Radio Broadcast (Nov 1926-Apr 1927)

FEBRUARY, 1927 ONE-METER RADIOPHONY 363 FIG. 5 A front view of the transmitter showing the antenna. along the wire until another point of maximum deflection is obtained. The distance between these two points is then measured, and this distance, in meters, is equal to one-half the wavelength. Therefore, the distance, in meters, between successive maximum deflections, multiplied by two, equals the wavelength at which the transmitter is oscillating. This method of wavelength determination is quite accurate, and Mr. Hallberg's experiments indicate that it gives the shape of the wave as well as its length. Soon after one begins work of this sort, it becomes evident that one of the most important things is to develop an efficient beam transmitter such as was described in the Bureau of Standards Scientific Paper No. 469. Mr. F. C. Jones, of 6 ajf-6 xm, in his article in the May, 1925, 057", described a reflector which he used on a wavelength of about three meters. Several polar diagrams were given showing the reflection with different numbers of wires, spaced various distances apart. These diagrams are valuable, and are reproduced here in Fig. 3. The curves are practically self-explanatory. In A and B there is a considerable back loop, which is due to reflection from a moist stone wall located near the reflector system. These curves show very well that the closer the wires are together, the sharper will be the beam, and also the greater the back leakage. The reflectors were arranged in the form of a parabola and their effect, as can be seen from the curves, is similar to that which is obtained through the use of a reflector in an automobile head light, for the purpose of directing the light along a certain path. The antenna is placed at the focus of the parabola. The reflector wires are one-half wavelength long minus a small amount due to the loading effect of the other nearby wires. For 3 meters, the wires may be 1.5 meters long, for work on the second harmonic. They may also be made 3 meters long if the fundamental is to be used. The antenna should be of the same length, with a single turn at the center to be used for coupling purposes. These curves are given because of their value to the experienced experimenter and will be especially valuable to those who desire to work using such a reflector system fed through an r. f. feeder line. With such reflectors, the received energy can be increased hundreds of times without any change in the input to the transmitter. If an intensive study is to be made, the reflector and antenna systems should be arranged so that many different combinations and sizes might be set up. We hope to do some of this in the Laboratory and the results will be published in a future article. However, a great deal of excellent experimental work can be carried on by merely using a simple antenna, as described in this article. The designing of a reflector system will be an interesting job after the simple antenna system has been brought up to its maximum efficiency. WEAF RE BROADCAST ON I METER THE tests and experiments that Mr. Hallberg has done to date using a simple antenna have proven very satisfactory, and the editors of circuit and the other in the plate circuit, are made of hollow brass tubing so arranged that one can slide within the other and in this way the length varied. Fig. 6 shows very well how simple the construction of this transmitter can be made. Amateurs will recognize from the schematic diagram, Fig. 4, the form of modulation as being similar to the so-called "loop modulation," in which a single turn of wire in series with the microphone is placed close to the transmitter inductance. Talking into the microphone causes the amount of power absorbed by the loop to vary in accordance with the sound waves. Fig. 8 illustrates the arrangement of apparatus in the self contained portable receiver, and Fig. 9 is the circuit diagram. The choke coils for use in the receiver are constructed in exactly the same manner as those used in the transmitter. Be sure that the turns are not too close together since it does not require much ■ capacity to produce an effective bypass for these high frequencies. The transmitter is not at all difficult to make up and practically all the information necessary can be obtained from the accompanying illustrations and diagrams. The grid and plate coils, or loops, should preferably be made up of copper tubing but satisfactory results will be obtained if a large size copper wire is used. The radio frequency choke coils can be seen in Figs. 4 and 6. They consist of about 10 turns of wire. They are easily made up by closely winding approximately this number of turns on a \" dowel Looking down on the transmitter. Radio Broadcast witnessed a very nice demonstration of rebroadcasting using a wavelength of 1 to \\ meters. The signals from weaf were picked up with an ordinary 610-kc. detector, and the telephones held near the microphone of the short-wave transmitter. The modulation in the transmitter was excellent and clear signals could be picked up with the 1 to i§-meter receiver without any difficulty. In these tests no antenna was used on the receiver, the pickup from the transmitter being merely by means of small loops that comprise the coils of the receiving set. On the transmitter an antenna is used, and a part of it can be seen in Fig. 5 accompanying this article. This antenna consists of two brass tubes which telescope one within the other. The length of the antenna is varied until it is brought into resonance with the oscillations in the tube. A great deal of development work on both the transmitter and receiver is necessary. It is possible that results will be better with the antenna coupled to the transmitter in some other manner. The range can be extended considerably if an antenna were to be used in conjunction with the receiver. The transmitter diagram is shown in Figs. 4, 5, and 6. Fig. 6 is a top view of the transmitter. The construction of the r. f. choke coils can be plainly seen. The two loops, one in the grid Photograph Note how simple is the construction Adjustable Loop Adjustable Loop |00000OOOO0Tj A. C. Supply FIG. 7 An a.c. operated transmitter. The signals from this transmitter can be picked up on an ordinary crystal. Ci and C> may be 0.001 mfd. condensers. C3 has a maximum capacity of 0.0002 mfd. The transformer supplies energy for the filament and plate circuits