Radio Broadcast (Nov. 1925-Apr 1926)

Cutting Out the Locals Simple and Efficient Wave Traps to Eliminate Interfering Near-lry Stations HOWARD E. RHODES CT" HE trend in broadcasting for some time has ■*■ been toward the use of higher powers, whereever that is possible, and this, combined with the concentration of many stations in large centers, has made the problem of selectivity a serious one for many listeners. It is easy to remedy most of the trouble encountered in the average receiver by simply adding a wave trap of good design, and the accompanying article reviews the subject in a very helpful fashion. The present article is concerned with types which can be made very easily, and a second article, to be printed soon, will describe a radio-frequency amplifier and detail how it functions as a wave trap. — The Editor. w; '1TH the increase in power of a great many of our broadcasting stations, listeners find need of some efficient method whereby undesired signals may be eliminated. These signals sometimes are caused by direct pick-up by the coils of the set so that its natural selectivity is of no avail in eliminating them. This occurs when the set is being operated in proximity to a broadcasting station with the result that the field strength in the vicinity is great enough to induce currents directly in the various coils of the receiver. Under these conditions it will be found possible to hear the program with the antenna disconnected. The only practical methods to be used in eliminating this type of interference are either shielding of the receiver or using some form of close field coil, such as a toroid. If the set is to be shielded, it is necessary to line the entire FIG. 2 One method of connecting a wave trap in circuit inside of the cabinet, including the cover and the panel, with thin copper. Any joints in the copper sheet are soldered together and connection is finally made between the shield and the minus A terminal which should, in this case, be connected to ground. This puts the entire shield at ground potential, thereby excluding from the coils any external electrical fields. It should also be pointed out that this shielding will also eliminate any interference caused by power lines, motors, etc., provided they are causing trouble by setting up currents directly in the coils. However, if this interference is reaching FREQUENCY FIG. I The impedance curve of a well-made wave trap the set by way of the antenna system, shielding will be of no aid. A second and rather recently developed method of eliminating this type of interference is by the use of toroid coils. This type of coil is now being widely employed in instances where there is need of an inductor that is unaffected by external fields. The reason why toroids are not sensible to magnetic fields is easy to understand. Suppose such a coil is placed in a magnetic field. Then a voltage is induced in practically every turn of the coil. However, the voltage induced in one half of the coil is exactly equal and opposite to the voltage produced in the other half of the coil. These two voltages react against each other and the resultant effective voltage is therefore zero. This type of winding can be placed in comparatively strong magnetic fields without having any appreciable voltage produced in it. Toroids are, therefore, very useful in eliminating that type of interference caused by some form of direct pick-up by the coil units of a receiver. These two methods which have just been outlined, first, shielding of the set, and secondly, using Toroid coils, are practically the only methods of eliminating this type of interference. There is a second type of interfering signal that also causes considerable trouble, and against this type the two methods so far described are useless. If the signals being induced in the antenna are sufficiently strong, it is possible that a receiver may not have a selectivity sharp enough to eliminate them completely, although its selectivity under ordinary conditions may be perfectly satisfactory, fake, for example, the curve of Fig. 10 which represents the resonance curves of a fairly good receiver. For ordinary reception this degree of selectivity is satisfactory, but under some unusual condition, even greater selectivity might be required. For instance, the receiver might be operated at a location quite close to a powerful broadcasting station so that the signal strength from this station, in comparison with the strength of the signals that it is desired to receive, is so great as to produce considerable interference, even though the receiver is considerably detuned from the interfering signal. It is apparent then that in some way this powerful signal must be impeded so as to decrease its strength. THE SIMPLICITY OF A WAVE TRAP THE most common piece of apparatus for use in this connection is the so-called wave trap which is actually a filter circuit tuned to absorb the interfering signals. These wave traps are very easily constructed, and cost little. They consist merely of an ordinary coil and a condenser. The traps are connected in the antenna circuit and are adjusted so that at the frequency of the interfering signal they have a very high impedance, the exact value depending on whether the unit is connected in series or parallel with the antenna. This high impedance prevents the signal from passing down through the primary of the antenna coupling and so into FIG. 3 Another method of connecting the trap to your receiver, by the inductive method. This very often produces satisfactory results without materially changing the tuning of the antenna circuit