QST/April 1916/The Sending Set

by Arthur A. Hebert

THE average amateur thinks that because he has a coil or transformer, condenser, gap and a helix, it is only necessary to make the connections, and, if a big noise is made in the gap, he must be radiating powerful currents in the antenna. It is, therefore, my intention to say a few words in this article on the different pieces of apparatus used in the sending set in the order of their importance.

THE CONDENSER: Known to many amateurs as Leyden Jar. In my opinion it is one of the most important parts of the apparatus, but it is seldom taken into consideration, so long has a discharge can be obtained, at the gap. A simple way to tell if there is enough Capacity (condensers in the circuit) is to watch the discharge at the gap,—if it is red or yellow (flaming) there are not enough condensers; if on the other hand the discharge is stringy and blue, it is quite certain there are too many condensers in the circuit. For use with a properly rated ¼ kilowatt transformer, which is a favorite with many, a condenser capacity of .0045 microfarads (three sections of Murdock’s Moulded Condensers will give approximately this capacity) should be found very satisfactory, and with 1¾ turns in the primary of the oscillation transformer of the “Pan Cake” type, which will be taken up later, will give a wave length of 200 meters in the closed circuit.

THE GAP: Be sure to keep the two electrodes clean in a plain gap, and in a rotary gap see that the teeth in the wheel are kept bright, as well as the electrodes (Emery Cloth or a fine file should be used for this work). In two cases which came to my knowledge the keeping of the gap clean increased the radiation by nearly ½ ampere. Do not be afraid to have the electrodes large enough—the less resistance in the path of the current the better the discharge—this is particularly so with a rotary gap. In connection with the rotary gap, bear in mind that the speed at which it revolves has a material effect on the oscillations radiated, and it is not the greatest speed that will give the best radiation. A simple method to determine the note or pitch of the wave emitted by a rotary gap is to hold lightly a piece of stiff paper over the revolving wheel touching it as though cleaning the teeth. If the speed of the Motor is varied, there will be different tones given out which can be compared to the frequency of the signals radiated and received by other stations. In making this test it will be found that better radiation takes place if the motor is so adjusted, that the number of revolutions is reduced just enough, so that the pitch of the note is two or three tones lower than when the motor runs at its highest speed. This applies particularly to motors whose speeds are two thousand revolutions or over, and having wheels not over six inches in diameters.

HELIX OR OSCILLATION TRANSFORMER: On account of the law, in order to obtain a pure wave, it is almost a necessity to use an oscillation transformer, although a plain helix can be used, but it requires very careful adjustment and very seldom can be made to radiate efficiently by the average experimenter and have a pure wave. One of the most satisfactory Oscillation Transformer for small sets, is the flat spiral type, commonly known as the “Pan Cake.” It can be constructed as follows: The primary winding is made of ten turns of flat brass or copper ribbon (copper preferably) placed edgewise on a Bakelite or Hard Rubber insulating support, spaced one-half inch. The ribbon is ⅝ of an inch wide by about 1-64 of an inch thick. When wound spirally, the inside diameter should be about 4½ inches, the outside diameter 14½ inches. For the Secondary winding use the same size ribbon and spacing, but have about 20 turns, which will enable one to experiment with waves of from 200 to 600 meters. The set will be more efficient, of course, if this Oscillation Transformer is constructed with just one-half the turns given above to work on the 200 meter wave. Either the primary or secondary winding should be so arranged that it can be drawn away from the other so that a certain amount of coupling can be obtained.

THE ANTENNA: Efficient radiation cannot be obtained if the antenna has a normal wave length of 300 to 400 meters, as great many have, and the gap circuit 200 meters. If such a condition exists the circuits are out of balance, and it is certain that no radiation will take place. The only remedy is to shorten the Antenna; for even though a condenser should be put in series with the ground, it is doubtful if the natural period of the Antenna could be reduced enough to work properly,—so that if one expects to obtain good results and keep within the law, it would be better to shorten the Antenna. In the event of shortening the Antenna, it becomes necessary to add more wires to give an effective capacity to take care of the current to be radiated. For instance: Supposing the Antenna to be 100 feet long, 4 wires spaced 3 feet apart and it is to be shortened to 50 feet. Good results in sending will be obtained if there are 8 wires spaced 18 inches apart. The Antenna must be considered in the light of a Condenser, similarly to the condenser charged by the transformer, good work cannot be expected if it does not take care of the charge imposed upon it.

 TUNING: There is only one thing to say:—Bunch all of the apparatus together, being sure that the leads in the closed circuit (condenser and gap circuit) are short, of large wire, and be sure that the wires of the low voltage side (house current of the transformer do not parallel the high voltage wires of the secondary side of the transformer, which will cause more trouble to the transformer and the house meter; and often set the house on fire.