Checking Interference from Power-line Leaks

Radio Record, Volume I, Issue 51, 6 July 1928, Unnumbered Page

Checking Interference from Power-line Leaks

Power Boards and Plants should Study Listeners E have received complaints from several listeners in some of the smaller towns as to interference occasioned by leakages of power from either power-lines or stations. This is one of the greatest bugbears of radio reception.. To the credit of the exigineers of a number of New Zealaud Power Beards, it may be stated, several of them have made rough and ready trouble-locating radio sets for tracking down this criminal of broadcast listening. Not only are power-line leakages a menace to the popularity of broadcast listening, but also are a clear indication of a certain degree of inefficiency in the electrical reticulation system. HE following article, taken from the New York "Popular Radio," requires no apology for its rather technical features, seeing that it will serve as a help to those officials who desire to locate and cure man-made electrical interference which incidentally has developed to such dimensions in some areas as to spoil many an hour’s enjoyment for our broadcast-listening readers. Where listeners are convinced that the fault lies with the power-line or station, we would recommend a frank approach to the engineer or board concerned, They are interested in such leaks because they spell loss.

IKE all other departments of radio research, the methods used in locating interference have, within the past five years, and especially within the past two years, been greatly improved and reduced to an almost exact science. Improvements have been brought about not only in the perfection of apparatus suitable for this work, but in the methods employed in operating inter-ference-finders for maximum efficiency. There was a time when the location of interference was a matter of haphazard guesswork, involving a great deal of time, effort and money. To-day vagrast sounds barely become audible before the "radio detective" corps has them traced down to their source.

Lhis article has been prepared principally for those interested in the most modern: methods of interference-finding, and professional service men, as well as power-station operators, will find in it many valuable suggestions that have come about through actual experiences in locating interference with an improved interference-finder operated from the seat of a sedan. Naturally a ear is a great asset in locating interference, since it provides for rapid transportation and ‘relieves the operator of the burden of carrying the apparatus. The make of car is not important, although a six-cylinder car is preferable to a four. A car with good springs and easy-riding qualities is desirable, both for the comfort of the operator, and the qviet operation of the set. It also is quieter mechanically, Since the operator will have to ‘be out in all kinds of weather a closed car is of advantage; _

a closed car also excludes street traffic noises. ‘Where possible, the upper portion of the car should not be of metal, Where an all-metal body type of car is used, it will probably be necessary to make provision for an external loop that can be fastened outside. Careful attention should be given to eliminate interference caused by the electrical apparatus of the car itself. Some methods used for this purpose are as follows. The generator should receive periodic inspection. The commutator should be kept clean and the brushes properly adjusted. This helps to keep it in perfect condition at all times. If sparking occurs a condenser connected from the positive brush to the chassis will be of help. The distributor also should receive careful attention, care being taken that the contacts are kept bright and properly adjusted, All wiring, with the exceptidn of the high tension leads, should be of the armoured type. A shield may be made of lead or copper to fit over the motor bluck, enclosing the distributor, spark plugs and ignition wiring. This should be made so that it is readily removable, as access to the parts enclosed is often necessary. The shield must be perforated or be made with screening so that air circulation is possible. A condenser may be connected from one side of the primary of the ignition coil to the car frame, either at the ammeter or directly at the coil. A choke may also be inserted in the primary lead. This should be of the same type as the one described elsewhere in this article, One or more of these methods will serve to eliminate most of the interference from the car. Slight

interference will always be present, due to the ignition system, This is not objectionable, as other forms of interference can be heard through it, and at the same time it serves as an indication that the set is in operating condition. RADIO EQUIPMENT _USED IN TROUBLE- . FINDING, HE sets for interference finding used are of many types. Some of them are made by the plant "radio bug.’ Other companies use sets manufactured for interference work, These sets have been dcsigned after several years of experimenting and as the result of the experience in the trouble-hunting field by hundreds of power com-

panies, One is a five-valve set, consisting of two stages of high-frequency amplification, a detector and two stages of low-frequency amplification. Pro-~ vision is made for the plugging-in of exploring coils or external loops by means of a jack which automatically disconnects the internal loop. The _ highfrequency stages and detector may be used without the low-frequency stages by means of a jack provided for this purpose. The volume and valve contrcls are equipped with calibrated dials to facilitate volume comparisons. A switching arrangement is provided which cuts out the high-frequency stages and .permits the use of the low-frequency stages alone. This has proved to be of great use in locating cases of trouble which are being radiated along an entire high tension or distribution line with equal intensity at all points. Cases of interference due to street lighting circuits which have been troublesome to locate in the past are also comparatively easy to locate with this set, Ear phones of sensitive type are used in preference to a loudspeaker in conjunction with this trouble finder, as they hélp to shut out extraneous noises and enable the operator to hear weak signals which vould hardly be audible on a speaker. A loop receiver is universally used, as it is mora compact and does not require the erection of an antenna. In most cases, the loop is mounted in a horizontal rather than a vertical plane, making it non-directional. The directional value of the loop is of very little use, although it has been greatly recommended by many writers in the past. The uselessness of a directional loop is easily understood when we consider that, from the point of view.

