226 BELL SYSTEM TECHNICAL JOURNAL 



principles. This discussion is applicable to all antennas regardless of how 

 they are made or used. When applied to radar antennas it deals chiefly 

 with those properties of the antenna which affect the radar range. 



Almost all microwave radar antennas are large when measured in wave- 

 lengths. When used as transmitting antennas they produce desired radia- 

 tion characteristics by distributing the transmitted energy over an area or 

 Svave front'. The relationships between the phase and amplitude of elec- 

 trical intensity in this wave front and the radiation characteristics of the 

 antenna are predicted by 'ivave front analysis. Wave front analysis is 

 essentially the optical theory of diffraction. Although approximate it 

 applies excellently to the majority of radar antenna radiation problems. 

 We shall discuss wave front analysis in Section 3. 



2. Transmission Principles 

 2.1 Gain and Effective Area of an Antenna 



An extremely important property of any radar antenna is its ability to 

 project a signal to a distant target. The gain of the antenna is a number 

 which provides a quantitative measure of this ability. Another important 

 property of a radar antenna is its ability to collect reflected power which 

 is returning from a distant target. The efectiie area of the antenna is a 

 quantitative measure of this ability. In this section these two quantities 

 will be defined, and a simple relation between them will be derived. Their 

 importance to radar range will be established. 



Definition of Gain. When power is fed into the terminals of an antenna 

 some of it will be lost in heat and some will be radiated. The gain G of 

 the antenna can be defined as the ratio 



G = P/Po (1) 



where P is the power flow per unit area in the plane linearly polarized elec- 

 tromagnetic wave which the antenna causes in a distant region usually in 

 the direction of maximum radiation and Po is the power flow per unit area 

 which would have been produced if all the power fed into the terminals 

 had been radiated equally in all directions in space. 



Definition of Effective Area. When a plane linearly polarized electromag- 

 netic wave is incident on the receiving antenna, received power Pr will be 

 available at the terminals of the antenna. The effective area of the antenna 

 is defined, by the equation 



A = Pn/P' (2) 



where P' is the j^ower per unit area in the incident wave. In other words 

 the received power is equal lo ihc j)ower flow through an area that is equal 

 to the effective area of the antenna. 



