Abstract
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.
Highlights
In 1995, Huynh and Lee showed the addition of a U-shaped slot significantly increased the otherwise narrow impedance bandwidth (BW) of a probe-fed microstrip patch antenna on a low permittivity substrate [1]
The probe has little effect in this geometry, we model it with arbitrary square cross section of 1 mm2
A first-principles mechanism of operation for the U-slot patch based on CMT, as revealed by characteristic mode analysis (CMA), has been presented
Summary
In 1995, Huynh and Lee showed the addition of a U-shaped slot significantly increased the otherwise narrow impedance bandwidth (BW) of a probe-fed microstrip patch antenna on a low permittivity (foam) substrate [1]. This mode behaves as a parallel LC resonator among Ls, Lp, Cs and Cp; e.g., increasing the probe diameter d and slot thickness tw and th reduces Lp and Cs, respectively, thereby increasing the uncoupled slot resonator frequency fslot. The success of the equivalent circuit model implies that the modes of the full geometry are coupled forms of the uncoupled patch and slot resonator modes; recall that the circuit element values of Fig. 11(b) are extracted from CMA data of each separate and uncoupled resonator and that the coupling coefficient is derived from the full geometry resonant frequencies via (2). It is reasonable to conclude that if CMT governs an equivalent circuit that accurately models the U-slot patch, CMT governs the U-slot patch
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