Radio broadcast .. (1922-30)

482 Radio Broadcast 2 will operate satisfactorily with all tubes except soft detector tubes, such as the UV-2oo; but will work best with UV-2OI-A or Western Electric 2 i6-A tubes. The use of WD-I2 tubes in the wavemeter shown in Fig. i and a small size B battery permits the construction of a very neat self-contained wavemeter of small dimensions. The milliammeter used should have a range of from o to 5 milliamperes. SIMPLE METHODS OF CALIBRATION MOST experimenters and amateurs, who are sufficiently interested to construct a wavemeter. understand its general principles of construction and use, but are puzzled to find a suitable means of calibration. The establishment of broadcasting stations and the accuracy with which the wavelength or frequency of the broadcasting station is set, has solved this problem. The accurate calibration of a wavemeter from a reading obtained on even one broadcasting station is now possible, provided, of course, that the broadcasting station chosen is one of the larger size broadcasting stations, which has had its frequency or wavelength carefullv fixed. V Any or Radio Frequency Receiver goi FIG. 3 Method of connection for calibration of receiving set using circuit of Fig. i. Also method of calibration of wavemeter circuit. Here the wavemeter is shown within the dotted lines Before going into the method of calibration it may be advisable to explain briefly the relation between frequency and wavelength. The electro-magnetic disturbance in the ether travels through the ether at the same speed as light, namely, 300,000,000 meters per second. The frequency of oscillation does not affect the speed at which the disturbance travels through the ether, so that all waves, regardless of length, travel the same distance in the same time. This being the case the amount of space transversed by a single cycle of the electrical disturbance can be obtained by dividing the number of cycles per second into the distance traveled per second. If a radio set transmits at a frequency of 50,000 cycles, or as it is now more commonly called 50 kilocycles, the distance traveled by one cycle will be 50,000 divided into 300,000,000 or 6,000 meters. Conversely, if the wavelength of a station is given, by dividing the wavelength in meters into 300,000,000 the frequency of the electrical disturbance can be determined. The wavelength and frequency are, therefore, dependent entirely on one another. With the facts above in mind, consider what means there are available for calibration using frequencies or wavelengths known to be associated with the various broadcasting stations. Take for example, station WEAF of the American Telephone & Telegraph Company, located in New York City and operated on a wavelength of 492 meters or 610 kilocycles. Assume we desire to calibrate the meter shown in Fig. i. Couple the wavemeter circuit as shown in Fig. 3 to a regenerative receiving set. Tune the receiving set to station WEAF very carefully. This is best accomplished by causing the receiver to oscillate strongly and then adjusting to the exact point where the howl between the frequency of the broadcasting station and the frequency of the receiver disappears. This is commonly known as the point of zero-beat. Allow the receiver to oscillate strongly on this adjustment, tune carefully with the condenser on the wavemeter until a deflection of the milliammeter is obtained.