Radio Digest (July 1924-Apr 1925)

February 7 , 1925 RADIO DIGES T— Illustrated Tuning in Broadcasters on Their New Waves Interference Problem Not Hard to Solve By G. M. Raley EVER since the third Radio conference when it was decided to reallocate wave bands among the broadcasters so as to create less interference and make room for more stations, the def partment of commerce Radio section has been hard at work attempting to execute the program mapped out by the conference. But the reallocation was much easier said than done. Not alone was it difficult to crowd the stations any closer together, but the expected co-operation of broadcasters in some cases was not forthcoming. Now the Radio administration officials have finally begun the active work of changing wave lengths here and there, raising some, lowering others, in a practical attempt at carrying out the program. As a result, Radiophans far and near are sending in complaints by the thousands. Radio supervisors, broadcasters and this publication have had their share of these. The trouble seems to be from two things: 1. Distant stations, with new waves spaced closer together, are difficult to tune in without interference, one from the other. 2. Old dial settings, once sure to bring \J7 AERIAL TOAERIAL POST 0F5ET 25 TO 35 TURN COIL .0005 MFD. (500 MMF.) TO GROUND POST -= GROUND OF SET Figure 1 in a certain station, mean nothing when that station's wave length has _ been changed. Solution of Problems Not Hard It is the purpose of this article to give the Radiophan some workable suggestions for the overcoming of these difficulties. Neither are hard to solve, but every listener might as well follow now the instructions outlined, as later, for the department of commerce assures that more than twenty 500-watt stations are awaiting their opportunity of going on the air — with a corresponding renewal of the chaos. The first problem — separation of stations without interference — depends upon the selectivity of the receiving set used by the Radiophan. Increased selectivity, to overcome the closer spacing of the stations, some of which are now only seven kilocycles apart in their frequencies, can be obtained in a number of ways. A brief resume of the various methods of increasing the selectivity of a receiver includes the use of short outdoor or indoor aerials, better ground connections, wave traps (pr high frequency by-pass circuits), good variable condensers, vernier controls, low loss inductances, shielded sets (when of the super sensitive type), and a great deal of patience. Patience in tuning a set is a remarkable aid to selectivity that is often overlooked by the beginning Radiophan. Better Aerials and Grounds Poor aerials and grounds mean a path of high resistance for the incoming Radio signals, with an increased loss of selectivity. In general it may be said that the lower the resistance of an antenna circuit (aerial and ground), the correspondingly higher will be the selectivity of that circuit. Short aerials mean low resistance aerials. It is often wise, when selectivity is desired, to use an aerial not over sixty or seventy-five feet long, including the ground lead and lead-in in this length. The ground connection is very important. This should be clamped or soldered to a cold water pipe which has been scraped or filed clean at the point of connection. The aerial lead should be insulated well from contact 'with the house or window still at the point where it is brought into the house'. Porcelain tube, or other effective insulators should be used. For the separation of local or nearby stations, without interference, the writer has used a short indoor aerial with much success. It might be well to have one of these available for use or experiment. Wave Trap Usually Sure Cure The wave trap, sometimes called high frequency by-pass or filter, about to be described, is one of the surest methods of separating and eliminating one of two closely tuning stations, be they both distant or one distant and the other near by. A wave trap consists essentially of a coil of wire (inductance) tuned either by switch taps (inductively) or by a variable condenser (capacitatively), which is placed in series or parallel with the coil. The device may either be placed in series with the receiver in the aerial lead, or in parallel "with the receiver across the aerial and ground posts of the set. One of the most efficient of these is shown in figure 1. It is connected in parallel with the receiver and is therefore known as the parallel type. The theory is that the wave trap circuit, when tuned to the wave length of the station not desired, will by-pass the signals from this station into the ground. Efficiency Depends on Farts Used The parts required for the trap are but two: a good low loss type variable condenser of .0005 mfd. (500 mmfd.) capacity and a 35-turn honeycomb coil, or equivalent low loss coil inductance. The two are connected in series across the aerial and ground binding posts of the receiving set, and should be placed several feet away from the cabinet of the set so as not to be inductively coupled with the receiving tuner. The efficiency of this arrangement is almost entirely dependent on the grade of apparatus used, so that it is advisable to use the best obtainable condenser and coil. The coil can be made by the Radiophan who cares to dabble in such things. This is done by winding in a single layer 25 turns of number 24 dec. wire on a 3inch diameter or 35 turns of the same size wire on a 2-inch diameter paraffined VT^ AERIAL TOAERIAL POST .0005 MFD. (500 MMR) GROUND Figure 2 OF SET cardboard, hard rubber or bakelite tube. To make the trap even more effective, wind fifty turns of the wire on a 2-inch diameter, tapping every 10 turns. See figure 2 for method connecting taps to switch points. "When using a trap of this type, always use as much of the coil as possible in tuning to eliminate an interfering station. How to Use Trap To use the trap, after having connected it as shown and placed it at least two feet away from the receiver's cabinet, set the dials of your receiver where the desired station is heard, but is interfered with by the undesired station. Next, adjust the variable condenser of the wave trap until the interference has been reduced to a minimum, at the same time readjusting the receiver so that the desired station comes in as strong as it is possible to make it. A little practice will soon surprise the most skeptical listener in. In logging or recording dial settings so as to come back to a station when desired, it will now be necessary to include the reading of the wave trap dial along with the others. Other Aids to Selectivity Low loss variable condensers and inductances, if not used in your receiver. should materially improve the selectivity if substituted for the less efficient variable condensers and inductances. Super sensitive sets, such as the superheterodyne or five tube Radio frequency designs, should have their cabinets weil shielded so as not to pick up interfering stations through the ability of their inductances to act as effective aerials. 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