RADAR ANTENNAS 225 



through the appUcation of electromagnetic theory. Maxwell's equations 

 provide a general and accurate foundation for antenna theory. They are 

 the governing authority to which the antenna designer may refer directly 

 when problems of a fundamental or bafHing nature must be solved. 



It is usually impracticable to obtain theoretically exact and simple solu- 

 tions to useful antenna problems by applying Maxwell's Equations directly. 

 We can, however, use them to derive simpler useful theories. These 

 theories provide us with powerful analytical tools. 



Lumped circuit theory is a tool of this sort which is of immense practical 

 importance to electrical and radio engineers. As the frequency becomes 

 higher the approximations on which lumped circuit theory is based become 

 inaccurate and engineers find that they must consider distributed in- 

 ductances and capacitances. The realm of transmission line theory has 

 been invaded. 



Transmission line theory is of the utmost importance in radar antenna 

 design. In the first place the microwave energy must be brought to the 

 antenna terminals over a transmission line. This feed line is usually a 

 coaxial or a wave-guide. It must not break down under the voltage which 

 accompanies a transmitted pulse. It must be as nearly lossless and reflec- 

 tionless as possible and it must be matched properly to the antenna terminals. 



The importance of a good understanding of transmission line theory does 

 not end at the antenna terminals. In any antenna the energy to be trans- 

 mitted must be distributed in the antenna structure in such a way that the 

 desired radiation characteristics will be obtained. This may be done with 

 transmission lines, in which case the importance of transmission line theory 

 is obvious. It may be done by 'optical' methods. If so, certain trans- 

 mission line concepts and methods will still be useful. 



While it is true that transmission line theory is important it is not nec- 

 essary to give a treatment of it in this paper. Adequate theoretical dis- 

 cussions can be found elsewhere in several sources.^ It is enough at this 

 point to indicate the need for a practical understanding of transmission line 

 principles, a need which will be particularly evident in Part II, Methods 

 of Antenna Construction. 



We may, if we like, think of the whole radar transmission problem in 

 terms of transmission line theory. The antenna then appears as a trans- 

 former between the feed line and transmission modes in free space. We 

 cannot, however, apply this picture to details with much effectiveness unless 

 we have some understanding of radiation. 



In the sections to follow we shall deal with some theoretical aspects of 

 radiation. We shall begin with a discussion of fundamental transmission 



1 See, for example, S. A. Schelkunoff, Electromagnetic Waves, D. Van Nostrand Co., 

 Inc., 1943, in particular. Chapters VII and VIII, or F. E. Terman, Radio Engineer's Hand- 

 book:, McGraw-Hill Book Co., Inc., 1943, Section 3. 



