We propose a new way to enlarge gain bandwidth of a miniaturized reflectarray antenna (MRA), a new type of reflectarray antenna (RA) with significantly reduced volume. To extend the gain bandwidth, we have used meander‐line based sub‐wavelength elements as reflectarray cells, which can fully cover a required reflection phase range ideally, 360°. As a result, we have accomplished a three times wider gain bandwidth than that of our previous MRAs. In addition, the proposed MRA can also provide high gain with good aperture efficiency even though a main beam directs toward a tilted direction. Furthermore, remarkably reduced electrical volume, which is 700 times smaller than the smallest conventional RAs, is still maintained. Measured aperture efficiency of the proposed MRA is 41.1% which is comparable with conventional RAs. And the measurement results show that the main lobe direction is very stable around the target direction within the given −1 dB gain bandwidth without any degradation in other performances. As a result, the proposed MRAs with wide gain bandwidth stands out as a good alternative of the conventional RAs.

We propose an electrically frequency tunable absorber (EFTA) with varactor diodes, which can provide good absorption properties even in much higher frequency bands than conventional EFTAs. In general, conventional EFTAs cannot guarantee high absorption rate beyond a certain frequency limited by high capacitance of commercial varactor diodes. In this letter, however, we suggest a simple but fairly effective way to overcome the high frequency limit by intentionally inserting parasitic capacitance introduced from multiple slits. The parasitic capacitance connected to the varactor diodes in series can decrease overall capacitance to an optimum value. Consequently, we not only can maintain good absorption rate in a very high frequency region, but also freely select a target absorption frequency band by modifying the number of slits. Experiments for normal and oblique incidence have been carried out to validate the proposed method. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:145–148, 2017

We propose a thin electromagnetic wave absorber using varactor diodes combined with intentionally introduced multiple slits, which enables continuous sweep of an absorption frequency band throughout relatively wide frequencies. The absorption frequency range of conventional electrically tunable absorbers has been restricted by high capacitance of varactor diodes. In order to overcome the problem, we introduce parasitic capacitance and connect them with varactors in series, which reduces the total capacitance dramatically. As a result, we can raise the operating absorption frequency up to the X-band region. Moreover, we can also control the operating frequencies by modifying the number of slits with little change in an entire frequency sweep range. Good agreement between simulated and measured results show the validity of our proposal.

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We investigate the overall performance of a miniaturized reflectarray antenna radiating in unusually inclined directions. In our previous work, we successfully reduced an overall electrical volume of a conventional reflectarray antenna by more than hundreds of times. In the previous work, beam scan directions were strictly limited on only one plane, i.e., an H‐plane which is a general beam scan plane for the used source half‐wavelength dipole antenna. However, for unrestricted beam control required to cover entire hemispherical radiation space, beam scanning on an E‐plane is also much required. Accordingly, we examine antenna gain and aperture efficiency for inclined radiation on an E‐plane, which is disadvantageous for the source dipole antenna. To minimize expected antenna gain decrease, each cell of the reflectarray is designed to present desired reflection phase which is required to provide constructive interference between direct and reflected waves coming from the dipole and the reflectarray, respectively. Consequently, even though the proposed antenna scans on an unusual E‐plane including a dipole axis, we can show our miniaturization method can still provide relatively high aperture efficiency, which is important to scan a main beam in any arbitrary directions collaborating on our previous work.