Radio Broadcast (May 1927-Apr 1928)

218 RADIO BROADCAST JANUARY, 1928 negative. Again, the key may be cut in on a through line, positive or negative either way, and these conditions are likely to be reversed many times in the course of a day through jack plugging operations at the switchboard. In most cases these clicks are unreadable; they occur at the break of the key, not on the closing of the circuit, but in the immediate vicinity of a telegraph office, radio interference may be noted from the local or sounder circuit. This is readable and can usually be eliminated by bridging the relay contacts of the local circuit with a one-half microfarad condenser without any series resistance. Occasionally, complications enter into the situation, making necessary the use of full surge traps. An obstinate case of key clicking was cleared up only after applying the surge trap described in Fig. i . Another difficult case, in connection with a set of repeaters, required choke coils as described in Fig. 2 in each line and 0.5-mfd. condensers, series arrangement midpoint grounded, across the repeater contacts. Duplex and quad circuits require individual treatment; much experimental work is required before standard eliminators can be recommended for these and the various types of stock tickers in daily operation. RADIO RECEIVERS MUCH of the so-called inductive interference has its origin in the broadcast listener's own set. It might be due to any one of the following causes: (1.) Loose antenna or ground connections will give rise to serious clicking noises when shaken by wind or other agency. (2.) Loose or corroded connections at the A battery will cause flickering of the tubes and consequent clicking. (3.) Internal defects in any one cell will cause harsh grating noises. This can be checked with a telephone transformer and pair of headphones, the secondary of the transformer being connected to the battery and primary to the phones. (4.) Defective B batteries cause a "chirping" or clicking interference. It doesn't follow that because a B battery registers full voltage, 22§ or 45, that it is in good condition. One high-resistance cell of the many incorporated in this battery can render the battery unfit for use although on open circuit it reads up to full strength. (5.) Oxidized contacts of the tube prongs or spring contacts of the sockets are fruitful sources of trouble. These should be scraped occasionally and the spring strips pulled up gently to insure that good contact is being made. (6.) Broken lead-in wires cause poor reception and clicking noises. Naturally, a lead-in wire must be insulated where it enters the building but the practice of using covered wire all the way down from the horizontal portion of the antenna is not to be recommended. From swinging in the wind, this wire often breaks inside the insulation but is held up by the fabric so it appears to be continuous whereas the wire itself is separated an eighth of an inch or more. (7.) Concealed house light wiring in the walls will cause humming noises if the radio set is installed close up to such partitions. This is especially noticeable in the case of singlecircuit receivers. In the case of abnormal humming it is probable that the main supply line (threewire system) is badly out of balance or the wrong side of the house lighting system is grounded. This latter will not necessarily blow the fuses. A and B supply devices designed for sixty-cycle supply will also give some similar trouble when used on twenty-five cycle lines. Various stray noises will, of course, result from defective tubes, shaky splices, poor construction, connections soldered with acid flux, etc. Before assuming that any given interference comes from the outside, broadcast listeners should disconnect the antenna and ground wires and note if the interference weakens or disappears. If it does, the source of the trouble is outside. If it remains the same and is not responsive to tuning, the trouble, nine times out of ten, is in the set itself. Unshielded sets may pick up outside interference without the antenna or ground being attached, but a decided weakening will be observed when these are disconnected and this is sufficient proof that the disturbance is not internal. As a test, the house lighting supply switch should be opened for a moment. If the interference disappears coincident with this, all lights and electrical apparatus in the residence should be checked over for possible faults. A lamp loose in its socket can cause quite a lot of trouble. Partial shorts in the interior wiring will cause trouble over a fairly wide area; such a case -iCr* A-H' B-%' C-H" D-2V E=%" Hardwood Bobbin. approx. 