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A gain-enhanced antenna consisting of a bandpass frequency selective surface (FSS) and a patch antenna is initially proposed in this communication. By utilizing the reflection characteristic of the FSS, a quasi-Fabry–Perot (F-P) cavity is formed between the FSS and antenna’s ground, which considerably improves the gain of the antenna. Meanwhile, the bandpass FSS has a little phase compensation for oblique transmitting waves, which also slightly enhances the gain. Then a low-radar cross section (RCS) and gain-enhanced antenna is proposed by using absorptive/transmissive frequency selective structure (ATFSS). It is implemented by mounting the ATFSS above a patch antenna. The ATFSS is constructed by stacking a lossy layer above a bandpass FSS. Both antennas are fabricated, assembled, and measured. They achieve a gain-enhancement of 5.27 and 3.84 dB, respectively. Besides, the low-RCS antenna achieves 10 dB monostatic RCS reduction from 5 to 5.33 GHz and from 9 to 13.4 GHz under the TE-polarization, and from 8.4 to 13.5 GHz under the TM-polarization, respectively. Simulated results show it also achieves a good improvement in bistatic RCS reduction as well.

A wireless multistate controller based on radio frequency identification (RFID) technology for remote control applications is presented. The controller consists of a configurable RFID tag which has multiply alternative paths connecting to a multiway switch component. The main part of the tag is formed from the combination of loop and dipole to ensure balanced three-dimensional current responses and relatively stable impedances during path switching. With the proposed structure, the controller achieves a compact dimension of only 49 × 47 mm, a stable operation frequency band around the Federal Communications Commission (FCC) band (902–928 MHz), a quasi-isotropic radiation pattern with a gain variation less than 3 dB, as well as a control distance of 6.5–9.1 m in different configurations and directions. These features make the proposed controller a promising choice for wireless multistate remote control applications.

This letter proposes a microwave planar sensor for the characterization of magnetodielectric materials. The sensor is designed by loading microstrip lines with complementary split-ring resonators (CSRRs). Two patches are connected to the microstrip lines to couple with the CSRRs, and a pair of 50- $\Omega $ resistors are in series with the lines to provide a notch. The designed sensor can extract the permittivity and permeability of the magnetodielectric materials simultaneously. A prototype of the designed sensor is fabricated and measured for validation of characterizing the magnetodielectric materials.

A novel frequency-selective rasorber (FSR) with a broad absorption band is proposed in this letter. The proposed FSR is realized by vertically cascading a two-dimensional (2-D) lossy layer with a 3-D band-stop frequency-selective structure (FSS). The 2-D lossy layer is composed of meandered square loops loaded with lumped resistors, and the 3-D band-stop FSS is constructed by parallel-plate waveguide arrays (PPWAs) stacked along the TE polarization direction (linear polarization perpendicular to the PPW). The band-absorptive FSR exhibits an absorption band (| S 11| ≤ –10 dB and | S 21| ≤ –10 dB) from 7.85 to 12.85 GHz and a low-pass band (| S 21| ≥ –1 dB) up to 3.8 GHz. Its transition efficiency of the transition band (FRap) is 2.1. A prototype is fabricated, and its measured results show that the proposed FSR has a low loss and flat low-pass band, a wide absorption band with high selectivity, and a steep transition band between the low-pass band and absorption band.

This paper presents a 3D band-absorptive frequency selective rasorber (FSR). Such an FSR is absorptive within a specific band and almost transparent below the absorption band. The definition and evaluation of the band-absorptive FSR is initially provided and followed by the design concept of 3D FSR. An example is then proposed, whose unit cell is composed of 3D absorber and 3D band-stop FSS. An equivalent circuit model is established for understanding its operating principle. A prototype of the proposed 3D band-absorptive FSR is fabricated. The measured results show that it has an absorption band ( $\vert S_{11}\vert \le -10$ dB and $\vert S_{21}\vert \le -10$ dB) from 4.21 to 6.51 GHz, with a thickness of 0.17 wavelength at 4.21 GHz. Moreover, $\vert S_{21}\vert $ is higher than −1 dB at the frequencies below 1.28 GHz. The proposed band-absorptive FSR can be used as radomes for low-frequency antennas in military communications or spectrum monitoring systems.

