CAIN OF DIRECTIVE ANTENNAS 



71 



tween couplets increases the gain only up to a certain point, after 

 which the formation of parasitic lobes decreases the effectiveness of the 

 array. The trend of these gains may be illustrated more effectively in 

 graphical form. 



In Fig. 6 calculated gain ratio is plotted against number of couplets 

 giving one graph for each separation considered. These ratios are not 

 based on the data given in Fig. 5, but were obtained from the integra- 

 tion of the equation of the directional diagram over an arbitrary 

 sphere by use of equation (27) below. It may be noted that for many 

 conditions the difference between these methods of calculating gain is 

 only moderate. These power ratios are for the most part linear, 



Fig. 6 — Antenna arrays. Calculated power ratios vs. number of couplets. 



indicating that such gains are proportional to the length of the array. 

 This is in keeping with the view that a receiving antenna can intercept 

 wave power more or less in proportion to its dimensions. It is also 

 interesting to note that the slope of the curve of X/2 is approximately 

 twice that for X/4, so that 16 couplets spaced \i wave-length give 

 approximately the same gain as eight couplets spaced yi wave-length. 

 This again shows that the length of the array is the most important 

 criterion in determining its gain. In Fig. 7 the same data have been 

 plotted in decibels. 



In Fig. 8 gains expressed in decibels are plotted against the separa- 

 tion between elements. This shows more definitely the trend of the 



