Perforated ground plane (PGP) structures have many interesting properties, such as low-wave effect, suppression of surface waves, high-impedance characteristics, and arbitrary stopbands (passbands). In recent years, PGP structures have attracted great interests in RF and wireless applications (such as passive and active circuits, antennas, waveguides, and packaging). PGP structures are created by assembling a periodic or nonperiodic structure composed of elements in ground plane which are small compared to the wavelength of the propagating electromagnetic wave. As a consequence, PGP structures alter electromagnetic waves propagate in the RF and wireless system.

The objective of this special issue is to provide an in-depth description of the state of the art of research and development in this area. The themes include the new challenges and recent applications in the field of PGP structures for RF and wireless components. Of 17 submissions, 8 papers are accepted for publication in this special issue. Each paper was reviewed by at least two reviewers and revised according to the reviewer’s comments. The accepted papers cover the following topics: analysis of the microstrip patches over ground planes with rectangular apertures, couplers with defected ground structure (DGS) microstrip line, DGS resonators, slotted magnetic-LC resonators, bandpass filter (BPF) with slot spurline technique, reconfigurable antenna, and heterostructure high-mobility transistors (HEMTs).

Microstrip patch antennas offer many attractive features such as low profile, light weight, low cost, and ease for integration with printed feeding networks and active circuits. The microstrip patch antenna can be fed through an aperture cut into a microstrip line ground plane. Several advantages, such as the weak parasitic radiation, tunable impedance and resonant frequency of antennas, have been obtained by using this feeding configuration. However, the algorithms developed for the analysis of these antennas should be able to account for the effect of possible apertures existing in the ground planes of the resonators. In A. Messai et al.’s paper, the authors present a rigorous full-wave analysis of high superconducting rectangular microstrip patch over ground plane with rectangular aperture in the case where the patch is printed on a uniaxially anisotropic substrate material. The dyadic Green’s functions of the considered structure are efficiently determined in the vector Fourier transform domain. The effects of uniaxial anisotropy in the substrate on the resonant frequencies of different TM modes of the superconducting microstrip antenna with rectangular aperture in the ground plane are analyzed. The accuracy of the analysis is tested by comparing the computed results with measurements and previously published data for several anisotropic substrate materials.

Microstrip lines with the defected ground structure (DGS), namely, DGS microstrip line or DGS line, can raise the realizable upper limit of the characteristic impedance of a microstrip line to around 200 Ω and can be used in unequal power dividers and couplers. While there has been a serious problem when they are packaged in metallic housing because the bottom ground plane of the microstrip lines, where DGS patterns are realized, makes direct contact with the metallic package, at this time, the advantageous effects of DGS are removed. In J. Lim et al.’s paper, the authors adopt a double-layered substrate structure to solve the ground contact problem of DGS lines. In the proposed structure, DGS patterns are realized on the ground plane of microstrip lines as in previous cases. However, the second substrate, of which dielectric material is exposed to the top plane, is attached to the ground plane of the first substrate, where the DGS patterns exist. Therefore, it is possible to remove the ground contact problem of DGS, while the advantages of DGS are preserved. At last, they have designed and measured a 10 dB branch line hybrid coupler using DGS line.

Recently, microwave circuits are popularly designed on the ground plane, such as defected ground structure (DGS) resonator, which fully utilize the printed circuit board and therefore reduce the overall circuit size, also with the high power capacity. It provides a novel way to realizing the microwave passive components. In this special issue, there are two papers on the topic of the DGS resonator or its application. In S. J. Lee et al.’s paper, the authors presented a flexible microwave tag system using a frequency-scanning type RFID with multiple DGS resonator. The DGS resonator is spiral shaped and implemented on the rear side of a transmission line, which has the advantages of excellent band notch characteristics as well as bit-error avoidance from the frequency selective reflection. In addition, the tag system is designed on a thin flexible substrate in order to be applicable for amorphous surfaces. In X. Guan et al.’s paper, the authors designed a triple-mode bandpass filter using a dual-mode DGS resonator and a microstrip resonator. The dual-mode characteristic is achieved by loading a defected T-shaped stub to a uniform impedance DGS resonator. A uniform impedance microstrip resonator is designed on the top layer of the DGS resonator and a compact bandpass filter with three resonant modes in the passband can be achieved. The authors have also given the coupling scheme and matrix for the structure.

Split-ring resonators (SRRs), capacitive loaded (C loaded) loops, folded stepped impedance resonators (SIRs), and their complementary counterparts have been extensively used for the implementation of metamaterials and many devices based on them. The magnetic-LC (MLC) resonator can be etched in the ground plane of a single-ended microstrip or a differential microstrip line. In J. Naqui et al.’s paper, the authors presented the MLC-inspired circuits where the MLC units are loaded in both single-ended microstrip and differential microstrip transmission lines. By loading an MLC with the differential microstrip line, for the differential mode, there is an electric wall at the symmetrical plane, and thus the MLC can be driven. Due to the symmetry property, the symmetrical plane of this MLC-loaded single-ended microstrip transmission line becomes a magnetic wall, and the MLC cannot be excited. They also demonstrated that the MLC resonators have many potential applications such as the implementation of balanced notch filters and stopband filters.

Dual-mode microstrip filters are widely used in wireless communications systems, which are based on the coupling of two degenerate modes and split by adding a perturbation element in a geometrically symmetrical resonator. Recently, resonator with slotline structure has attracted more attention because of its compactness and high power capacity. In H. Liu et al.’s paper, the authors proposed a miniaturized dual-mode bandpass filter using slotline resonator, slot spur lines, and sagittate stubs. The type of the filter characteristic (chebyshev or elliptic) can be controlled by the location of the spur lines. In addition, the filter achieves large reduction in overall size by using the sagittate stubs instead of the traditional ones.

With advantage of multipattern and multipolarization, the reconfigurable antennas are popular and timely to address complex system requirements by modifying their geometry and electrical behavior, which can increase the capabilities of wireless integrated information systems, expand their functionality, or widen their bandwidths, with efficient spectrum and power utilization. In A. Chen et al.’s paper, the authors discussed different patterns and polarizations of reconfigurable antennas according to current research work in this area. The radiation pattern states of antennas include beam direction, shape, and gain, and the polarization states of antennas consist of horizontal/vertical linear, ±slant 45° linear, left-hand, or right-band circular polarized. Different multi-pattern and multi-polarization antennas with various structures and working mechanisms are compared and discussed in the paper.

By the great development of the material quality and device processing techniques, AlGaN/GaN HEMT has been much improved in both DC and RF performances. It will become an ideal candidate for high-power, high-frequency, and high-temperature electronic devices. There are many researches about improving its DC and AC performances by changing material or structure of epitaxial layer of AlGaN/GaN HEMTs since the performances of monolithic microwave integrated circuits (MMICs) are influenced by the characteristics of active devices. In L. Jin et al.’s paper, the authors presented the impact of layout sizes of Al_0.27Ga_0.73N/AlN/Al_0.04Ga_0.96N/GaN HEMTs based on SiC substrate on its characteristics that include the threshold voltage, the maximum transconductance, characteristic frequency, and the maximum oscillation frequency. The changing parameters include the gate finger number and the gate width per finger. It is significant for designing AlGaN/GaN HEMT with excellent performance.

These papers represent an exciting and insightful observation into the state of the art, as well as emerging research topics, in this important field.

Acknowledgments

We hope that this special issue would attract a major attention of the peers. We would like to express our appreciation to all the authors and reviewers for great support to make this special issue possible.

Haiwen  Liu
Lei  Zhu
Dal  Ahn
Amin  Abbosh
Sha  Luo