  Patch antennas incorporating a U-shaped slot are well-known to have
relatively large (about 30%) impedance bandwidths. This work uses
Characteristic Mode Analysis to explain the impedance behavior of a classic
U-slot patch geometry in terms of Coupled Mode Theory and shows the relevant
modes are in-phase and anti-phase coupled modes whose resonant frequencies are
governed by Coupled Mode Theory. Additional analysis shows that one uncoupled
resonator is the conventional TM01 patch mode and the other is a lumped LC
resonator involving the slot and the probe. An equivalent circuit model for the
antenna is given wherein element values are extracted from Characteristic Mode
Analysis data and which explicitly demonstrates coupling between these two
resonators. The circuit model approximately reproduces the impedance locus of
the driven simulation. A design methodology based on Coupled Mode Theory and
guided by Characteristic Mode Analysis is presented that allows wideband U-slot
patch geometries to be designed quickly and efficiently. The methodology is
illustrated through example.
