changed according to the frequency Band operated. RFC are built at a compromise value that fits all the frequency bands the transmitter handles. Applied voltage, circuit impedance, frequency of operation are the main factors to be considered here. The resistor/capacitor values change only if there is a change in the supply voltage. 

It is always nice to keep different different stages in a transmitter, separated in well shielded boxes if its own. It also is user-friendly if all the modules are arranged in a single chassis. A cabinet, stages layout for QRP home brewers are given in C-26/3. It includes AC/Batt. power supply circuit too. The details of the sockets also are not given because any coupling mechanism with strong contact arrangements are usable here. 

All the components used must be of maximum quality features to ensure best performance. The size of the box depends upon the size of the PC Boards and other hard components used along with. Also keep the size at a standard size. The present arrangement has given a AC/Batt change over facility too. 

Module Layout, Cabinet Design and Controls  - Jim (VU2JIM). If relay power supply is given through a slide switch at the back of the cabinet, while transmit/receive change over, this relay switch also needs to be changed. This is used as a safety switch. Most important thing is that the PCBs should not change the positions as shown in the lay out. Another important thing is that all wires used should be strong wires at minimum length. The VFO case should be strong Aluminium and in no way the power supply shall be added to the same chassis. Do it in a separate box. 

Chapter 25                                                            Chapter 27