RADIO BROADCASTING TRANSMITTERS 127 



suppress effectively the beatnotes likely to be encountered under any 

 practical operating condition. Developed in this manner the propo- 

 sition is more or less self-evident, but due to the rapidity with which 

 the audio spectrum of broadcasting apparatus is being extended, 

 some emphasis on the matter seems desirable. 



Still another factor of importance from a system standpoint is 

 control of radio harmonics. Spurious radiation of all types is inimical 

 to intensive development and must be avoided. The harmonic prob- 

 lem presents unusual difficulties since efficiency requires that the 

 tubes in the final power-amplifier stage be used in such a manner that 

 relatively large harmonic voltages are impressed on the output circuit, 

 yet the harmonic power radiated must be held to an extremely small 

 amount. A measure of the purity of wave form required may be 

 gained from the fact that a 5-kw. transmitter operating on a good 

 antenna is capable of establishing an electromagnetic field of approxi- 

 mately 0.5 V per meter at a distance of one mile. Under the circum- 

 stances, a harmonic of 0.1 per cent represents a field intensity of 

 500 ^v per meter at the same distance. Acceptable service in many 

 areas is being obtained with field intensities of this order of magnitude. 

 To bring the interfering field down to the static level would probably 

 require reduction of harmonics to 0.01 per cent or less. From an 

 apparatus standpoint, such performance represents a very difficult 

 problem and it is questionable if it can be justified at the present 

 time. Practice on this point is still in a state of flux, but there is 

 reason to believe that some intermediate value, such as 0.05 per cent, 

 will prove to be the proper solution, and will be applied to all broad- 

 casting stations in the near future. 



One circumstance that has undoubtedly contributed to the delay 

 in formulating definite requirements concerning the control of har- 

 monics has been the difficulty of obtaining suitable apparatus for 

 the evaluation of such components in quantitative terms. Field 

 strength measuring sets have recently been made commercially avail- 

 able, however, which are capable of covering the necessary range in 

 frequency and intensity. A photograph of one of these sets is shown 

 in Fig. 3. It consists essentially of a sensitive, stable superheterodyne 

 receiver incorporating a calibrated attenuator at the input of the 

 intermediate-frequency amplifier and a supplementary radio-frequency 

 oscillator from which a voltage of the frequency of the station under 

 measurement can be introduced in the antenna circuit. The oper- 

 ating characteristics of such an instrument have been described by 

 Friis and Bruce.^ By means of a series of removable loops and coils, 



*H. T. Friis and E. Bruce, "A Radio Field-Strength Measuring System for 

 Frequencies up to Forty Megacycles," Proc. I. R. E., 14, 507-519; August, 1926. 