A 3D dual-polarized frequency selective rasorber (FSR) using magnetic material and meander-line is presented in this paper, which consist of the absorption and transmission channels. The wide absorption band is produced by the absorption channel consisting of the short-ended parallel-plate waveguide (PPW) loading the magnetic material and the meander-line, while the transmission band is produced by the open-ended lossless PPWs. The magnetic material is lossless in the transmission band resulting in the low-loss in transmission window, therefore the incoming EM waves cannot be absorbed by the absorption channel. An absorption band from 5.02 GHz to 9.60 GHz and a transmission band from 13.6 GHz to 16.4 GHz are obtained in the FSR presented, with the fractional bandwidth of 62.65% and 18.67% respectively. The thickness is 0.18λ a at 5.02 GHz. The experimental results shows that the presented FSR exhibits the advantages of dual-polarization and wide absorption. And no lumped components required reduced the fabrication cost greatly.

A switchable absorber/absorptive frequency selective reflector (AFSR) is proposed in this paper. This structure is switchable by enabling an ideal PIN diode or not. When the PIN diode is enabled, the structure works as an absorber with an ultra-wide absorption band. And the structure works as an AFSR when the PIN diode is disenabled. Simulated results show that the achieved absorber has a wide absorption band $(\vert \mathrm{S}_{11}\vert \leq-10\text{dB})$ with a fractional bandwidth of 150.3%, which is from 1.92 GHz to 13.42 GHz. The realized AFSR has two absorption bands, and one reflection band $(\vert \mathrm{S}_{11} \geq-1\text{dB})$ . The lower absorption band is from 1.98 GHz to 4.86 GHz (84.2%) and the upper absorption band is from 8.37 GHz to 13.37 GHz (50%). The simulated reflection band is from 4.86 GHz to 8.37 GHz (46%). The relative thickness of the proposed structure is 0.128.

A three-dimensional (3-D) bandstop frequency selective structure (FSS) with an ultra-wide stop-band is presented in this paper. It is comprised of two stepped-impedance parallel-plate waveguide (PPW) channels, which are stacked along the E-field direction. Simulated results show that the proposed FSS achieves a wide stop band (|S 11 |≥–1dB) with a fractional bandwidth of 138.8%, ranging from 3.17 GHz to 17.55 GHz. Moreover, the stop-band is quite flat, across which the fluctuation is less than 0.4 dB. The simulated reflection coefficients of the FSS under the oblique incidence up to 60° are also given, which indicate good stability of the frequency response. The proposed band-stop FSS has potential in radomes of radars to suppress the higher order harmonics of certain communication systems.

This communication presents a 3-D frequency-selective rasorber (FSR) that has a passband with a low insertion loss and two absorption bands at both sides of the passband. A lossless and two lossy resonators are employed in the unit cell of the periodic structure to produce the passband and the absorption bands, respectively. Utilizing the techniques of stepped impedance resonator and capacitor loading, the thickness of the resonators is reduced. Moreover, the capacitor loading also frees the tuning of each resonant frequency of the lossy resonator, which facilitates the FSR design. The multiresonances of the lossy resonators are also employed to obtain a wide upper absorption band. An equivalent circuit model is established for understanding the operating principle. A prototype of the proposed FSR is fabricated and measured. Good agreement between measured and simulated results validates our FSR design.

A circular-polarised antenna with omni-directional radiation is presented in this study. The proposed antenna consists of four bended dipoles, excited simultaneously by integrated baluns. The E φ and E θ components, generated by the horizontal and vertical parts of the bended dipoles, are shown to have a phase difference of 90°. The operating principle of achieving circular polarisation (CP) with bended dipoles are described and derived, according to which guidelines for designing CP antennas are provided. Following the guidelines, a prototype of the compact omni-directional CP antenna is fabricated, with a size of 0.27 λ 0 × 0.27 λ 0 × 0.13 λ 0 , a fractional bandwidth of 9% at 2.43 GHz and an average left-handed circular polarisation (LHCP) gain above 0.15 dBic in the operating band.

