Radio Broadcast (May 1928-Apr 1929)

AJi EAST SET TO BUILD Little explaining need be done if this illustration iscompared with Figs, i and 2. The "loop tap" indicated by the pencil is not used in this set but is shown merely to indicate how this type of tap is made. In this coil, the tap employed is near the detector and cannot be seen in this photograph. A Qood Crystal Receiver for the Beginner By KEITH HENNEY Director of the Laboratory THE receiver illustrated above is simple to built, costs but little, delivers high-quality signals, and is reasonably selective. It provides an excellent receiver for the beginner to try his hand at — after having put together this array of apparatus he has the whole world of radio home construction at his feet. There are many people who would like to listen to local broadcasting without a great expenditure of cash, either for the radio set or for its upkeep. This set will provide good signals from broadcasting stations not too far away at small cost. The receiver consists of a tuner, a crystal detector, an amplifier and a pair of head phones. Anyone with a soldering iron and a pair of pliers can assemble it in an hour. The disadvantages of the receiver are few: it is not selective enough to distinguish between stations operating within 30 kc. of each other; it will not receive the "coast"; but for reception from locals or powerful stations up to 100 miles away it is excellent. Because of the special electrical characteristics of the crystal — in this case, a piece of carborundum— it is possible to use a high-ratio audiofrequency amplifying transformer. The step-up 1 as between primary and secondary circuits in this case is 6:1. Any present-day transformer of 1 such high ratio, used with a vacuum tube detector circuit would give comparatively poor quality — the low notes especially would suffer. The coil can be made at home, or any commercial coil may be employed provided it has the proper number of turns to cover the broadcasting band with the condenser used and provided it is not too difficult to solder a few taps on it. The condenser may be any assembly that happens to be in the builder's junk box. Naturally, the better the coil and the condenser the better the final result. A good way to wind the coil is to place a ' rat tail file, a pencil or a piece of dowel rod through the spool containing the wire which is to be wound on the coil form and to place the spool on the floor. Two holes are drilled in the ends of the coil form and one end of the wire from the spoil is looped through one of these holes. A weight, one's feet for example, is placed on the rat tail file, and the wire wound on the form. The purpose of the weight is to keep the wire taut so that it goes on the form tight enough that it will not fall apart under temperature or humidity changes. When the proper number of turns has been wound, the wire is cut and the end looped through the second hole in the coil form. The diagram, Fig. 1, indicates the exact number of turns recommended for this receiver. Taps should be made at three places, dividing the coil into four equal parts. These can be made by twisting a loop of wire when it is wound on the form or by soldering short lengths of wire to places where the insulation has been scraped from the wire after winding. The loop method is shown in the photograph above. Strongest signals will be obtained with the antenna wire attached to one end of the coil and the ground to the other. At the same time the selectivity will be poorest. To improve the selectivity, the antenna may be tapped on to the coil as shown in the accompanying illustration, or an additional winding of about 10-20 turns may be wound about the larger coil and the antenna and ground attached to it. The antenna should be about 75 feet long. Still greater selectivity may be obtained by tapping the detector circuit to only a part of the coil. Note that this was done in this receiver. See Fig. 1. This is because the crystal is a low impedance detector and increases the effective resistance of the tuned circuit consisting of the coil and condenser. When tapped across part of the coil this increase in resistance, and resulting decrease in selectivity, is not so marked. The arrangement used in the Laboratory is shown in Fig. 2. In the Laboratory, signals freer from outside noise or "interference" were secured by not grounding the crystal circuit. This may not be the case generally and for this reason the constructor should try grounding the circuit as shown in the dotted lines. With this receiver, tested in our Laboratory, and using the tapped arrangement, it is possible to hear wjz 30 miles away when weaf is operating 8 miles away although with bad interference. With a wave-trap tuned to weaf, considerable improvement in wjz's signals is noted, wjz cannot be heard at all if the detector is connected across the entire coil. The parts actually used in the set photographed follow, and any similar apparatus may be used. It is even possible to hear signals with a crystal, home assembled, such as galena or silicon — a very cheap detector. The home constructor is advised against such procedure. The Carborundum unit is recommended because it is a compact, stable, and sensitive unit, and because it is possible to use a biasing voltage on it to increase its sensitivity. The constructor may use a small flash-light cell as the biasing battery or he may use the voltage obtained from dry cells used to light the filament of the amplifier tube. Using the extra cell is simpler, but has the disadvantage that it is an additional unit which needs replacement. If the dry cells are used, the dotted lines in the diagrams should be followed, or as in the insert in Fig. 2, where the voltage drop across the rheostat is used. It does not matter where the various parts are located on the base board. One arrangement is shown in the photograph. The picture wiring diagram, Fig. 1, shows where the wires go. The signals from this receiver may be amplified by any of the power amplifier units now readily obtainable. If a two-stage amplifier is used, such as is made by Samson, Amertran, SilverMarshall or others, the output of the detector may be used and the amplifier tube, transformer, and accessory apparatus shown in this model may be eliminated. (Dotted lines in Fig. 2) 97