Wireless communication and/or wireless power transmission are highly desired in some of the practical environments fully enclosed by conducting walls. In this paper, a semi-analytical modal analysis is conducted for the purpose of characterizing wireless channels in a fully-enclosed space. The modal analysis is based upon an integral equation method. The cavity Green's function in the spectral domain (that is, expressed in term of cavity modes) is employed in the integral equation. The analysis results indicate that, when a transmitter and a receiver are symmetric to each other with respect to a certain cavity mode, the load of the receiver could be coupled to the transmitter with little dispersion, leading to excellent wireless channels with the potential of accomplishing efficient wireless communication and/or wireless power transmission. A cubic cavity with a side length of 1 meter is analyzed as a specific example, and the modal analysis results are verified by experiments. Measurement data agree with the theoretical analysis very well. As predicted by the theoretical analysis, excellent wireless channels associated with the TM220 mode (with a bandwidth of 40 MHz), TM310 mode (with a bandwidth of 10 MHz), and TM311 mode (with a bandwidth of 20 MHz) are demonstrated inside a cubic box with side length of 1 meter.
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