Đang chuẩn bị liên kết để tải về tài liệu:
Electromagnetic Waves and Antennas combined - Chapter 15

Một khái niệm hữu ích là một tản nhiệt đẳng hướng tản nhiệt có cường độ là như nhau trong tất cả các hướng. Trong trường hợp này, tổng bức xạ Prad quyền lực sẽ được phân bổ đều trên mọi góc độ vững chắc, đó là, trên tổng số góc khối của một Ωsphere lĩnh vực = 4π steradians, và do đó, cường độ bức xạ đẳng hướng sẽ được: | 15 Transmitting and Receiving Antennas 15.1 Energy Flux and Radiation Intensity The flux of electromagnetic energy radiated from a current source at far distances is given by the time-averaged Poynting vector calculated in terms of the radiation fields 14.10.4 p Re X FZ I jk Re 0Fg 4 FA X F - Noting that 0 X f we have ớ c I ờ x ớ r c - I f FA0 ộ 2 Therefore the energy flux vector will be ỉ r ỉ rtf F e 2 15.1.1 32rt2r2 Thus the radiated energy flows radially away from the current source and attenuates with the square of the distance. The angular distribution of the radiated energy is described by the radiation pattern factor FA0 2 Fg 0 4 2 Fộ 0 2 15.1.2 With reference to Fig. 14.9.1 the power dp intercepting the area element ds r2dii defines the power per unit area or the power density of the radiation dp _ dp tik2 . 12 dS r2dQ r 32tT2r2 I I power density 15.1.3 The radiation intensity U 0 4 is defined to be the power radiated per unit solid angle that is the quantity dP dil r2dP dS r2Tr-. U e dQ r2Pr 3 radiation intensity 15.1.4 602 1 5. Transmitting and Receiving Antennas The total radiated power is obtained by integrating Eq. 15.1.4 over all solid angles di2 sin 0 d0drf that is over 0 0 7T and 0 2tt f TT f 2n Prad I I U 0 ộ dQ Jo Jo total radiated power 15.1.5 A useful concept is that of an isotropic radiator a radiator whose intensity is the same in all directions. In this case the total radiated power prad will be equally distributed over all solid angles that is over the total solid angle of a sphere iTsphere 4tt steradians and therefore the isotropic radiation intensity will be rr dp Prad Prad 1 p f27Tĩĩíữ ƯỈ -7 U 3 p du du i - sphere 4tt 4tt Jo Jo 15.1.6 Thus Ui is the average of the radiation intensity over all solid angles. The corresponding power density of such an isotropic radiator will be dP ds I Ul _ Prad r2 4rtr2 isotropic power density 15.1.7 15.2 Directivity Gain and Beamwidth The directive gain of an antenna system towards a given direction Ỡ is .