Radio Broadcast (May 1927-Apr 1928)

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Suppressing Radio Interference Every Conceivable Source of Radio Interference Is Considered, Remedial Offered — Farm Lighting Plants, Railway Signals, Telegraph Lines,, Railways, and Interference Originating in the Receiver Itself, Are Taken A. T. LAWTON gestionis Being Tickers, Street in This Chapter THE studies made by the author in the elimination of interference have been so extensive that it would be hardly possible to combine the various chapters in one issue. An endeavor has been made, however, to eliminate cross references so that each article in the series may be complete in itself. The forms of interference covered in the two previous chapters, printed in the September and November, 1927, issues of Radio Broadcast, dealt with interference originating at the following sources: Oil-burning furnaces, X-ray equipment, dental motors, motion-picture theatres, telephone exchanges, arc lamps, incandescent street lamps, flour mills, factory belts, electric warming pads, and precipitators. The information printed results from a two-and-a-half-year study by the author in more than 132 cities. The first form of interference considered in the present chapter, is that originating in farm lighting plants. FARM LIGHTING PLANTS INTERFERENCE from this source is confined to rural districts. The characteristic click, click, corresponding to the ignition spark, reveals the source at once; rarely do we get trouble from the commutator. This continuous clicking is very loud on nearby radio receivers and because of the large number of plants in operation in communities not served by electric power companies, the total interference is very annoying. Complete elimination of the disturbance created is a relatively simple matter. Nearly all these plants, being bolted down to a wooden platform, are insulated from the ground and two 2-microfarad condensers in series, midpoint connected to the engine frame, and bridged across the outgoing d.c. feeders, will clear up the clicking. Ground connection of the series wire should not be made to a water pipe or earth rod in this case. Even i-microfarad condensers will be found suitable for small plants. If the engine bed is of concrete or the plant is grounded in some other way, complete elimination is not obtained by the above method although the reduction (about 80 per cent.) is material. The remaining interference is not likely to be serious. However, where it is desired to clear this out also, choke coils should prove effective. It is not uncommon to find the exhaust pipe of such plants carried to a muffler drum in the ground; such a connection to earth offsets the effect of the condensers and more satisfactory results will be obtained if this pipe is cut and a section of asbestos or other suitable piping inserted. Further experimental work is required in the cases where Electrical Inspections call for the grounding of small lighting plants. Possibly a fine wire choke coil, say, of No. 26 wire, bridged by a standard lightning arrester, would be satisfactory, the coil preventing the accumulation of any static charge and the arrester taking care of any abnormal surge superimposed on the system from outside sources. As a precaution, all leads around the plant proper, either high or low-tension, should be cut as short as possible. Where one or two of the storage cells are used to operate a radio set as well as light the residence, complete elimination of the trouble for that particular set becomes difficult. Prior to arriving at suitable preventive measures in the various plants investigated, experiments were carried out directly on the ignition system but all methods tried proved of no avail. In the case of "make and break" type ignition, condensers applied directly to the ignition system stalled the engine, apparently neutralizing the effect of the inductance coil, but were quite effective on the outgoing lines and, of course, did not interfere with normal operation. In all cases attachment of the condensers is made at the switchboard, under the same terminals to which the two lighting mains are connected. RAILWAY SIGNALS RAILWAY "wig-wag" signs and crossing bells give rise to heavy clicking radio interference. Fortunately, their periods of operation are limited, but where the trouble is material it can be cleared up by shunting the operating contacts with a resistance of about 350 ohms. This applies to low-voltage d.c. operated bells and signals. Chokes:' For types operating on 400-600 volts it is necessary to bridge each individual set of contacts with a resistance of the above order. The vibrating reed type battery charger used in conjunction with these signals is a real offender. Interference from this source has given rise to a large number of complaints but, generally speaking, railway companies are averse to tacking on any surge traps to this equipment and in the dozen or so cases cleared up, the operating company took the chargers out bodily and substituted a type which is silent so far as radio interference is concerned. n LAND LINE TELEGRAPH AND STOCK TICKERS RAPID clicking interference from the above equipment is a serious matter in towns and smaller cities. Not that there is less telegraph activity in the larger centres, but in the places referred to a comparatively greater number of residences and radio dealers are located in the vicinity of the telegraph offices. Normal operation of the keys and repeaters set up a vigorous highly damped wave which breaks in on practically any setting of the dials of the average radio set. The clicking from stock tickers is severe even where the wires are enclosed in lead-sheathed cable. Since it is necessary to apply suppressive measures to each individual line, the factor of cost looms large when we consider the number of lines entering the average city office. Where only a few lines are concerned it is usual to place a 1 -microfarad condenser across the key contacts inserting in each condenser lead a resistance of about 20 or 30 ohms. Strictly speaking, only one resistance unit is required but it makes a very great difference which contact of the key this resistance lead is connected to. If the condenser alone is used, reduction of the interference may be noted but the resultant arcing at the contacts is prohibitive. It is ordinarily supposed that the condenser in this case should absorb the spark; rather, it turns the spark into an arc and resistance is required to overcome this. We must remember that many variables enter here; at a given time, one leg of the key may be to ground and the other leg to line, positive or Line FIG. I The circuit employed to eliminate interference caused by key clicking. An obstinate case was eliminated by application of a surge trap, as shown in this diagram Chokes130 turns No.26 D.C.C. wire on wooden core 1^'diam. 217