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BELL SYSTEM TECHNICAL JOURNAL 



dotted outline is the diagram of the unit rhombic antenna character- 

 istic enlarged 16 times. The complete diagram is of course a solid. 

 Figure 46 is an attempt to show how the middle lobe of Fig. 4a looks 

 when viewed from the ground plane at a horizontal angle of 45° from 

 the line of the antennas. The contour lines on this leaf-shaped figure 

 are lines of equal reception. All of these diagrams are for a frequency 

 of 4,700 kc, near the low end of the range of received signals. At 

 higher frequencies the lobes will be more slender and the angle of 

 maximum reception will be lower. Fortunately the latter corresponds 

 to the trend of the received signals. 



When the outputs of several antennas are connected to a receiver 

 and added in the proper phase the total signal can be made equal to 



Fig. 4 — Polar diagrams of musa antenna system, (a) Showing three possible 

 locations of the major lobe, {b) Solid polar diagram of middle lobe shown in (a). 



the sum of the signal voltages. The set noise, however, adds at 

 random phase with the result that there is an improvement in signal- 

 to-noise ratio over a single antenna equal to the square-root of the 

 number of antennas, or 10 logio n in decibels. The theoretical im- 

 provement of a 16-antenna system is, therefore, 12 decibels. On the 

 assumption that received noise comes from a random direction with 

 respect to the signal a similar result is obtained. At a particular time, 

 however, the principal noise may be arriving from such a direction as 

 to allow of no discrimination by the antenna system, or at another 

 time to allow much more than 12 db discrimination. 



The received signal in an antenna is a result of both the direct wave, 

 arriving from some angle above the horizon, and the same wave front 



