Abstract
Recently, water has been proposed as an interesting candidate for use in applications such as tunable microwave metamaterials and dielectric resonator antennas due to its high and temperature-dependent permittivity. In the present work, we considered an electrically small water-based dielectric resonator antenna made of a short monopole encapsulated by a hemispherical water cavity. The fundamental dipole resonances supported by the water cavity were used to match the short monopole to its feed line as well as the surrounding free space. Specifically, a magnetic (electric) dipole resonance was exploited for antenna designs with a total efficiency of 29.5% (15.6%) and a reflection coefficient of −24.1 dB (−10.9 dB) at 300 MHz. The dipole resonances were effectively excited with different monopole lengths and positions as well as different cavity sizes or different frequencies in the same cavity. The overall size of the optimum design was 18 times smaller than the free-space wavelength, representing the smallest water-based antenna to date. A prototype antenna was characterized, with an excellent agreement achieved between the numerical and experimental results. The proposed water-based antennas may serve as cheap and easy-to-fabricate tunable alternatives for use in very high frequency (VHF) and the low end of ultrahigh frequency (UHF) bands for a great variety of applications.
Highlights
Modern technology calls for increasingly smaller designs, which in turn requires increasingly smaller antennas
Even though electrically small antennas have been studied for decades [1,2,3,4,5,6,7,8,9], their inherently poor matching, low radiation efficiency, and narrow bandwidth pose a serious design obstacle
The matching issue is traditionally tackled with appropriate matching networks; such solutions suffer from narrow bandwidths, high tolerance requirements, and modest efficiencies [2]
Summary
Modern technology calls for increasingly smaller designs, which in turn requires increasingly smaller antennas. While some recent approaches toward efficient small antennas have involved either specific shaping of the radiator or addition of conductors around it [3,4,5,6], as well as exploiting artificial metamaterials (MMs) and MM-inspired structures [7,8,9], a more traditional approach to reduce the antenna size has been to load it with a high-permittivity material. Such antennas, known as dielectric resonator antennas (DRAs), rely on resonances supported in high-permittivity structures. With its relatively high frequency and temperature-dependent permittivity in the microwave frequency range [12], water holds a great potential as an inexpensive, abundant, and
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