A rectangular cavity for cell phone antenna efficiency measurement

Electromagnetic cavities can be used to measure antenna efficiency. Enclosing an antenna within a conducting rectangular or spherical cavity can eliminate the radiation resistance from the impedance without changing the loss resistance of the antenna. The cavity size should avoid both the disturbance of the field near the antenna and the cavity´s resonance. The efficiency of the antenna is calculated from the measurement of its input resistance inside the cavity and in free space. The paper uses the same approach to compute cell phone antenna efficiency. First, the size of the rectangular cavity is chosen and its resonant frequencies closest to the US cellular band (824-894 MHz) are determined. Then, a monopole antenna is placed inside the cavity in order to excite a coupling between the antenna and the cavity at those resonant frequencies. A simple model of a bar cell phone is then placed inside the cavity, and the changes in the coupling are analyzed. Finally, a cell phone with its antenna connected to a coaxial cable is placed in the cavity, and the input resistance is measured. Efficiency values measured from the cavity are compared to the values obtained by the radiation pattern integration method. The results show that if a cavity is chosen appropriately, it can effectively be used for measurement of cell phone antenna efficiency.