Radio Broadcast (Nov 1926-Apr 1927)

362 RADIO BROADCAST FEBRUARY, 1927 Mic roam meter fig. 2 A diagrammatical sketch showing how the instruments are arranged. The technique of measuring is explained in the article. The lead connecting from the Lecher wire to the thermocouple should be as short as practical, not over 1" long. The twisted leads may be several feet long so as to permit the microammeter to be placed on a table below the Lecher wire. The r.f. chokes are of the same construction as those used in the transmitter and receiver mA. This gives an input power of 1.3 watts. With this power the signals could be picked up and were sometimes as loud and clear as with a 210 tube, but were not very dependable. For more consistent results a 210 tube was used with 6 volts on the filament and 220 volts on the plate. The plate current varied between about 18 and 30 milliamperes, which corresponds to input powers of 4.4 and 6.5 watts respectively. Some experiments were tried with a plate voltage of 400 and 7.5 volts on the filament but there was no improvement in reception. The fact that results were practically the same with either 400 or 200 volts on the plate of the 210 tube seems to indicate that with 200 volts there is plenty of power available and that the problem lies in suitably utilizing it and preventing any great amount of absorption by surrounding objects. The problem of absorption is one that causes a great deal of trouble when the work must be confined to an apartment or small laboratory. As mentioned above, it is very much better to conduct the experiments in the open. Lecher wires. It is not necessary to use two wires, and in this work on very short wavelengths Mr. Hallberg has found that the single wire gives better results and that the readings are much more accurate than can be obtained using two wires. After some experimenting, apparatus was set up as illustrated in Fig. 1. Fig. 2 is a diagram of the set-up. The single No. 18 bare copper wire was tightly strung up between the two supports, A and B, the end of the wire at A being brought within about two inches of the oscillator inductance. The end of the wire could have been formed into an open loop to afford more pick-up but it was not found necessary to do this. One terminal for the heater of the thermocouple is hooked over the wire and the other end of the thermocouple is left open. The microammeter is connected to the output terminals of the thermocouple by a pair of twisted leads. In determining the wavelength the hooked wire is slid along the Lecher wire until a maximum deflection is obtained. This point is marked on the wire. The instrument is then slid further A • 14 reflector wires, 4!8" long, spacing 1'. Wavelength 3 meters. Back loop mainly due to reflection from moist stone wall. B 14 reflector wires, 4!4"long, spacing 1'. Wavelength 3 meters. Back loop mainly due to reflection from moist stone wall. C • 14 reflector wires, 4!6"long, spacing 1'. Wavelength 3 meters. Back loop smaller since reflector further from wall. D • 14 reflector wires, 4'-7"long, spacing 1', Wavelength 2.94 meters., E14 reflector wires, 47"long, spacing l1. Wavelength 3 meters. (Compare with C and D.) F 8 reflector wires, 47"long, spacing 2' Wavelength 2.96 meters. (Compare with CD and E.) G • 3 reflector wires, 4!9.5"long, spacing >£ wavelength. Wavelength 3 meters. H 5 reflector wires,4;9.5"long, spacing \ wavelength. Wavelength 3 meters. FIG. 3 Eight valuable polar curves reprinted from QST, showing the energy distribution from a transmitter when using different reflectors. The reflector wires were in all cases arranged in the form of a parabola.energy being fed to the antenna, located at the focus, by means of an r.f. transmission line reference to the transmitter, but the experiments were not carried any further. The best place to do experiments of this nature would be in the open country where there are not so many objects capable of energy absorption in the vicinity of the transmitter. The results mentioned above were obtained when a 210 type tube, with a plate voltage of about 220, was being used as an oscillator. The receiver was a portable affair with self-contained batteries. Some care is necessary in selecting the tube for the receiver to be sure to obtain one with a sufficiently low inter-electrode capacity. In the experiments which Mr. Hallberg made, he found the Magnavox tube especially suitable because its grid-plate capacity was in general very much lower than that of the other tubes with which he experimented. A small antenna is used on the transmitter and the construction of this antenna will be described in detail further on in the article. No counterpoise is used on the transmitter. The receiver uses neither antenna nor ground, although they may be used if desired. Various types of tubes were used in the transmitter with various plate voltages. With a 201-A best results were obtained using a plate voltage of 130 and a plate current of about 10 In ultra short-wave work of this sort it is almost essential that the receiver and transmitter be calibrated together since very slight changes in inductance or capacity will make it impossible to tune the two sets to the same frequency. In one test we had tuned-in the signal at about 50 on the condenser dial of the receiver, but, on shortening the grid inductance fth of an inch it was not possible to pick up the signals. In measuring the wavelength, Mr. Hallberg used a special form of Lecher wire which is very interesting. Perhaps all of our readers are not familiar with the Lecher wire. Lecher wires usually consist of two parallel bare copper wires spaced by about four inches and, at one end, the two wires terminate in a small helix which is coupled to the oscillator. The other ends of the Lecher wires are left open. When energy is induced in these wires, it creates what are termed "standing waves," similar to the effect that would be produced if' a rope were to be attached to some fixed support and the other end moved up and down at a certain definite rate. When the rate at which energy is given to the rope is correct, waves start to run along the rope and finally there are created a series of stationary waves similar to those induced electrically in ..Hollow Brass Tubes-. FIG. 4 An ultra short-wave transmitter. The two adjustable loops are the inductors. They are made of hollow brass tubing arranged so that one piece can slide within the other. Details for making the r.f. chokes are given in the text. Variable condenser Ci has a maximum value of 0.0002 mfd.