Equivalent Circuits and Analysis of a Generalized Antenna System [Antenna Applications Corner]

It is well known that the maximum amount of power in an electrical or electronic system can be transferred to a load when its impedance is the complex conjugate of the source impedance, although, the power transfer efficiency is only 50%. Is this fact true for a radio system, as well? The question has existed for decades. In this article, the equivalent circuits of antenna systems are reexamined. It is demonstrated that classical equivalent circuit models based on the Thvenin and Norton theorems are not suitable for a receiving antenna system since the antenna cannot be simply approximated by a voltage or current source. A constant power source is required but not readily available. Thus, two new general power source models are introduced and examined. It is found that they may be suitable for a transmitting antenna system but not accurate enough for a receiving antenna when the antenna loss is taken into account. A novel special constant power source model is therefore proposed and shown to accurately simulate the behavior of a receiving antenna. When the load impedance is the complex conjugate of the antenna impedance, maximum power is obtained, and the power transfer efficiency is simultaneously maximized (up to 100% if the antenna system is lossless).