300 turns No.22 (about 125 ft.) FIG. 2 occurred recently where a long nail driven in an attic floor brought a concealed power wire in contact with the grounded plumbing. ELECTRIC STREET RAILWAYS INTERFERENCE from this source presents a 1 problem for which no really satisfactory solution has been found. Without question, many radio sets are unnecessarily interfered with by this agency and in such cases relief is possible. The steady interference under a street-car line is violent; fifty feet either side of this it is almost negligible. Only when cars go by or some very abrupt alteration of the line energy values occurs do we get trouble at this distance. The following actual test is interesting: On a radio set, the antenna of which ran parallel to a car line, broadcast programs were rendered valueless as mediums of entertainment but when this antenna was changed to run at right angles to the line, good reception became possible. The effect was decided. If your antenna is at present installed at the front of the building, facing the car line, shift it to the rear as far as possible. If conditions permit, run it at right angles to the line of interference. It is surprising what a difference one or two feet variation in the plane will make here and we suggest when installing, permanently fixing one end of the horizontal portion and temporarily attaching the other end of the horizontal part to a pole which may be carried backward and forward on the roof to test out the exact position of minimum interference. Relative intensity of the interference is noted at each setting and a permanent pole fixed at the proper point. We must remember that lighting and power circuits nearer the residence than the trolley feeder are concerned in transferring this disturbance to the antenna, and distortion of direction is pretty sure to occur. Sparks at the trolley wheel cause clicking interference; large arcs cause none. The bigger the flash, the less the interference. Cars going up grade on full power cause decidedly less interference than cars coasting with practically all power off, only the lights or heaters being in operation. Trolley shoes are no improvement over wheels. Catenary suspension makes no difference. !f the positive lead of the station generator goes direct to the trolley wire, interference will probably result. If this lead goes to the series field winding first, the free end of the series field going to the line, the choking effect of the coils tends to suppress any commutator interference. In addition to checks on the usual 600-voIt d. c. systems, observations were carried out on: (a.) Pantograph System, 6600 volts a.c. copper trolley wire. (b.) 1500-volt d.c. system using shoes. Copper trolley. (c.) 2400-volt d.c. pantograph. Copper trolley, (d.) 1500-volt system d.c. shoe contact. Entire system steel trolley wire. A great many variable factors entered into the results and for scientific purposes the data obtained are not considered satisfactory. Much time and effort and expense have been devoted to the whole problem of interference from electric railways but, so far, results have not been very encouraging. It must not be concluded, however, that the proposition is hopeless; greater efforts are being made in every city to keep the rail bonding in good shape and it is probable that more careful attention to car motors and equipment generally will tend to alleviate the situation. Peculiar effects are often observed on radio sets installed in the vicinity of a car line. It is not uncommon to find that a passing car, even when causing little or no radio interference, will "take away," temporarily, the program being received. As soon as the car moves away a few hundred feet the program comes back, without any alteration of the set tuning whatever. Again, in two widely separated instances, and some distance away from the car lines, the disturbance on residence radio receivers amounts to a continuous loud roar. Immediately outside both residences no interference can be picked up on a standard six-tube super-heterodyne receiver. Evidently this was a case of electrolysis since an exploring coil on the water piping indicates fluctuating current in the plumbing system. Concealed pipes and wiring in these residences carry the surge also and loop reception is practically no better although the ground wire to the water pipe is cut off. One of the most curious and perplexing cases of radio interference that we know of had its origin in an electric street railway system. This particular noise affected only three residences, all fairly close together. All efforts to locate the trouble failed but the plumber solved the problem. It so happened that the drain pipe of the centre house became blocked; on digging this up it was found to be full of a fibrous growth and at a crack in the pipe a mass of roots had come through and grown out in the direction of the railway tracks, close up to the rails, in fact. This offshoot evidently served as a conductor to electrify the plumbing fixtures of the three houses concerned and as the radio set grounds were attached to this, every variation in the railway line voltage set up a disturbance on the radio receivers. When the drain pipe was cleaned and replaced, all interference disappeared.