Abstracts:In this letter, a wideband transition for exciting the second higher order mode on microstrip line is presented. Different from the traditional excitation method, a systematical design approach is first conducted. The field distribution of higher order modes could be well matched and transformed into quasi-TEM mode by a taper structure. Based on the study of the orthogonal principle of different modes, each mode could be effectively excited by a specific amplitude&#x002F;phase condition of the feeding network. To meet the wideband amplitude and phase radio requirements among different ports of the feeding network, a multimode resonator-based phase shifter could be utilized to achieve 180&#x00B0; phase difference. To validate this proposal, a wideband EH₀-to-EH₂ mode transition with 60&#x0025; bandwidth is proposed, designed, and implemented. Both the extracted propagation constant and the back-to-back <italic>S</italic>-parameters show good wideband response over the concerned frequencies.

Abstracts:In this letter, transmitting metasurface using single dielectric substrate for orbital angular momentum (OAM) generation is presented in <italic>Ku</italic>-band. A dimension extension method is introduced in designing the unit cell of the proposed transmitting metasurface to achieve required phase shift and transmission magnitude for generating OAM. A prototype of the proposed transmitting metasurface is fabricated and measured. Measured results suggest that the proposed transmitting metasurface can generate OAM vortex wave at 18 GHz. The proposed method can be applied in wireless communication systems requiring low-profile transmitted OAM generator.

Abstracts:This letter proposes a circularly polarized patch antenna with a compact volume of 91.9 mm³ for implantable applications. Foremost, the input impedance of patch antenna with reduced ground inside lossy muscle tissue is analyzed. Then following the traditional way of realizing circular polarization (CP) performance, crossed slots are introduced into the patch center. However, as the working environment is lossy tissue rather than traditional air, additional <italic>LC</italic> elements are needed to meet the impedance matching of 50 &#x03A9;. Thus, besides crossed slots in the center, arc-shaped slots and shorting pins are loaded on the antenna edge. Simulated results show that a CP bandwidth of 18.3&#x0025; is obtained, proving the antenna is suitable for biomedical devices.

Abstracts:A dual-polarized omnidirectional antenna with simple feed and ultrawide operational bandwidth is presented. The antenna is a combination of four vertical monopoles and four horizontal dipoles. The four vertical monopoles are printed on two thin FR4 substrates arranged orthogonally to each other. This arrangement not only makes the design easy to fabricate, but also significantly simplifies the feed and design of the horizontal dipoles. To avoid the feeding intersection, the feedline of the horizontal dipoles is placed at an off-center position. This allows placing three additional metallic posts, which makes the system mechanically stable while preserving the design symmetry for a high isolation. The final design with 0.23<inline-formula><tex-math notation="LaTeX">$lambda _{text{min}}$</tex-math></inline-formula> profile was realized (<inline-formula><tex-math notation="LaTeX">$lambda _{{min}}$</tex-math></inline-formula> is the free-space wavelength at the minimum operating frequency). An overlapped bandwidth of 78.6&#x0025; (1.70&#x2013;3.90&#x00A0;GHz) with a measured isolation of more than 34&#x00A0;dB is achieved. Satisfactory omnidirectional patterns in the horizontal plane in both modes are obtained with the peak gains of 6.9 and 5.4 dBi, respectively.

Abstracts:A transmit-array lens antenna is proposed with a view to optimally treat patients with tumors deep inside their bodies. The proposed antenna produces a destructive interference of the electric field that can reduce the heat generated at the skin by the heat-nulling technique. Simultaneously, it concentrates microwave energy on the deep-seated tumor. It is possible to heat an area of 114 mm² at a 50 mm depth below the skin without damage to normal cells. The specific absorption rate (SAR) at a depth of 50 mm corresponds to 50&#x0025; of the maximum SAR induced in the human body. These results show that the proposed transmit-array lens antenna is suitable for treating tumors located deep inside the body, such as kidney cancer.

