Radio broadcast .. (1922-30)

66 Radio Broadcast tions within the Metropolitan area, with promises of even more in the near future. In the spring of this year, when the new schedule of wavelengths was established, and some of the more important stations were relocated, radio engineers were importuned on every hand for some means by which these various stations could be better separated. "Distance fans" wanted to be able to cut out the local stations entirely so as to reach out to programs from stations in the West and South. This had become an impossibility for those located in the center of the City, very near to stations WJZ and WEAF. In fact, in many instances it was found, depending somewhat upon the location of the receiving set, that whenever these powerful stations started to operate, it was practically impossible to eliminate either one or both of them and hear any other station at all clearly. I had been conducting experiments for some time upon the inductive wave-trap described in this article as offering the most practical solution to the problem ; and the acuteness of the situation made it imperative that something be done at once. SOME INTERFERENCE CANNOT BE ELIMINATED IN CONSIDERING interference between broadcasting stations, it is very necessary to know that which can properly be eliminated, and that for which there is, to date, no known remedy. The most common form of inter FIG. 2 How the unit looks when assembled ference which cannot be effectually eliminated is that caused by two stations which are operating on identically the same wavelength bands. This will produce a steady heterodyne note, or whistle, in the headphones whenever this particular wavelength is tuned in. This type of interference can be distinguished by the fact that it can be heard without the receiving set oscillating (contrary to the usual heterodyne beat note), and does not change pitch as the receiver is adjusted. This interference is, of course, the fault of the transmitting stations, and they should be notified of the fact that they are on each other's wavelengths. ADVANTAGES OF THE INDUCTIVE WAVE-TRAP WAVE-TRAPS have been used more or less successfully for years, generally by commercial operators on telegraph signals and later by broadcast listeners. The wave-trap which was in use until somewhat recently, however, was not altogether efficient. It filled the main purpose of trapping out the interfering signal, but at the same time, frequently reduced the signal strength of the station which the operator wished to hear so seriously as to make the remedy of little practical use. The inductive wave-trap which we are considering is a great improvement over its predecessor, which, in more cases than not, consisted simply of a single coil and variable condenser in series with the antenna. The idea itself is not a new one, although it has never had any great commercial application to radiotelephone reception. Well-built inductive wavetraps overcome the defect which I have mentioned and operate with much greater efficiency on the broadcasting wavelengths than the old style wave-trap. In fact, in many cases where the interfering station is a powerful one, the weak station received through it will be materially strengthened when the wave-trap is set to cut out the former. HOW TO BUILD YOUR OWN WAVE-TRAP THE inductive type of wave-trap is particularly useful when used as a rejector circuit — that is, in series with the antenna between the lead-in and the set. It consists of a coil of approximately 45 turns of medium-size copper wire, generally wound on a cylindrical form about 3^ inches in diameter, with a variable condenser of about .0005 mfds. capacity connected across it, the combination forming an oscillating circuit. A second coil of about 10 turns is wound on top of the first winding so