528 ANTENNAS AND RF COMPONENTS 



between the slot and its driving coaxial line. Further, this wasping effect 

 capacitively loads the slot at its center, allowing for a physically smaller 

 slot and cavity than would have been possible otherwise. This is important 

 at low frequencies where components are becoming heavy and cumbersome. 

 The particular antenna shown in Fig. 10-8 is seen to have a secondary 

 antenna consisting of an array of dipole elements at a high frequency and 

 lower power. Because of the frequency difference, they do not couple to 

 one another and the two arrays are conveniently suspended by the same 

 structure. 



10-8 TWO-ARM SPIRAL ANTENNAS 



A final antenna type to be described, finding application to passive radar 

 direction finding and semiactive missiles, is logarithmic in nature. Typical 

 of antennas in this class are the spiral antennas which are wide band and 

 wide beamed. 



Geometry. Flat spiral-wound antennas have been studied extensively 

 by the University of lUinois^^ and others. This antenna in its pure form is 



i2"The Equiangular Spiral Antenna," Technical Report No. 21, Sept. 15, 1957, Electrical 

 Engineering Research Laboratories, University of Illinois, Contract AF33(616)-3220, Project 

 6(7-4600), Task 40572, W.A.D.C. 



