A MULTIPLE UNIT STEERABLE ANTENNA 



351 



(2) It is theoretically possible to construct a suitable unit antenna 

 in the space provided for it when (1) is satisfied. 



III. Description of the Experimental MUSA System 

 Antennas and Transmission Lines 



Any type of unit antenna whose directional pattern suppresses the 

 undesired principal lobes over the required wave-length range is 

 basically suitable for use in a MUSA system. The rhombic antenna ^^ 

 does not fulfill this requirement as well as the linear array of subunits 

 discussed in the preceding section. It was, however, selected on 

 account of its advanced state of development. The manner in which 

 it fits into the MUSA array factor will be discussed later. 



The coupling or "cross talk" between antennas need not be of neg- 

 ligible magnitude in a MUSA system. For, to a first approximation, 



ANTENNA TERMINATION 

 RESISTANCE 



MATCHED 

 OSCILLATOR 



MATCHED LOAD 



Fig. 8 — Measurements of cross talk between adjacent antennas in the MUSA as 

 made from the transmitting point of view. 



the coupling is confined to adjacent antennas and is similar for all 

 pairs so that only the end antennas could be expected to fail to combine 

 properly with the others. At the ends, "dummy" antennas, not con- 

 nected with the receiver but terminated like the others, could be 

 erected to supply the coupling necessary to make all antennas alike. 

 Measurements made on the experimental MUSA (Fig. 2) indicated 

 that the cross talk is small enough to be neglected, however, so that 

 dummy antennas ahead of or behind the six regular ones were con- 

 sidered unnecessary. The performance of the system in subsequent 

 tests corroborates this conclusion. 



The crosstalk measurements yielded the results indicated on Fig. 8. 

 The small amount of crosstalk current (0.0017) measured at the trans- 



" Bruce, Beck, and Lowry, "Horizontal Rhombic Antennas," Proc. I. R. E., 

 vol. 23, pp. 24-46, January, 1935; Bell Sys. Tech. Jour., January, 1935. 