Simple and low cost tag antenna with long reading distance in various environments is in great demand with its wide applications. This paper describes the design and optimization procedure of a single-layer tag antenna, which covers relatively long reading range when mounted on both metallic surface and various dielectrics. The antenna is based on the loop structure, with a small $ka$ of 0.66. Characteristic mode analysis is first performed to inspect how a typical loop structure radiates in free space and on PEC. Antenna structure is then designed based on the characteristics of the loop. Afterward, equivalent circuits of the tag antenna on dielectrics and on metal are extracted, in order to provide guidance for antenna optimization. The proposed tag antenna achieves the maximum reading distance of 6.1 m and 14.1 m, respectively, when mounted on metal and dielectrics with low permittivity. Measurements of the tag antenna were carried out in different environments, including outdoor scenario, indoor scenarios with low and high reflections. The measured results agree reasonably well with the predicted ones. The tag antenna is proved to be competitive in perspective of simple structure, low cost, long reading distance, and tolerance on various background materials.

An omnidirectional antenna array with wide band and low gain variation in the horizontal plane is proposed in this letter. The antenna unit of the array is composed of two vertically arrayed omnidirectional elements and the associated feeding network. 3-D dipoles with vertical arms and parasitic dipoles are introduced in the antenna unit to significantly enhance the bandwidth and decrease the gain variation. An array with three vertically installed units is fabricated. Each unit is 120 ° rotated compared to the neighboring one for a lower gain variation. The array exhibits a broad bandwidth of 56.4% (1.54-2.75 GHz) for VSWR ≤ 1.5, a gain around 7 dBi, and a low gain variation around 1.5 dB, which may find applications in base station for wireless communication or spectrum monitoring/jamming systems .

Radio frequency identification (RFID) has shown its potential in human&ndash, machine interaction thanks to its inherent function of identification and relevant physical information of signals, but complex data processing and undesirable input accuracy restrict its application and promotion in practical use. This paper proposes a novel finger-controlled passive RFID tag design for human&ndash, machine interaction. The tag antenna is based on a dipole antenna with a separated T-match structure, which is able to adjust the state of the tag by the press of a finger. The state of the proposed tag can be recognized directly by the code received by the RFID reader, and no complex data processing is needed. Since the code is hardly affected by surroundings, the proposed tag is suitable to be used as a wireless switch or control button in multiple scenarios. Moreover, arrays of the proposed tag with rational tag arrangements could contribute to a series of manual control devices, such as a wireless keyboard, a remote controller, and a wireless gamepad, without batteries. A 3 &times, 4 array of the finger-controlled tag is presented to constitute a simple passive RFID keyboard as an example of the applications of the proposed tag array and it refers to the arrangement of a keypad and can achieve precise, convenient, quick, and practical commands and text input into machines by pressing the tags with fingers. Simulations and measurements of the proposed tag and tag array have been carried out to validate their performances in human&ndash, machine interaction.

A printed Vivaldi cell, which covers from 3GHz to 18GHz and can scan ± 45° at E-plane/H-plane, is proposed in this paper. The single printed Vivaldi antenna is $0.07\lambda_{\text{low}}$ wide and $0.38\lambda_{\text{low}}$ high, where $\lambda_{\text{low}}$ is the wavelength at low frequency. The proposed antenna only performs well in array. $1 \times 8$ array and $3 \times 10$ array are simulated to observe its performances. The average active VSWR can less than 3 when $3 \times 10$ array scan 45° at E-plane in 3GHz to 18GHz.

A novel dual-polarized band-absorptive frequency selective rasorber (FSR) is proposed in this communication. The FSR is constructed by two-layer cascaded printed circuit boards, in which meander-line square loops with and without lumped resistors loaded on the top and bottom surfaces of the two substrates. Its operating principle is analyzed, and an accurate equivalent circuit model is presented. A prototype has been fabricated, assembled, and measured. The measured absorption band is from 4.8 to 6.81 GHz (34.6%), with a thickness of $\lambda _{\mathrm {a}}$ /8 at 4.8 GHz. Moreover, it is almost transparent to electromagnetic waves below 1.54 GHz. The measurement results achieve a good agreement with the simulated results, which verifies the effectiveness of the design.

