New EM structures are demonstrated to realize low radar-cross-section (RCS) antennas by making effective use of frequency selective absorber (FSA). According to the well-known reciprocity principle, the two-layered FSA can be considered as an actual receiving antenna and then transformed to a circularlypolarized (CP) antenna. At the radiation state, a truncated patch resonator on the bottom layer is fed by a coaxial probe so as to produce CP wave, and it further excites the slots on the upper layer, resulting in its radiation towards free space; when the antenna at stealth state, the detective incident wave can be effectively absorbed outside the radiation band, thus achieving RCS reduction. The design strategies are then explained and verified with the aid of the corresponding equivalent circuit models. Two examples of 2x2 and 4x4 antenna arrays were designed to validate the flexibilities of the proposed design method. In final, the 4x4 antenna array was fabricated and measured, and reasonable agreement is achieved.

This paper presents the proposal, synthetic design, and implementation of a class of wideband bandpass frequency selective structures (FSSs) with multiple in-band transmission poles. Each FSS element is formed by introducing parallel-coupled stepped-impedance slotline resonators (SISRs) on printed circuit board (PCB). By periodically printing the FSS elements in long PCB pieces and then stacking them together with a certain distance, the incident spatial waves in free space can be converted into guided waves propagating along the slotline resonators of our proposed FSS, and once again be reconverted from guided waves into spatial waves. Therefore, the slotline resonators can be designed to manipulate the incident spatial waves by controlling the filtering response of guided waves within FSS. An equivalent transmission-line (TL) model is then established to explain the manipulation mechanism. In order to systematically synthesize the filtering response, the transfer function of the equivalent TL model is mapped to the generalized Chebyshev equal-ripple response. The closed-form equations are derived, such that all of the electrical and physical parameters are determined by the design specifications of in-band equal-ripple return loss (RL) and fractional bandwidth (FBW). To verify the theoretically predicted performances, three examples with different RLs (20 dB, 30 dB, and 10 dB) and FBWs (90%, 60%, and 120%) are designed, fabricated, and measured respectively. Experimental results demonstrate that the proposed FSSs can achieve desirable and stable wideband bandpass performance under oblique incidences. Our proposed method can also be applied to the design of other high-order wideband FSSs.

This paper presents a printed wideband linear-to-circular polarizer for X-band applications. The proposed polarizer consists of a periodic array of orthogonally inserted slot-line structures, which are treated as two independent transmission paths for two orthogonal components of a linearly polarized incident plane wave, respectively. In order to effectively realize linear-to-circular polarization conversion, the orthogonal slot-line structures are loaded with inductive strips and pairs of open-circuited strips, respectively. This arrangement can maintain equal magnitude and 90° phase difference between these two transmission paths in a wideband frequency range. With the aid of equivalent circuit models, operation principle of the proposed polarizer is extensively explained, and a design example is then conducted based on the prescribed specifications. Finally, the designed polarizer is fabricated and measured. The measured results confirm the theoretical predictions that the insertion loss is less than 1 dB from 8.27 GHz to 13 GHz, and the axial ratio is less than 1 dB over entire band ranged from 7.92 to 12.16 GHz. This kind of polarizer indeed features great design flexibility, low insertion loss, and wide polarized conversion bandwidth.

In this paper, a novel frequency-selective surface (FSS) with polarization selection and dual-band polarization conversion characteristics is proposed for terahertz applications. The FSS is composed of two layers of metallic periodic arrays separated by a polymer dielectric spacer. The top and bottom arrays provide bandpass and bandstop characteristics for TE- and TM-polarized incidences, respectively. Geometrical parameters of the top and bottom arrays are tuned such that the phase differences of the TE and TM transmissions at the lower (f ₁ ) and upper (f ₂ ) crossover frequency points are -90° and +90°, respectively. Therefore, for normal incidence, a 45° linearly polarized incident wave is converted into a right-handed circularly polarized and left-handed circularly polarized transmitted wave at f ₁ and f ₂ , respectively. Equivalent circuit models are established to explain the operation principle of the proposed FSS. A prototype is fabricated using micromachining with its transmission characteristics, namely, polarization selectivity and dual-band polarization conversion, validated by time-domain and frequency-domain measurements.

A generalized synthesis technique for a new class of high-order and wideband 3-D frequency selective structures (FSSs) is presented in this paper. First, a general circuit model with shunt short-ended stubs and an i-section non-uniform transmission line is discussed, whose frequency response can be synthesized with Chebyshev functions over a wide frequency range. Following the synthesis procedure, all the circuit parameters can be derived from the specifications of the filtering response, such as filtering order N (≥ 4), fractional bandwidth (FBW), and equal-ripple return loss (RL). Then, the general circuit model is applied to design high-order and wideband 3-D bandpass FSS, which is physically realized by using stacked printed circuit board pieces etched with slotlines and microstrip lines. As an example, a fourth-order 3-D FSS with FBW = 80% and equal-ripple RL = 10 dB is designed to show the establishment of good relationship between the circuit model and the physical model. To further improve the frequency selectivity, another two coupling paths independent of the main transmission path are created by simultaneously adjusting the FSS thickness and the separation of slotline sections, thus introducing two transmission zeros at both skirts of the passband. Both design examples are fabricated and measured. Good agreement between measured results and simulated ones indicates that the proposed synthesis method can provide an efficient and accurate solution to design such a class of wideband and high-order 3-D FSSs.