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A compact frequency band reconfigurable dual-mode antenna utilizing both Koch fractal geometry and circular complementary split ring resonators (CSRRs) is presented in this paper. The proposed antenna can be electrically switched into both dual- and single-band resonance modes. A Koch curve derivative novel fractal curve is used to design the patch boundary of the antenna. Two optimized circular CSRRs are loaded inside the antenna geometry to achieve double-band resonance. The antenna operates at both 5.1GHz (upper WLAN band) and 9.6GHz ($X$-band) in dual resonance mode and resonates only at 8GHz ($X$-band) in single resonance mode. A properly biased RF-range PIN diode placed at the slotted ground plane reconfigures the antenna in two operating modes by switching into its ON–OFF states. The antenna is fabricated on cost-efficient epoxy (FR-4) substrate consuming the maximum dimension of $0.34\lambda \times 0.32\lambda$ which is much more compact than the conventional rectangular patch antennas designed for similar bands. Stable radiation characteristics with reduced cross-polarization are achieved at all the resonant bands of the antenna. A prototype of the proposed antenna is fabricated and measured. The simulated results are in good agreement with the measured ones.

In this paper, a third-order, high selectivity Substrate Integrated Waveguide (SIW) bandpass filter (BPF) loaded with Circular Complementary Split-Ring Resonator (CCSRR) is proposed for C band applications. The SIW cavity BPF comprises two stepped resonators ($R_1$ and $R_3$) and one CSRR ($R_2$) engraved on the top metal plate. Initially, the cavity loaded with stepped resonators ($R_1$ and $R_3$) alone does not make a good selectivity on the lower out of band. Hence, the CCSRR is introduced between the stepped resonators ($R_1$ and $R_3$) to improve the filter’s selectivity and enhance the filter’s bandwidth. The introduction of CCSRR enhances the selectivity on the lower out-of-band at 5.5GHz, and the extra pole increases the bandwidth. The metamaterial behavior of CCSRR is validated by permittivity extraction. The 3dB bandwidth of the third-order SIW filter is 840 MHz with a center frequency of 6.18GHz. The designed filter uses RTDuroid 5880 substrate of ${ϵ}_r$ = 2.2 with a size of 28.4 × 28.4 × 0.51mm³. The proposed filter measurement and simulated results agree with each other.