RADAR A NTENNAS 231 



may require corrections for the effect of the "ground", and for the effect 

 of the transmission medium, which are beyond the scope of this paper. 



2.6 The Plane, Linearly Polarized Electromagnetic Wave 



In the foregoing sections we have referred several times to 'plane, linearly 

 polarized electromagnetic waves'. These waves occur so commonly in 

 antenna theory and practice that it is worth while to discuss them further 

 here. 



Some properties of linearly polarized, plane electromagnetic waves are 

 illustrated in Fig. 7. At any point in the wave there is an electric field and 

 a magnetic field. These fields are vectorial in nature and are at right 

 angles to each other and to the direction of propagation. It is customary 

 to give the magnitude of the electric field only. 



If we use the M.K.S. system of units the magnitudes of the fields are 

 e.xpressed in familiar units. Electric intensity appears as volts per meter 

 and magnetic intensity as amperes per meter. The ratio of electric to 

 magnetic intensity has a value of 1207r or about 377 ohms. This is the 

 'impedance' of free space. The power flow per unit area is e.xpressed in 

 watts per square meter. We see, therefore, that the electromagnetic wave 

 is a means for carrying energy not entirely unlike a familiar two wire line 

 or a coaxial cable. 



Electromagnetic waves are generated when oscillating currents flow in 

 conductors. We could generate a plane linearly polarized electromagnetic 

 wave with a uniphase current sheet consisting of a network of fine wires 

 backed up with a conducting reflector as shown in Fig. 7. This wave could 

 be absorbed by a plane resistance sheet with a resistivity of 377 ohms, also 

 backed up by a conducting sheet. The perfectly conducting reflecting 

 sheets put infinite impedances in parallel with the current sheet and the 

 resistance sheet, since each of these reflecting sheets has a zero impedance 

 at a spacing of a quarter wavelength. 



A perfectly plane electromagnetic wave can exist only under certain ideal 

 conditions. It must be either infinite in extent or bounded appropriately 

 by perfect electric and magnetic conductors. Nevertheless thinking in 

 terms of plane electromagnetic waves is common and extremely useful. In 

 the first place the waves produced over a small region at a great distance 

 from any radiator are essentially plane. Arguments concerning receiving 

 antennas therefore generally assume that the incident waves are plane. In 

 the second place an antenna which has dimensions of many wavelengths can 

 be analyzed with considerable profit on the basis of the assumption that it 

 transmits by producing a nearly plane electromagnetic wave across its 

 aperture. This method of analysis can be applied to the majority of micro- 

 wave radar antennas, and will be discussed in the following sections. 



