24 



ANTENNAS 



direction as indicated in Figure 20. Here we ha\-e a 

 long-wire antenna with tlie input at the left and the 

 characteristic impedance (resistance) at the right. 



A traveling-wave V antenna uses two of these 

 elements (see Section 3.3.2) and a rhombic is com- 

 posed of four elements (see Section 3.3.3), with the 

 elements arranged at angles A\hich produce maximum 

 directivity of the combinations. 



Antennas of the nonresonant or tra\-eling-wa\'e 

 types are used both for longer and for very short 

 waves. (However, there is an intermediate fre- 

 quency region extending from about 100 to 3,000 mc 

 where the half-wave dipole is of such con\'enient 

 size that standing-wave dipoles or dipole arrays are 

 most frequently employed.) 



In the microwa\'e band ^vhere transmission is 

 effected by wave guides it is possible to terminate a 

 wave guide with a horn which "matches the imped- 

 ance of the wa\'e guide to that of free space" and 

 acts as a directive antenna (Section 3.7). A slot 

 or a series of slots in the side of a wave guide may 

 also act as an antenna at these frequencies. 



3.1.7 



Radiation Resistance 



The radiation resistance Rr of an antenna is the 

 ratio P, of the total power radiated in all directions 

 to the square of the current at the point of measure- 

 ment. The power may l)e computed by integrating 

 the radial component of the Poynting vector over a 

 spherical surface surrounding the antenna. Then 

 if li is the effective value of the input current, 



(1) 



Rr = 



Ir 



The radiation resistance of the doublet antenna is 

 stated in equation (9) in Chapter 2 to be 



fi, = 80,fj 



ohms. 



(2) 



3.1.8 



Influence of Near-by 

 Condiictinfi Bodies 



The impedance of an antenna is affected by the 

 presence of conductors in the vicinity and depends 

 upon the mutual impedances between the conductors 

 and the antenna. The mutual impedance decreases 

 ^vith increasing distance so that for conducting 

 bodies of comparable size the effect is negligible for 



distances greater than, perhaps, 2 to 3 wavelengths. 



But for conductors set less than a wavelength 

 apart, such as an antenna and reflector (or director) 

 combination or as antenna arrays, the mutual 

 effect plays an important role and modifies the 

 input impedance of the antenna. 



For an antenna set near a large conducting body, 

 such as a large metallic sheet or the earth, the mutual 

 effect is cared for in a different way. If the earth, for 

 instance, is assumed plane and perfectly conducting, 

 its effect is the same as that of the mirror image 

 of the antenna in the gromid. As shown in Figure 4, 



1 ANTENNA- 



PLANE EARTH 



PERFECT CONDUCTOR 



-*• IT/ IMAGE 



VERTICAL HORIZONTAL 



Figure 4. Method of images. 



the image of a vertical antenna is a similar antenna 

 with current m the same direction, while the current 

 is reversed for a horizontal antenna. The radiation 

 field at any point above ground is obtained by 

 summing the radiation fields of antenna and image. 



3-! STANDING- WAVE ANTENNAS 



3^' Linear Antennas 



A linear antenna is a straight thin rod supplied 

 with alternating current. According to whether the 

 connection to the antenna is made at the middle or 



END FED 

 ALTERNATE CURRENTS 



CENTER FED 

 CO-PHASED CURRENTS 



Figure 5. Distribution of current amplitutles with 

 linear antennas. 



