1.2 How to Quantitatively Evaluate an Antenna

After designing an antenna, we cannot say whether it is good or bad by simply looking at it. We must find a way to quantitatively evaluate it. In cellular antenna's designs, the frequently used parameters are the reflection coefficient, the voltage standing wave ratio (VSWR), efficiency, gain, and bandwidth. The contents of this section are only a brief review of frequently used parameters. More comprehensive materials and detailed deductions can be found in some classical textbooks [3, 7, 8, 9, 10].

From the circuit point of view, an antenna is a single port device. A transmission line can be used to feed the antenna, as shown in Figure 1.7. An input signal takes the form of an incident wave traveling along the transmission line. It flows from the signal source toward the antenna. Assuming the amplitude of the incident wave is V_incident. At the antenna port, some of the energy carried by the incident wave is radiated by the antenna. In the meantime, the residual energy is reflected at the port and travels back along the transmission line. The amplitude of the reflected wave is V_reflected.