Chapter 3 

 ANTENNAS 



*' FUNDAMENTALS 



^ ' ' Function of Antennas 



ATRAXSMiTTiNG AXTEXXA convert:^ the power 

 delivered to it into electromagnetic radiation 

 (neglecting losses); a receiving antenna abstracts 

 power from an incident electromagnetic wave and 

 delivers to the receiver that part which is not re- 

 radiated or lost in the antenna. In the short and 

 microwave region the power conversion is effected 

 with a very small loss so that for most practical 

 purposes the power loss inside the antenna may be 

 disregarded. Apparent losses caused bj' reflection 

 owing to mismatch between the antenna and its 

 input circuit are of a different nature and are not 

 included herein. 



For many purposes it is desirable to concentrate 

 the power radiated into a beam of comparatively 

 small angle as in this way the field strength in the 

 preferred direction is enhanced. The gain of a 

 directional antenna is defined by means of a com- 

 parison of the given antenna radiation pattern with 

 that of an electric doul^let. 



The gain of an antenna is the ratio of power that 

 must be supplied to a doublet to the power that must 

 be supplied to the antenna considered in order that, 

 at a gi\'en large distance, the electric field at the 

 maximum of the antenna pattern is equal to the 

 field at the same distance in the equatorial plane of 

 the doublet. From the reciprocity principle it is 

 foimd that the gain of a recei\'ing antenna is eciual 

 to the gain of the same antenna used as a transmitter. 

 A discussion of antenna gain and reciprocity is given 

 in Chapter 2, Section 2.1. 



^•'•^ Directive Antennas 



Polar plots of antenna radiation patterns are of 

 two kinds: either the relative magnitude of the 

 Poynting vector (power per unit area) is plotted 

 along the radius vector, or the relative magnitude of 

 the radiation electric field strength is plotted in the 

 same wav. Usuallv the value of the radius vector at 



the maximum of the pattern is taken equal to unity. 

 The Poynting vector plot is obtained from the field 

 strength plot b.^- squaring the radial distances 

 (Figure 1). 





HALF 

 POWER PT 



Tzo? 



Figure 1. Antenna radiation patterns. 



If an antenna system is designed so that most of its 

 power is concentrated into a comparatively small 

 cone, the corresponding part of the radiation pattern 

 is called the main lobe. Commonly there are a num- 

 ber of secondary maxima (side lobes) much smaller 

 than the main lobe. The \\idt.h of the main lobe is 

 measured by the angle lietween half-power points. 

 Half-power points are those points in the polar 

 diagram of the antenna pattern where the power 

 per unit area is equal to one-half that at the maxi- 

 mum, the field strength being I/V2 = 0.707 times 

 that at the maximum. This angle is also referred 

 to as the beam width. The beam width varies from a 

 degree or less for some specialized radar antennas to 

 very large angles such as 50 to 60 degrees, depending 

 on the design and purpose of the antenna. The 

 larger the beam \\'idth the smaller the gain. 



It should be noted that an antenna radiation 

 pattern may have high directi\'ity with respect to 

 one plane going thr(jugh the antenna and little or 

 no directivity in another plane. Thus a doublet 

 antenna (for definition see Section 2.1.1) is directive 

 in a plane which contains the antenna itself but is 

 nondirective in the equatorial plane perpendicular 

 to the antenna (see Figure 11). 



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