WAVEGUIDE TRANSMISSION 327 



The sharpest radio beams now in general use are only a few tenths of a 

 degree across. We conclude that for these sharp beams a small but neverthe- 

 less appreciable curvature remains in the radiated wave packet. This means 

 that, when the wave front has arrived at a distant receiver, it is still many- 

 times larger than any receiving antenna it may be practicable to construct, 

 and accordingly the latter can intercept but a small portion of the total 

 advancing wavepower. This implies a considerable loss of power, which is 

 indeed the case. 



In the process of radio reception, one may think of the antenna structure 

 as a device that cuts from the advancing wave front a segment of wavepower 

 which it subsequently guides, preferably without reflection, to the first 

 stages of a nearby receiver. To be efficient, the wavepower intercepted should 

 be large. This, in turn, calls for a receiving antenna of considerable area. It 

 will be remembered that a large aperture was also a necessary feature for 

 high directivity at the transmitter. This is consistent with the accepted view 

 that the processes of reception and transmission through an antenna are 

 entirely correlative and that a good transmitting antenna is a good receiving 

 antenna and vice versa. The directive properties of an antenna are some- 

 times specified in terms of its effective area. (See Section 10.0.) 



The term uniform plane wave is a highly idealized entity assumed in 

 many problems for purposes of simplicity but never quite attained in prac- 

 tice. In an idealized wave front, the electric and magnetic components 

 E and H are not only everywhere mutually perpendicular but both com- 

 ponents are exclusively transverse. That is, there is no component of either 

 E or H in the direction of propagation. Such a wave belongs to a class 

 known as transverse electromagnetic waves (TEM). These may be com- 

 pared with others, to be described later, known as transverse electric waves 

 (TE) and transverse magnetic (TM) waves. Waves guided along parallel 

 conductors are also TEM waves, but except in the case of infinitely large 

 conductors they are not uniform plane waves. 



6.4 Reflection of Space Waves from a Metal Surface 



One of the early triumphs of the electromagnetic theory was its ability 

 to account satisfactorily for the reflection and refraction of light. This 

 theory was so general as to include not only a wide range of wavelengths 

 but also a wide range of surfaces as well. According to this theory, re- 

 flections may occur whenever electromagnetic waves encounter a dis- 

 continuity. This may happen, for example, when waves fall on a sheet of 

 metal, in which case the discontinuity is due to the sudden change in 

 conductivity. Reflection may also occur when waves are incident on a 

 thick slab of glass or hard rubber, in which case reflection is due to a sud- 



