In this letter, an ultrawideband (UWB) antipodal tapered slot antenna with reflectionless notched band is proposed and investigated. The proposed antenna uses an integrated absorptive bandstop filter, consisting of two quarter-wavelength open-circuit stubs and a 50 Ω resistor, to form a reflectionless notched band. Consequently, the input energy at the notched frequency is absorbed by the resistor rather than reflected. An antenna prototype is designed and fabricated to validate the design. Both simulated and measured results present that the realized gain drops over 40 dB at 5.5 GHz, and the reflection coefficient is below –10 dB from 3.1 to 10.6 GHz. Besides, a low ringing level of –23.4 dB of the measured reception pulse shows a good time-domain performance. Therefore, the proposed antenna could be applied in UWB short-pulse system to inhibit the 5 GHz WLAN interference.

An endfire antenna with sum and difference beams is designed based on phase transforming of a proposed asymmetric spoof surface plasmon polariton (SSPP) structure. The asymmetric arrays of grooves, with different groove depths, periods, and/or duty ratios, serve as transmission structure and transmit electromagnetic (EM) waves in SSPP modes. Due to the distinct phase constants for the EM fields on the two sides of the asymmetric structures, the transformation of phase difference for the EM waves is realized. Superposition in the endfire direction for the in-phase and out-of-phase EM fields forms the endfire radiations with sum and difference beams. A design procedure is proposed where operating frequencies of the endfire antenna are predicted with the dispersion curves of the asymmetric SSPP structures and allocated by phase transformation. A prototype is implemented to validate the proposed design principle. Sum beam operating frequencies of the endfire antenna are at 5.50, 6.60, and 7.09 GHz, while difference beams appear at 6.10 and 6.90 GHz. Good agreement of the results validates the feasibility of the proposed mechanism and design procedure. The proposed endfire antenna with sum and difference beams has broad prospect in direction finding, tracking, and anti-interference applications.

A compact decoupling structure, implemented by the postwall slotline (PWS) based stepped impedance resonator (SIR), is proposed for enhancing the isolation between patch antennas. A pair of SIRs are placed between microstrip antennas to cut off the coupling path, which enhances the decoupling performance greatly at resonance frequency. Meanwhile, designed with a combination of conventional slot line (CSL) and the PWS, the decoupling structure has a compact profile and thus is suitable for miniaturized element space. A two-element patch array is designed and fabricated to verify the decoupling effectiveness of the proposed structure, by providing simulated and measured results. Realized gain of the antenna with proposed structure is about 5.4 dBi. High isolation of 30.4 dB at 5.7 GHz is achieved and the edge-to-edge distance is 0.059 ${{\lambda}_{0}}$ with the decoupling structure. Moreover, with the decrease of element distance, isolation improvement will be significantly enhanced. The compact size and isolation performance show the potential of the structure to be applied in antennas arrays including multi-input-multi-output (MIMO) systems and adaptive arrays.

Novel resistor-free differential microstrip lines prototype loaded with lossy metamaterial is proposed to achieve wideband common-mode (CM) absorption. The metamaterial is constructed by a chain of lossy complementary split-ring resonators (CSRRs), which are etched on the ground plane. Different from the CM absorption realized by lumped resistors, this work utilizes the inherent dielectric and conductor losses introduced by the differential microstrip lines and CSRRs to absorb CM energy. Furthermore, CSRRs are designed with gradual perturbation to broaden the CM absorption band. The lossy even-mode equivalent circuit models are established and analyzed to reveal the CM absorption mechanism. In order to verify the proposed strategy to design resistor-free differential microstrip lines with CM absorption, a sample was fabricated and measured. The measured $S_{\mathrm {cc21}}$ and $S_{\mathrm {cc11}}$ are kept below −10 dB simultaneously from 4.3 to 5.65 GHz with 27.4% fractional bandwidth, where the CM energy is absorbed over 80% by the dielectric and conductor losses. Meanwhile, the low insertion loss of $S_{\mathrm {dd21}}$ below 6 GHz as well as the good differential-mode eye diagrams demonstrates that good signal integrity of differential signals is kept.

Radiation loss of a typical spoof surface plasmon polaritons (SSPPs) transmission line (TL) is investigated in this paper. A 325 mm-long SSPPs TL is designed and fabricated. Simulated results show that radiation loss contributes more to transmission loss than dielectric loss and conductor loss from 2 GHz to 10 GHz. Radiation loss of the SSPPs TL could be divided into two parts, one is caused by the input mode converter, and the other is caused by the corrugated metallic strip. This paper explains mechanisms of radiation loss from different parts, designs a loaded SSPPs TL with a series of resistors to absorb electromagnetic energy on corrugated metallic strip, and then discriminates radiation loss from the input mode converter, proposes the concept of average radiation length (ARL) to evaluate radiation loss from SSPPs of finite length, and concludes that radiation loss is mainly caused by corrugated structure of finite length at low frequency band and by the input mode converter at high frequency band. To suppress radiation loss, a mixed slow wave TL based on the combination of coplanar waveguides (CPWs) and SSPPs is presented. The designed structure, sample fabrication and experimental verification are discussed.