In this letter, a compact, low-profile, bandwidth enhanced, dual-cavity substrate integrated waveguide (SIW) filtenna is demonstrated. Two SIW cavities are stacked vertically on top of each other. A complementary split ring resonator (CSRR) slot is etched in the top surface of the uppermost cavity, causing the top surface to act as a patch antenna. The operational impedance bandwidth is significantly enhanced by merging the three resonances that arise from this configuration. One is introduced by the patch and the other two are inherently generated by the two cavities. A metallized coupling post is introduced from the ground plane through both cavities to the upper surface to excite the fundamental resonant mode of the patch, as well as to electromagnetically couple the two cavities. The optimized filtenna was fabricated by standard printed circuit board (PCB) technology and tested. It has a low profile 0.03λ 0 and compact size 0.62 λ 0 ×0.62 λ 0 at its center frequency, f 0 = 2.95 GHz. The measured results agree well with their simulated values. They demonstrate a 6.3% fractional bandwidth, a maximum realized gain of 6.73 dBi, a flat gain profile within its passband, and excellent out-of-band selectivity. Index Terms-Bandwidth, compact antenna, filtenna, low-profile antenna, patch antenna, substrate integrated waveguide (SIW)I. INTRODUCTION riven by the increasing demand for smaller, cheaper wireless communications and sensor devices, platform miniaturization has become a key consideration. This trend requires size reductions in both the radio frequency (RF) antennas and the associated front-end circuits. While progress in printed-circuit board (PCB) technologies and electrically small antennas has occurred, the use of multifunctional components has proven to be another effective methodology to

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Compact, low-profile, linearly (LP) and circularly polarized (CP) patch-based filtennas are realized with a custom-designed coupling probe. It introduces a deep null at both the lower and the upper band edges of the filter response. These two nulls facilitate a quasi-elliptic bandpass behavior and can be independently controlled to achieve sharp band-edge skirts and high out-of-band suppression levels. The CP version evolves from the LP design by introducing a T-shaped near-field resonant parasitic (NFRP) element near the probe to create two transmission paths with an inherent 90° phase difference. Its presence facilitates the simultaneous excitation of the TM 10 and TM 01 modes of the patch without the need for any power divider or phase delay line, reducing the design complexity and lowering the insertion loss. Prototypes were fabricated, assembled, and tested. The measured results agree well with their simulated values. They are low profile (0.03 λ 0 height) and compact in size (0.04 λ 0 2 footprint). The LP and CP prototypes exhibit, respectively, a 10-dB fractional impedance bandwidth of 7% and an overlapping axial ratio fractional bandwidth of 4.5%. Excellent measured performance characteristics are demonstrated including flat passband realized gain values and filter responses with sharp roll-off rates and high out-of-band suppression levels. Index Terms-Circularly polarized antennas, compact antennas, filtennas, linearly polarized antennas, low-profile antennas, near-field resonant parasitic elements I. INTRODUCTION ith the rapid expansion in modern wireless communication technologies, progress continues towards antenna systems that are more compact, multifunctional with high efficiencies, and more easily integrated with the electronics. The recent emergence of

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