TRANSOCEANIC TELEPHONE SERVICE 277 



to the plane of the conductor. In other directions destructive inter- 

 ference reduces the radiation from the vertical members. The system 

 is equivalent to four Hertz oscillators driven in phase, and arranged in 

 two groups one half wave-length apart, the two oscillators of each group 

 being placed one above the other. Both computation and experiment 

 have shown that with this system of radiation there is an improvement 

 of approximately 6 db. In other words the same signal intensity in the 

 chosen direction is obtained with one fourth of the power required by a 

 one-element radiator. A second similar conductor system placed 

 directly behind the first in a parallel plane one quarter wave-length 

 away, will be excited parasitically from the first conductor and will act 

 as a reflector, thereby creating a unidirectional system. It has been 

 found that the reflector further reduces by 3 db the power required to 

 maintain a given signal intensity in the desired direction, thus bringing 

 the total gain for the system up to 9 db. This is also in agreement with 

 the theoretical computations. 



It is obvious that the system in Fig. 3 can be extended vertically to 

 include more radiating elements by increasing the length of the con- 

 ductor and it can be enlarged horizontally by placing several units 

 alongside each other, care being taken to obtain the desired phase rela- 

 tions by transmission lines of the proper length. In this way large 

 power savings may be effected. At Lawrenceville the maximum gain 

 is about 17 db (a power ratio of 50) over a vertical halfwave oscillator. 

 The enlarged system lends itself readily to mechanical support and 

 forms so-called exciter and reflector "curtains" which are suspended 

 between steel towers appropriately spaced. Aside from other con- 

 siderations, which will be mentioned in connection with station layout, 

 the size of the antenna is influenced by the complex and variable nature 

 of the wave propagation through space. At present this determines 

 the degree of directivity which is most useful for the average conditions.^ 



The closed loops of each unit corresponding to Fig. 3 greatly facilitate 

 the removal of sleet. In addition to loading the antenna mechanically, 

 ice, having a dielectric constant of 2.2 at these high frequencies, ad- 

 versely affects the tuning. At Lawrenceville sleet is removed by heat- 

 ing the wires with current at 60 cycles. This is accomplished without 

 interfering with the service by employing one of the less familiar 

 properties of a transmission line. The same property also is used to 

 effect impedance matches wherever the transmission lines are branched. 

 If a line, exactly one quarter wave-length long, of surge impedance Z^ is 

 terminated with a load Z^, the sending-end impedance Z, is equal to 



*J. C. Schelleng, "Some Problems in Short Wave Telephone Transmission." 

 Presented to the Institute of Radio Engineers at a meeting Nov. 6, 1929. 



