Major advances in millimeter-wave antennas have been made in recent years, in particular in two areas. A new class of leaky-wave antennas based on open millimeter waveguides has been proposed and investigated, and substantial progress has been achieved in integrated antennas where active and passive circuits, possibly in monolithic form, are combined with the radiating elements in one compact unit. Interesting developments have taken place also in a third group of millimeter-wave antennas, that of microstrip antennas and printed circuit antennas in general. An attempt is made to summarize these developments. >

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Millimeter wave antennas

A hybrid modal solution is developed for the circular semi-infinite dielectric rod antenna excited in the HE_{11} hybrid mode. Numerical results for the near-zone and far-zone fields, as well as the gain, beamwidth, and percentage of power radiated, are determined for three rods of different diameter and dielectric constant by enclosing the rods with a concentric metal pipe and allowing the radius of the pipe to become large. Comparison is made between these results and previous approximate methods of solution that have been applied to the dielectric rod antenna.

A modal analysis of the dielectric rod antenna excited by the HE 11 mode

The problem of the dielectric rod antenna is studied numerically. The surface integral equation is used to simulate the problem in its actual form. From the surface currents induced on the antenna surface, the far-field radiation patterns are calculated. Different parameters are considered, such as the extension of the dielectric from the waveguide aperture, the waveguide and the dielectric diameter, the dielectric constant and finally the effect of shaping of the dielectric end. The performance of the antenna as a feed for the reflector antenna is also studied by calculating the gain factor, spillover efficiency and the phase center location of the feed. Some of the results are compared with the available experimental data of Dombek. The agreement is found to be excellent.

Radiation characteristics of the short dielectric rod antenna: A numerical solution

Ultrawide band (UWB) nondestructive testing and evaluation techniques can provide the detection of defects and damages in construction materials and members. The development of UWB antennas with proper dimensions and high performance for these systems is one of the major challenges. In this letter, an antipodal Vivaldi antenna (AVA) has been developed to be used in UWB techniques for nondestructive detection of cracks in concrete members. To obtain a desirable antenna performance including the wider impedance bandwidth, the higher directivity, and higher front-to-back ratio rectangular-shaped slits (RSS) and a half-circular-shaped lens (HCSL) have been added in the design of a conventional tapered slot AVA. A parametric study on the dimensions of the slits is provided to optimize performance and size of the proposed antenna. The optimum size of the proposed antenna is 40 × 80 × 0.508 mm3, and it operates over the frequency range from 3.4–40 GHz. It is shown that the RSS increase impedance bandwidth especially at the lower frequencies. Directivity of the proposed antenna at higher frequencies has been increased by the HCSL. The feasibility of the proposed antenna for nondestructive evaluation of concrete members is demonstrated by investigation into its performance with concrete specimens possessing an air-gap inside a concrete specimen. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1573–1578, 2015

Development of the antipodal Vivaldi antenna for detection of cracks inside concrete members

This letter proposes a dielectric helical antenna made of three-dimensional (3-D) printed zirconia ceramic, which possesses a better performance than the conventional metal one. On the one hand, the radiation principle of the proposed antenna is investigated by regarding its structure as a dielectric waveguide. It is found that a dielectric waveguide operating at the cut-off state of higher mode, here is TM01 mode, can radiate electromagnetic (EM) waves. The propagation constant of EM waves in waveguide is extracted to demonstrate that the dielectric waveguide indeed works as a fast-wave status at this moment. On the other hand, both the far-field and scattering performances between the proposed and conventional helical antennas are comparatively studied. It is observed that the structure composed of dielectric helix and metal ground can achieve similar radiation gain, axial ratio (AR), and AR bandwidth as the conventional all-metal helical antenna, but possessing lower radar cross section at high frequency. Benefited from a ceramic stereolithography apparatus, the proposed structure can be fabricated precisely, and measured results have reasonably validated the simulated ones.

Radar Cross-Section Reduction of Helical Antenna by Replacing Metal With 3-D Printed Zirconia Ceramic

Present-day theory of uniform cylindrical dielectric-rod antennas rests on the three theoretical treatments of Horton, Bouix and Fradin (leaky-guide theory), and on Brown's end-fire theory A digression on the general relationship between Schelkunoff's equivalence principle and the vector Kirchhoff formula reveals that these radiation formulas may only be applied to regions in which the vector potentials are known to satisfy the inhomogeneous wave equation On applying this axiom to the theories, all but Brown's are shown to be fallacious A detailed application of the vector Kirchhoff formula to uniform cylindrical dielectric rods supporting the HE11 mode produces an expression for the far-field radiation pattern, which, when computed, gives good agreement with existing experimental data The analysis allows for dielectric losses and can also be extended to nonuniform dielectric antennas with gradual tapers Fresnel interaction of the apertures and lens effects are considered Finally, some design recommendations for dielectric rod antennas are made

J.R. James

Papers

Theoretical investigation of cylindrical dielectric-rod antennas