320 BELL SYSTEM TECHNICAL JOURNAL 



it is necessary, therefore, that the resonance characteristic be studied 

 and means provided to eliminate excessive frequency discrimination 

 within the desired band. On the other hand, the wave-antenna is an 

 aperiodic structure and, in consequence, its transmission-frequency 

 characteristic is so flat that it need not be considered. 



3. Sensitivity. There are two factors which require that the output 

 from the receiving antenna for a given field strength be as large as 

 possible. First, if the receiving station be located at any position 

 other than that at the terminal of the antenna, which is necessarily 

 the case if more than one antenna be used in an array, the signal on 

 the transmission line from the antenna to the station must be much 

 greater than the noise currents induced into the transmission lines. 

 If the antenna output be excessively small, it is impossible to balance 

 the transmission lines so completely that this requirement is met. 

 Second, the amount of gain that can possibly be used at the radio 

 receiver is ultimately limited by the noise produced in an amplifier. 

 (This is discussed more fully under "Power Output Required from the 

 Radio Receiver" later in this paper.) To the first approximation, the 

 sensitivity of each of the antenna classes under consideration is a 

 direct function of its physical dimensions. There is, however, a limit 

 to the sensitivity of each antenna class, for mechanical limits govern the 

 maximum size of a vertical antenna, distributed capacity and mechan- 

 ical considerations limit the loop, and in the wave-antenna a restriction 

 occurs because of the peculiarity that the sensitivity reaches maximum 

 values at definite lengths. 



Since cost is likewise a factor governing the ultimate selection of 

 an antenna system, the sensitivities may well be compared for antennas 

 of equal cost. On this basis, a loop or a vertical antenna of effective 

 height of fifty meters is directly comparable with a wave-antenna one 

 wave-length long. By reference to Fig. 5, where the scale is the same 

 for all the directional diagrams, it becomes evident that the sensitivities 

 of all three classes of antennas are of the same order of magnitude, 

 being slightly greater for the vertical antenna and the loop than for the 

 one-wave-length wave-antenna. 



4. Stability. The sensitivity and frequency-transmission char- 

 acteristics of the antenna must be substantially constant during 

 changes of weather and seasonal conditions. The antenna classes 

 which require tuning are slightly poorer than the wave-antenna in 

 this respect. 



5. Reproducibility. Further improvement in directional discrimina- 

 tion against noise is obtained by using several similar antennas in an 

 array. The loop and the vertical antennas probably are best for 