In this paper, a dual-polarized broadband band-absorptive frequency selective rasorber (FSR) is proposed. The band-absorptive FSR exhibits a wide absorption band; meanwhile it is almost transparent to the incident electromagnetic (EM) waves at the frequencies below the absorption band. The FSR is comprised of a lossy layer and an absorptive transmission/reflection surface (ATRS). The lossy layer is realized by a square loop loaded with lumped inductors and resistors. The ATRS is different from the conventional reflective surface - it is comprised of two lossless square loops and one lossy square loop. Those loops are all achieved by loading lumped elements. Simulated results of the FSR is shown in the paper. It exhibits a broad absorption band from 4.02 GHz to 9.34 GHz (79.64% fractional bandwidth), while the cut-off frequency of the passband is 905 MHz ( $\vert S_{21}\vert \ge {-1}$ dB)

A type of broadband 3D frequency selective rasorber (FSR) was presented, which has a passband with low insertion loss and two absorption bands at both sides of the passband. By loading lumped resistors, inductors and magnetic materials, two broad absorption bands were obtained. Equivalent circuit models illustrate the physical mechanism of the proposed FSRs with high selectivity, low insertion loss at passband and broad bandwidth at absorption bands. It shows that the proposed FSR has bandwidth of 4.3∼15.8GHz with reflectivity of −10dB, which includes a bandwidth of 1.24:1 for the insertion loss less than 3dB, a bandwidth of 1.28:1 for the lower absorption band and a bandwidth of 2:1 for the upper absorption band.

This paper introduces a wideband 3D frequency selective rasorber based on ferrite absorbers. The ferrite absorber provides an ultra-wide absorption band, while a lossless stepped impedance resonator produces a pass-band within the absorption band. With the help of a series L-C circuit in front of the ferrite absorber, the ferrite absorber can be short-circuited within the pass-band, resulting in a low insertion loss. The proposed structure is suitable for stealthy radome applications below 1 GHz.

A capacitively loaded, inductively coupled fed loop antenna with an omnidirectional radiation pattern for UHF radio frequency identification (RFID) tags is proposed. The inductively coupled feed structure is beneficial for conjugate impedance matching. The loaded capacitors in the radiating and feeding loops allow for easy control of the resonant frequency and the antenna reactance, respectively. The omnidirectional radiation pattern on the horizontal plane makes the proposed tag insensitive to be mounted on different target objects. With a small size of 43 × 43 mm2, the measured maximum reading range of the prototype is 9.5 m with a total transmitted power of 4.0 W effective isotropic radiated power (EIRP).

This paper presents a 3D frequency selective rasorber consisting of 2D periodic array of open-circuited and short-circuited parallel-plate waveguides. The passband is obtained by the lossless open-circuited parallel-plate waveguide, while two absorption bands located at both sides of the passband are produced by the lossy short-circuited parallel-plate waveguide loaded with a chip resistor. A capacitor is inserted in the short-circuited parallel-plate waveguide, creating an additional resonator, which greatly expands the upper absorption bandwidth. The geometrical details of the proposed FSR are depicted, and simulated performances as well as some analysis are given.

The antenna array with non-symmetric U-strip stubs antennas is proposed for the wideband and omnidirectional radiation. More specifically, single radiation element with four optimized non-symmetric U-strip stubs antennas provides nice bandwidth characteristics. Three same radiation elements with 30 degrees rotated between adjacent ones are configured in vertical direction to compensate for the radiation depression at higher frequencies. As a result, our proposed antenna array is ensured bandwidth while greatly enhancing the omnidirectional radiation in the whole frequency range. For example, this antenna array achieves an impedance bandwidth of 60.9 % (1.6–3.0 GHz) for VSWR < 2. A well-formed omnidirectional pattern with gain variation less than 1.5 dB and the maximum antenna gain from 4.5 to 7.3 dBi are obtained in the horizontal plane across the operating frequency band.

