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The transmitter was built on 1/8" thick double clad printed circuit board. Figure 2
shows the layout of the bottom of the
board. The top of the board was left with
the copper clad sheet on except in places where leads come through- This allows the copper on the top side to be used as a common ground connection that is easy to solder to anywhere on the top of the board. This double clad board is also very mechanically rigid and leaving the copper on provides a means of structural connections by soldering.
The only unusual component is the relay, K1. This is a sensitive reed relay that is ideal for this purpose. The current for the coil is only a few percent of the total current that the transmitter draws. Other types are available in the catalogs, and the only major requirements are compatible size, low coil current, and fast switching, Good quality contacts are also necessary to minimize clicks.
The board can be processed by the methods outlined in any good printed circuit board kit, or the builder may wish to use his own methods. An easy method is to use paint as the resist and obtain Ferric Chloride from a chemical supply house for the etch-ant. The paint can be applied with a small brush where the conductor is to remain. The parts to be used should be kept handy to check for size and position. Clean the copper before applying the resist by rubbing with steel wool. Keep the steel wool away from any electrical equipment. During etching, the etchant and board are placed in a shallow pan or tray. The solution must be kept warm to hasten the chemical reaction. This can be done with a heat lamp or very carefully on a stove. A beginner should start with a good kit and develop his own methods after a little experience.
The capacitors are soldered directly on to the coils and are placed wherever the shortest leads will result. Components are soldered in with Little trouble, and placement and layout are not critical.
The transistors must have heat sinks because of the power that must be dissipated. Since there is little room for the commercial type heat sinks with radial fins, homebrew heat sinks had to be made. I hey can be simply bent from copper as seen in the pictures. Use a copper that is stiff enough to retain a good grip on the transistor case. Silicon grease could be used to provide a good thermal contact, but this has not been necessary. If the transmitter is to be used in dark surroundings (inside a cabinet) the heat sinks can be painted black
WB6BIH QRP transmitter.
Fig. 2. Printed circuit board. Layout is not critical, this is merely a suggestion, to increase heat radiation. If they are in the sun, they will absorb the heat, of course.
The construction of this unit is not complete as shown in the pictures, ends will be added and the unit will be installed in a cabinet with batteries for portable operation. I'm sure that each builder has different ideas of what he wants to build.
The tests conducted on the final version showed tuneup to be non-critical on the oscillator. Oscillation will result with the slug of LI in almost any position, but there is a point that will produce maximum stability. Adjust the slug of LI while keying the final to check the loading effects of the final on oscillator stability. When the oscillator is oscillating the oscillator current will be considerably smaller than the non-oscillating condition. The oscillator can be tuned for a dip in collector current, but it will be necessary to detune from the dip to provide optimum stability. There is plenty of output from the oscillator to drive the final. The amount of drive to the final can be estimated by the amount of final collector current, Tune for maximum final collector current with good stability. The tuning of the final tank coil should produce a shaip peak when tuned through, resonance. The link coupling was designed experimentally for optimum power transfer to a 50£i load. Other final tank circuits provided more output but this circuit was easiest to tune and provided good selectivity against harmonics.
While tuning avoid prolonged key down periods because the transistors can become overheated in a few minutes. In a test, the input to the final was about three watts with output about one watt. This means that the transistor must dissipate the remaining two watts into the air, which is the maximum power dissipation rating of this transistor with a good heat sink. The use of CW allows the transistor to cool during key up periods but the transistor will still run quite warm,
A listen to the keying shows it to be quite sharp and free from chirp. A look at the scope pattern confirms this and the keying tends to be "clicky/? This has not been a serious problem, however, and the simplicity of design and freedom from chirp makes it an acceptable compromise.
This transmitter will make an amusing weekend project for the builder with average experience, and QRP is an increasingly popular sport. Forty meter CW is very popular with a frequency of 7040 kHz, most popular for QRP use. This is a portion of our hobby with unlimited challenge. Contacts of thousands of miles have been established with much less than a watt, and one million miles per watt can be achieved with microwatt transmitters over short distances. Anyone can buy a kilowatt and work the world, but doing it with milliwatts is a personal achievement that can provide real satisfaction -
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