Useful Wave Trap to Cut Out Local Stations

Radio Record, Volume I, Issue 37, 30 March 1928, Page 13

Useful Wave Trap to Cut Out Local Stations

(Reprinted from Journal of Radio Engineers’ Institute.)

mea HOPLE living very near to ¥i a powerful broadcasting statiou may find that this station comes in loudly Mee! enough to interfere with Sete reception of other stations, even when the receiving set is most carefully tuned to the station that they want to hear. Particularly is this the ease in large cities like New York (which city has no fewer than twentytwo broadcasting stations, operating ou powers up to 5000 watts, and in general with many hundreds of thousands of people living within a few miles of each of these stations). With a receiver having poor selectivity, it may be that the nearest or most powerful station will be heard no matter how the set is tuned, but with a receiver of very great selectivity, only perhaps two or three stations of yery nearly the same -wave-length as the local station will be interfered with. ‘Thus the seriousness of the interference depends upon the selectivity of the receiver, the distance from the interfering station, and the power of the latter. In the following the methods of eliminating or greatly reducing such interference at minimum expense and trouble are given.

"Fortunately, in most cases the interference may be reduced to a point where it is umobjectionable by the use ef one or more wave traps. These are simple devices that can be bought fairly cheaply, and can be made at home very easily for almost no cost beyond that of a variable condenser,

Successful Elimination, "In the great majority of cases the interference can be eliminated by the use of what is called a series wave trap "The series wave trap has two binding posts, one of which is connected to the antenna post of the receiver, and the other is connected with the antenna, (‘The antenna is thus disconnected from the set and the signals have to go through the trap to reach the set. ‘This is why it is called a ‘series trap.) In addition to the wave trap « .0005 microfarad fixed condenser shonid be connected across the antenna and ground binding posts of the receiving set. "The series trap offers a very great obstruction to the interfering signal, and thus reduces the amount that gets through the ‘set, but offers comparative. ly ale obstruction to the desired signals. Construction of a Series Wave Trap. ‘A satisfactory series trap can be made of the following parts:-One 0005 microfad variable condenser, ons cylindrical cardboard cover off an |

old dry cell (about 24 inches in diameter), a spool of No. 24 double cot-ton-covyered wire, a smaJi hoard to mount the parts on, two binding posts of Fahnestock clips for connection to antenna and to receiving set. (See Figure 2.) Wind a coil with the wire on the cardboard tube, with the turns

close. together, about sixty turns. Twist a few loops in the wire for connections at several points, say, turns number 5, 10, 18, and 30, and also at the last turn. "Remove. the cotton covering from the wire on the coil at these points, so that connection can be made to any one of them by meaus of the flexible piece of wire marked ‘A.’ Such places, where connection may be made to certain turns on the coil, are called ‘taps.’ "Mount the variable condenser and coil on a wooden board, provided with two binding posts, as shown in Figure 2 Connect the first and last turns of the coil to the variable condenser, as shown in the figure. Then connect one of the binding posts or the hoard to one of the binding posts on the variable condenser, as shown in the figure. To the other binding post connect a piece of flexible wire (the No. 24 cottcn-covered wire can be used, but a piece of stranded flexible insulated wire would be better), and remove the insulation from the free end, so that it may be connected to one of the ‘taps’ which were made on the coil, as will be described in the second paragraph below.

Operation. "Remove the antenna wire from the ‘antenna’ binding post on your receiying set, and connect it to the righthand binding post on the wave trap, and connect the left-hand binding post’ on the wave trap to the binding post on your receiver, to which the antenna wire previously went (see the figure). Next, connect the .0005 microfarad fixed condenser between the-antenna and #Zround binding posts of the receiving set. "Now twist the ‘flexible’ wire connection on the wave trap around the ‘tap’ at the thirtieth turn, being sure to make a good connection. ‘Then start np the receiving set, set the wave trap condenser pointer at zero, and tune for some desired station. ‘This may come in at a different place on the receiver dials from those found previous-

ly. If the interfering station is now heard along with the desired one, turn the knob of the wave trap condenser very slowly uutil the interference disappears. "If the desired station goes out along with the interfering one, change the flexible connection wun the wave trap to the eighteenth turn, and repeat the operation. If the same thing still occurs, try ten turns and tive turns iu succession In each case, before changing the tap counectior, try retuning or readjusting your receiver, to see whether or not the desired station can be brought in, and also readjusting the wave trap slighily to keep out the interfering signal, A certain amount of back and forth adjustment between the receiver and wave trap may be necessary. "If with the tap connection on the thirtieth turn the interfering signal can still be heard under tie desired one, when the wave trap is tuned to give its maximum reduction of interference, change the tap connect to the sixtieth turn, and repeat the operation described J above.

a4 Certain amcunt of experimenting will be necessaty, in order to learn the effect of the wave tap on the receiver adjustments, and in order to learn how to adjust the wave trap as well as what tap on the coil is hest En

for your particular receiving :et end antenna, "If the trap does not work when first made and connected, inspect if carefully to see that it was made in accordance with the foregoing instructions. ‘Traps such as ike one described have actually been made, and have been used successfully with many bundreds of receivers, of the widest variety of manufacture, Loop Sets, "Less interference will be found usu. ally when using loop sets, because the loop is more selective than un anteuna, Also the loop can be turned into a position where the interference is very much reduced. And dnally, a wave trap can easily be made that will cause a great decrease in interference, as tollows :- Wind about twenty turns of double cotton-covered number 24 wire in a bunch around a regular size 45-volt ‘‘B"

battery (which is about 7 inches by 8 inches). Slip the coil off, and tie or tupe it together, to keep from falling apart. Connect it to a variable condenser. Then hold the coil near the loop, and adjust condenser to mvke the interference as little as pc asible." —