An ultra-wideband (UWB) substrate-integrated-waveguide (SIW) cavity-backed slot antenna covering the lower part of the 3.1-10.6 GHz block allocated to UWB transmission systems, being 3.1-3.6 GHz, is designed, constructed and validated. Owing to its planar topology, low profile and the use of cork substrate material, the proposed antenna may be integrated unobtrusively in any cork surface. Prior to the antenna design, the cork substrate material was characterized in the frequency band of interest. The design is conducted based on the average properties, while maintaining some impedance bandwidth margins to allow for varying cork material properties. A prototype is validated in free-space conditions, confirming the high performance observed in simulation. An impedance bandwidth of 700 MHz (20.9%) is measured. At the center frequency 3.35 GHz, a radiation efficiency of 78%, a front-to-back ratio of 17.2 dB, and a maximum gain of 4.9 dBi are obtained. The maximum gain varies only by 1.4 dB within the frequency band of interest. The other far-field properties also vary only negligibly, which is the most important requirement to UWB antennas.
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