RADIO BROADCASTING TRANSMITTERS 139 



It is interesting to contrast this situation with that existing in 

 some of the large rural districts as exemplified by the recent survey of 

 conditions in the Middle West by Jansky.^ Here over large areas 

 acceptable service is being obtained with field strengths of 50 and 

 100 /iv per meter. Giving due consideration to the difference in noise 

 levels, which is undoubtedly a factor of great significance, such a dis- 

 crepancy can only be reconciled on the basis of a vast difference in 

 service standards. That such conditions will be allowed to continue 

 for any considerable period of time is very doubtful. This is further 

 evidence indicating that the movement toward more powerful stations 

 is technically sound. 



One phase of the transmission problem which deserves increased 

 attention is antenna performance and design. It is an interesting 

 circumstance that while the accurate rating of broadcasting stations 

 is a matter of great practical concern to the industry, to date con- 

 sideration has been confined to the power delivered to the antenna. 

 Variations in the efficiency of the latter have been almost entirely 

 neglected in spite of the fact that, due to this cause, the power actually 

 radiated can be shown to vary through a range of four to one, or 

 greater. There is little doubt that stations should be rated, either 

 directly or indirectly, in terms of field intensity measurements. That 

 such a system of rating has not already been put into effect is probably 

 due to the lack of suitable measuring apparatus. With such equip- 

 ment now available, rapid progress in this direction is expected. 



An interesting feature of current American practice with respect 

 to broadcasting antennas is a definite tendency toward the use of 

 higher supporting structures. For the past few years, most of the 

 towers erected have been from 150 to 225 ft. in height. Several of 

 the more recent stations are employing 300-ft. towers, and it is not 

 improbable that some 400-ft. structures will be put up in the near 

 future. Since the natural frequency of grounded steel towers of these 

 dimensions falls in the broadcasting band and may approximate the 

 assigned operating frequency, low-capacity porcelain insulators are 

 inserted at the base. The latter effect a considerable increase in the 

 natural frequency of the towers and preclude serious distortion in 

 the field intensity pattern due to heavy induced currents in the steel. 

 The antennas themselves are of such dimensions that the current 

 antinode is positioned well up on the vertical section. The effect is 

 to concentrate the radiated power along the ground plane and to 

 increase materially the field intensity in the local service area. Such 

 antenna systems promise a better economic balance between the in- 



* See attached list of references. 



