Читать книгу Antenna and EM Modeling with MATLAB Antenna Toolbox - Sergey N. Makarov - Страница 20

After the antenna has been matched to the generator, the legitimate question to ask is how to find the antenna efficiency, E. Since R_a = R_r + R_L > R_r, only a part of the power delivered to the antenna is really radiated; another part is dissipated in the antenna itself and makes it a heater.

The corresponding equivalent circuit of the antenna with losses includes two resistors in series: the radiation resistance R_a and the loss resistance R_L. Since the same current i(t) flows through both resistors at any time instant, we can find the radiation efficiency or simply the efficiency of the antenna in the form of the ratio of two powers: the radiated power and the total power delivered to the antenna. Those powers can either be given in terms of average values or by their instantaneous values. For example, at any time instant

(1.10)

In other words, the efficiency is a sole function of the ratio of two resistances. When the loss resistance is zero, the efficiency percentage is 100%; when the loss resistance is equal to the radiation resistance, the efficiency percentage is 50%. The efficiency definition Eq. (1.10) holds for any antenna, whether resonant or not.