A miniaturized printed antenna with dual inductor-loaded monopoles is proposed in this paper. The two monopoles contribute two adjacent resonances, which result in a broadband performance of the antenna. Each monopole is embedded with a chip inductor. The inductors introduce large inductance, which greatly decrease the resonant frequencies of the monopoles. Some important parameters are studied for better understanding of the working mechanism of the antenna. Both simulated and measured results are given, which shows the antenna is a good candidate for mobile terminals.

A compact omnidirectional antenna of circular polarization (CP) is presented for 2.4-GHz WLAN access-point applications. The antenna consists of four bended monopoles, which are simultaneously excited through an associated feeding network. The radiated horizontal and vertical field components are derived based on an analytical model, and the 90° phase difference between them is shown to be naturally guaranteed. With a size of 0.22λ0 × 0.22λ0 × 0.076λ0, the antenna exhibits a bandwidth (|S11| ≤ -10 dB and AR ≤ 3 dB) of 3.56% and a left-handed CP average gain of 1.39 dB in the azimuth plane at 2.44 GHz.

An electrically small frequency-reconfigurable antenna with a very wide tuning band is proposed. Three varactor diodes are used to achieve the tunable capacitance. With a modifled feeding structure, the measured tuning range of the fabricated antenna reaches from 457.5 to 894.5 MHz, and the tuning band enhancement is also explained through an equivalent circuit analysis.

A simple and compact slot antenna with a very wide tuning range is proposed. A 25 mm (roughly equal to λH/8, where λH corresponds to the highest frequency of the tuning range) open slot is etched at the edge of the ground. To achieve the tunability, only two lumped elements, namely, a PIN diode and a varactor diode are used in the structure. By switching the PIN diode placed at the open end of the slot, the slot antenna can resonate as a standard slot (when the switch is on) or a half slot (when the switch is off). Continuous tuning over a wide frequency range in those two modes can be achieved by adjusting the reverse bias (giving different capacitances) of the varactor diode loaded in the slot. Through optimal design, the tuning bands of the two modes are stitched together to form a very wide tuning range. The fabricated prototype has a tuning frequency range from 0.42 GHz to 1.48 GHz with Sll better than -10 dB, giving a frequency ratio (fR = fu/fL) of 3.52:1. The measured full spherical radiation patterns show consistent radiation characteristics of the proposed antenna within the whole tuning range.

The paper presents two capacitively loaded electrically small loop antennas (ESLA): one is with an inductive coupled feed and another with a capacitively coupled feed. Equivalent circuits are developed to help better understand the working mechanisms. A summarization of the ESLAs with coupled feeds is given to indicate that both inductively and capacitively coupled feeds can be modeled as transformers, which are the keys to impedance matching. A design procedure is also given to guide the design of the capacitively loaded ESLAs.

A compact omni-directional circularly polarized antenna is proposed in this paper. The antenna consists of four bended dipoles and an integrated feeding network simultaneously exciting these four dipoles. Horizontal and vertical polarized field components are generated through the corresponding parts of the bended dipoles, and 90° phase difference between them are naturally guaranteed. With a size of 0.35λ 0 ×0.35λ 0 ×0.13λ 0 , the antenna exhibits a bandwidth of 9.8% (|S 11 |≤ -10 dB and AR≤ 3 dB) and a left-handed CP average gain of 0.68 dB in the azimuth plane at the center frequency 2 GHz.

In this paper, an electrically small normal-mode helical antenna (NMHA) with capacitive coupling feed has been proposed and studied. An equivalent circuit model of the antenna is proposed and analyzed to understand the working mechanism. Simulated results show the proposed the NMHA has pretty good impedance matching, radiation efficiency and good circular polarization.

A compact omni-directional antenna of circular polarization (CP) is presented for 2.4 GHz WLAN access-point applications. The antenna consists of four bended monopoles and a feeding network simultaneously exciting these four monopoles. The electrical size of the CP antenna is only λ 0 /5×λ 0 /5×λ 0 /13. The impedance bandwidth (|S 11

A high gain conical beam antenna consisting of two concentric open-ended coaxial apertures is proposed in this paper. With a diameter of 2.474λ 0 , the aperture antenna achieves a conical radiation beam with a gain of 12.1 dB and a maximum beam-pointing angle of θ=18° at 10 GHz. Simulated results are presented and discussed.