FUNDAMENTALS 



23 



3.1.3 



Antenna Pattern Factors 

 in Ground Reflection 



With highly directive antennas the magnitude 

 of the direct wave maj' differ appreciably from that 

 of the ground-reflected wave owing to their differ- 

 ence in angle of emergence from the antenna (Fig- 

 ure 2). This must be taken into account bj' using the 



^ sTPt.NCi'^'' OlRtCT RAY 



,,OONOJS!^^^^° 



RAY 



Figure 2. Antenna pattern factors. 



antenna pattern factors Fi and Fo in computing the 

 interference pattern above the line of sight. This 

 subject is dealt with in Section 5.2.6. 



3.1.4 



Standing-Wave Antennas 



An important class of antennas is that in which 

 standing waves of the currents and the voltages are 

 set up. In a transmitting antenna of this type, for 

 instance, a progressive or traveling wave is supplied 

 from the connected source of power. This is re- 

 flected from the end of the antenna and the inter- 

 action of the two sets of waves moving in opposite 

 directions results in a standing-wave system. 



In this event the current amplitude is zero at the 

 ends of the antenna and assumes differing values 

 at the other positions on the antenna. The dis- 

 tribution of current amplitudes is usuallj^ assumed 

 to vary sinusoidally with the distance from the end 

 of the antenna. This is a good approximation where 

 the diameter of the antenna ^^'ire is small compared 

 with the length, but may be seriously in error for 

 thick-wire antermas. 



The simplest, and one of the most commonly 

 used, standing-wave antennas is the half-wave 

 dipole antenna, discussed in Section 3.2.3. 



3.1.5 



Resonant Antennas 



Many antennas are operated at or near resonance, 

 which means that the reactive component of their 

 impedance vanishes or is very small. 



Two types of resonant antenna may be dis- 

 tinguished: either (1) the radiating element as a 

 whole is resonant, as in the case of the half-wave 



dipole, shortened the right amount; or (2) the an- 

 tenna system is made resonant by adding suitable 

 reactive components to the radiative elements. To 

 illustrate, the center-fed half-wave dipole of exactly 

 half-wavelength, assuming sine distribution of cur- 

 rent, has an inductive reactance; it may be made 

 resonant by the addition in series of a capacitive 

 reactance. This is known as antenna loading and is 

 common at the longer wavelengths where half-wave 

 dipoles would be too cumbersome. Another example 

 is that of a dipole radiator shorter than the half- 

 wave dipole and having the form of a metallic tube; 

 this is combined with a tunable cavity resonator 



•-LESS THAN RESONANT LENGTH - 



TUBE 



Figure 3. 

 impedance. 



INPUT 



f 



_L 



j_ 



INNER COAXIAL LINE 

 *-METAL SUPPORT 



Antenna tuned to resonance by a shunt 



inside the tube that acts as a shunt impedance, the 

 whole system being tuned to resonance (Figure 3) . 



Although the actual antenna impedance is made 

 up in a complicated waj'' of distributed capacitances 

 and inductances, the input impedance of the simpler 

 types of antennas for a limited frequency band 

 containing the resonance frequency is essentially 

 that of an ordinary series resonant circuit [the resist- 

 ance at resonance being essentially the radiation 

 resistance of the antenna (see Section 3.1.7)]. The 

 input impedance of certam other antennas is essen- 

 tially that of parallel-resonant circuits with very 

 large shimt resistances at resonance (see Section 

 3.2.2). For illustration, see Figure 6. 



^ * * Traveling- Wave Antennas 



In this type of antenna there is no standing-wave 

 system set up since the progressive or traveling 

 wave of current fed into the antenna is absorbed, 

 without reflection, by a terminal resistance placed 

 at the end of the antenna, which is equal to the 

 characteristic impedance of the antenna regarded 

 as a transmission line. Such antennas are necessarily 

 nonresonant. 



The traveling-wave antenna radiates most strongly 

 in the general direction of the wave motion. The 

 major lobe makes an angle a < 90 degrees with this 