of interference-finding in congested areas, we are dealing with signals that come from antenna systems miles in length that may completely surround the interference-finder, It is obvious, of course, that the directional value in such eases is of no use, since the loop will always "throw back" to the nearest power-line. The only real place where it may be of value is at corners or places where the lines branch in all directions. By getting directly beneath the lines at this point it may be possible +o locate the line carrying the heaviest interference. METHODS USED IN TROUBLE-FINDING, THE method more or less universally followed in locating trouble is to trace the source solely by volume indications. A careful survey is made,

and the centra!t area of tne aistulrp~ ance is located. This is done by cutting down the sensitivity of the set until the interference is barely audible. Thé line or lines are patrolled in all directions until a point is reached where the volume again approaches maximum, Following this procedure narrows the search down within a small. area. For an experienced operator this is comparatively easy. Further search in this area, by using the method outlined above, will in most cases disclose the source of trouble. The reason for cutting down the sensitivity of the set is that it is easier for the ear to distinguish between zero and some degree of maximum volume than between various degrees of maximum volume. Where the trouble i: being caused by a motor-driven appliance of any type it is usually due to defective brushes or commutator. If arcing at the brushes

occurs, first see that the brushes are properly adjusted. If the commutator is dirty or grooved it may be cleaned by sandpapering it with very fine sandpaper. Under no condition should emery cloth be used, since more harm will be done than good. Where the commutator is badly grooved it should be turned down in a lathe. This is an operation which will require the services of an experienced electrician. Another means of eliminating interference of this type is to use one of the methods shown diagrammatically in»Figure 1. Usually most. cases can be cleared up by using the methad shown at "A" In more stubborn cases the methods shown at "B’? or "C" will be necessary. There are several types of filter units shown in these diagrams manufactured by well-known manufacturers that are now available. These are properly designed and their use is recommended. If these are not obtainable condensers of suitable rating can be puz-hased and the choke coils can be constructed according to the data given below. CHOKE COIL DATA. OR a single-layer, air-core solenoid whose length is considerably greater than its diameter, substitution of the proper values in the following approximate formula will give a two millihenry induetance: a2 n2 equals 20,000 -(Continued on p. 2.)

— Checking Interference

‘(Continued from coyer.) where a is the radius of the coil in inches, n is the number of turns, and b is length of coil in inches. For a multiple-layer, coil of the | gaine type, the following formula, | also approximate, will serve: an2 (a-1/3c) . b . *here a, n, and b are as before, and « is the depth of the winding in inches. A 200-turn coil wound on a 38-inch non-metallic spool will give roughly two millihenries. In vibrating types of appliances, uch as battery chargers, a one-half or one microfarad condenser, conmected across the contacts, will ustially eliminate sparking and stop euch interference. It is essential in hunting trouble by volume indications that the same @istance from the line be maintained at all times. Approaching closer to the line will naturally cause more volume at such points. Frequently the volume will drop, and no doubt the operator will find that instead of going’ away from the trouble as he suspects, the line has crossed from one side of the street to the other. iWhere lines change from overhead to underground, or at poles where ground wires lead down from pot heads or other equipment, volume will also be greater. Low service wiring or coupling to telephone cables, trolley tracks, etc., or any metal object will in most cases produce the same result. At street corners where branch wiring exists or places where there are more wires volume may also be greater. It is essential, therefore, that allowance be mmade for these conditions. = 20,000 HERE is still another condition that is worth mentioning. Peaks in signal strength or "bumps," @s they are called in the parlance of the linemen, will be noticed in following a line. These will usually eccur in the centre of a span and e equally spaced. If a eareful eheck is made it will be noticed that they increase in volume as the source of interference is approached or deereased when going irom the source ef interference. | A survey of many thousands of €omplaints on the part of the power eompanies has brought to light another interesting fact. Most severe finterference occurs in cold dry weather. In wet weather, contrary to general belief, trouble is far less bothersome; and in many cases wevere interference will practically disappear. ' »