RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 641 



and 8 reflectors in a parallel plane. This type lends Itself very well 

 to construction for ultra-short-wave operation. The directivity of any 

 antenna is a function of its breadth measured in wave-lengths and 

 when the wave-length is short it is more economical and easier to 

 construct the antenna with the larger dimensions up and down the 

 pole or at right angles to the pole than to stretch it along the ground. 



Figure 21 indicates one of the first directive antennas to be used. 

 This is known as the "Beverage" ^° antenna and is used on long- wave 

 telephone circuits which operate between 60 and 70 kc. Such an 

 antenna is located at Houlton, Maine, for receiving from England. 

 The antenna structure appearing in this diagram may be several miles 

 in length. It receives best from the upper right-hand direction, as 

 indicated by the arrow. The incoming wave produces currents in the 

 antenna which experience gradual build-up along the line to the 

 receiving end where they operate the receiver and deliver the signal 

 to the telephone line. This type of antenna has a horizontal directional 

 pattern as indicated in the figure. Its maximum direction as used is 

 northeast, as that is the direction signals arrive from England. It also 

 happens that more static and strays reach this part of the country 

 from the southwest than from other directions and with the antenna 

 so oriented there is what is sometimes called a "blind eye " faced in the 

 southwest direction so as to receive a minimum of interference from 

 that direction. 



Some of the difficulties involved in transoceanic short-wave reception 

 may be explained by reference to Fig. 22; this shows a diagram of 

 the earth and the ionized region of the atmosphere called the "iono- 

 sphere." Signals from the transmitting station in England may reach 

 the receiving station in the United States by more than one path. 

 One path indicated has two reflections from the ionosphere and the 

 other has three reflections. Careful measurements on this diagram 

 indicate that these two paths are not equal in length, with the result 

 that signals received in the United States from across the ocean coming 

 over the two paths may be out of phase. Not only may there be two 

 paths but sometimes there are three, four or more so that the inter- 

 ference caused by signals coming over the several paths can give rise 

 to bad fading and distortion. The lower diagram in this figure shows 

 the vertical directive pattern of an ordinary directive antenna. It 

 shows this directive pattern to be large enough to receive simul- 

 taneously both incoming signal components from the two paths. 

 If it is desired to eliminate the undesirable effects produced by the two 

 signals coming in out of phase, one should be eliminated, 'i'his can 

 be done provided an antenna is constructed having a sharp c' recti ve 