Abstracts:In this letter, a novel design of compact self-triplexing antenna using half-mode substrate integrated waveguide (HMSIW) cavity for triple-band applications is presented. In the proposed antenna, an inverted V-shaped slot is inserted on the top surface of a rectangular HMSIW cavity and three individual microstrip feedlines are used to excite three separate modes at three different wireless local area network (WLAN) bands around 4.95, 5.3, and 5.9&#x00A0;GHz. Due to use of HMSIW cavity, the size of antenna has become compact. The measured isolation of the antenna in between any two ports is better than 20.5&#x00A0;dB in such a compact structure. The measured gains at three frequencies 4.95, 5.3, and 5.9&#x00A0;GHz are 4.5, 4.9, and 6.1&#x00A0;dBi, respectively. The efficiency of antenna is better than 84&#x0025; in all the bands. The separation between co-to-cross-polarization levels is more than 19&#x00A0;dB, and front-to-back ratio (FTBR) is better than 14&#x00A0;dB at each band. Moreover, any band of antenna can be tuned independently by keeping other bands almost intact.

Abstracts:Polarization converters play an important role in modern communication systems, but their wide and multiple band operation to facilitate volume and size reduction is quite challenging. In this letter, a dual-band linear polarization (LP) to circular polarization (CP) converter is presented using a novel design procedure based on geometric parameters optimization of a meta-array. For dual-band operation, the concept of mutual interaction of two distinct resonators is used. Step-by-step design procedure is proposed and an example is presented to convert incident <italic>x</italic> (<italic>y</italic>) polarization into transmitted left-hand circular polarized (LH-CP) [right-hand circular polarized (RH-CP)] in <italic>Ku</italic> band (15.15&#x2013;19.5&#x00A0;GHz) and RHCP (LHCP) in <italic>Ka</italic>-band (33.1&#x2013;39.05&#x00A0;GHz). After performing model-based theoretical paradigm analysis, and full-wave simulation and optimization, the converter is fabricated and the measurements are performed with a vector network analyzer. The measured results, close to simulation results, depict the validity and reliability of the design. The proposed meta-array is ultrathin, wideband, and polarization diverse, which is very suitable for wide range of applications including millimeter-wave-communication.

Abstracts:In this letter, we propose a generic design method of spiral multiple-input&#x2013;multiple-output (MIMO) antenna arrays for short-range ultrawideband imaging applications. This approach offers superior uniformity and low element shadowing properties in an effective aperture. Moreover, several variables are provided to further tune the positioning of elements. The proposed method avoids a disruptive array topology that exists in the curvilinear array for certain transmit and receive array combinations. Two spiral MIMO arrays are developed and shown to have better focusing property than curvilinear and rectangular arrays. Experimental results are also demonstrated to verify the imaging capability of the proposed spiral array.

Abstracts:This letter proposes a wide transmission band frequency-selective rasorber (FSR) based on a convoluted resonator (CR). The FSR consists of a two-layered metallic structure with an air gap in the middle. At the top layer, the lumped-resistor-loaded metallic dipole with a CR inserted into the center is applied to generate wide absorption and transmission bands. At the bottom layer, the wideband frequency-selective surface is realized by a triple-layer metallic structure, in which two layers of identical square patches are coupled through a layer of the square aperture. The full-wave simulation results show that the wide transmission band with low insertion loss and wide absorption band are realized. To validate the performance of the proposed FSR, the prototypes are fabricated and measured, and reasonable agreement between the simulated and measured results is obtained. The measured results show that the transmission band with insertion loss less than 1&#x00A0;dB is covered from 9.00 to 12.63&#x00A0;GHz with 33.5&#x0025; relative bandwidth. Compared with the previous FSRs with wide transmission band, the proposed FSR yields the widest relative bandwidth, better oblique performance, and simpler design.

Abstracts:A dual-band electromagnetic absorber based on double-layer frequency selective surface (FSS) is proposed for broadband applications. The layer I is designed with an absorption performance at higher frequency and a transmission window at lower frequency. Simultaneously, the layer II is designed with an absorption performance at lower frequency and a strong-reflection window at higher frequency. Due to the low-pass property of the layer I, this layer is on the top of the absorption structure, allowing the incident waves to transmit to the layer II at lower frequency and be absorbed. Meanwhile, the incident wave is absorbed at higher frequency by the layer I since the layer II acts as the strong-reflection substrate in the bottom of the absorption structure. The reflectivity of absorber below &#x2212;10&#x00A0;dB covers a bandwidth of 1.3&#x2013;4.1&#x00A0;GHz and 7.2&#x2013;10&#x00A0;GHz at normal incidence. Furthermore, the absorber can still maintain an absorption bandwidth below &#x2212;10&#x00A0;dB at 1.4&#x2013;4.1&#x00A0;GHz under the 30&#x00B0; oblique incidence. The good agreement between simulation and measurement validates the proposed absorber.