Radio broadcast .. (1922-30)

174 Radio Broadcast that the rotor and stator arc connected, leaving only two open wires from each variometer. These variometers m.iv be added in almost any non-regenerative set, by connecting individual variometers in place of the rotor and slator coils indicated in the tickler hook-up (i. e., one variometer in the grid circuit, and one in series with the telephone receivers). Tuning 'with a variometer set requires considerable practice, but once the operator becomes accustomed to the peculiarities of his apparatus, the remarkable reception will repay him for his efforts. The grid variometer will require certain definite settings for different wavelengths (which must be determined by trial), and should be first set on the wave adjustment for the signal it is desired to receive. The plate variometer is set at any non-oscillating position, and the station tuned by varying the antenna condenser or inductance. When the station is tuned in, generation is controlled by manipulating the plate variometer. The final adjustment is a very delicate tuning of the grid variometer. FIG. 3 A method for eliminating powerreceiving sets line noises in * S. W . B., Davenport, Iowa is bothered by neighboring arclight circuits, reception being practically impossible after sundown. "Every evening, when the arc circuit is turned on, we get a continued roar in the receivers that sounds as though we were in the power house standing alongside one of the generators. This trouble is not only at our place, but affects an area of about one hundred r.ity blocks. . . . The current which is supplying these arcs is direct current, and each circuit has about 80 lights on it." THIS problem has been a bugbear of radio experimenters since the early days of ^wireless, and many more or less successful solutions riJcve been offered. Arc circuit interference is reducible, for tif-e purpose of dealing with.it systematically, to one of two causes: audio frequency variation of the supply current; oY radio frequency. This last, however is often modulated ..•! a sound frequency. If the interference is in no degree tunable, it is probably interference of the first mentioned class. An audiofrequency disturbance may be due to many causes — generator hum, other line fluctuations, leakage in wet weather, arc sputtering, and sometimes, owing to certain values of capacity and inductance, the arc oscillating at audio frequency. Radio-frequency variations are slightly tunable, but will probably force oscillations at any wavelength due to the proximity of the line. If the annoyance comes under the first classification, (audio frequency) it will affect only single-circuit sets, and the solution is obvious. An inductively coupled receiver, such as a variometer short wave set, should be installed. Here, the inductance in the primary and secondary of the coupler (which is virtually a radio frequency transformer) is not sufficiently high to effect a transfer of pure audio frequency energy. The simplest, but not always successful, solution to radio frequency interference is to vary the position of the aerial in relation to the arc line, making, if possible a right angle. In some cases, the erection of a fan antenna has reduced the annoyance to a negligible hum. A logical, and in several instances successful system is the installation of a wave trap, a method, of course, only applicable when the interfering wave is considerably above or below the wave on which reception is desired. A wave trap is a circuit in resonance with the disturbing signal and in inductive relation to the antenna, which absorbs the undesired frequency, while not appreciably affecting signals on other waves. The most successful systems employ the principle of "bucking" the disturbing oscillations by similar oscillations in the opposite direction. It is known that two sets oi oscillations of the same frequency, but differing in phase by 1800, that is, one reaching its positive peak at the moment the other attains its negative peak, will nullify each other if they are approximately the same strength and vibrating in a common circuit. This effect may be occasionally secured by erecting a counterpoise, one end of which is free to swing so that the relative position of the counterpoise to the arc line may be varied. This system, however, is not so effective as a separate bucking circuit shown in Fig. 3. X and Y are respectively the secondary and primary of a variocoupler. The secondary is connected between the ground and a wire running as far as feasible (several hundred feet if convenient) parallel and near to the supply line. The primary is in the antenna circuit of the receiver. Arrow A indicates the momentary direction of the current in the arc line; B and C the current simultaneously induced in the parallel wire and the aerial. The dotted arrow shows the direction of the current induced from X to Y (provided X or Y is connected in the right direction). This last indicated current, by varying the coupling between X and Y, can be made to nullify the interfereing current, designated by arrow C. As was suggested, it may be necessary to reverse the connections to one of the coils, X and Y, for, if connected improperly, the disturbance will be magnified. The amount of winding on Y should be kept at as few turns as is compatible with the elimination of the interference, so as to increase the wave of the receiving set as little as possible.