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This letter presents a new circularly polarized (CP) active antenna for global navigation satellite systems (GNSS) operating in the L1‐band (1.164–1.28 GHz) and L2‐band (1.525–1.615 GHz). The active antenna consists of a passive antenna and a two‐channel low‐noise amplifier (LNA) circuit. To achieve wideband coverage of the L1/L2 bands while maintaining a low profile, a new electroplating process is utilized in fabricating the passive antenna. Additionally, to realize CP performance, the passive antenna is sequentially fed by four ports with a wideband feeding network. To minimize phase variations, a 90° phase shifter loaded with open‐short‐stub is integrated into the feeding network. Furthermore, to improve noise immunity, out‐of‐band rejection, and amplify GNSS signals, the LNA circuit cascades two‐stage dielectric bandpass filters and three‐stage LNAs. Consequently, the circuit features a low noise figure of below 1.65 dB, a maximum out‐of‐band rejection level of 60 dB, and a stable active gain of 30 dB. The performance of the active antenna is evaluated in an outdoor open field using a GNSS receiver. [ABSTRACT FROM AUTHOR]

Wideband omnidirectional antennas are essential components in radio monitoring and communication systems, enabling the reception of signals from all directions over a wide bandwidth. This paper presents a novel wideband omnidirectional antenna design that achieves a 1-dB gain variation across its azimuthal plane within a bandwidth of 1.8 GHz to 7.77 GHz. The antenna's exceptional performance is attributed to two flower-bud-shaped monopoles that, through pattern superposition, generate a wideband omnidirectional radiation pattern. Analysis shows that the use of a circular ground plane also reduces the azimuthal gain variation. Additionally, an embedded matching structure integrated into the antenna's base enhances the impedance bandwidth without compromising its compact size. Analytical investigations demonstrate that the matching structure effectively behaves as a five-order LC circuit, explaining its wideband matching capabilities. Furthermore, structural modifications effectively reduce side lobe levels, ensuring minimal interference. Experimental measurements corroborate the antenna's omnidirectional radiation pattern and confirm that the azimuthal gain variation remains within 1-dB throughout its bandwidth, while maintaining an S11 below −10 dB from 1.8 GHz to 7.7 GHz. The antenna's bandwidth overlaps with the spectrum intensively used in mobile communication technologies, such as LTE, Bluetooth, and IEEE 802.11be, as well as radiolocation applications, making it a promising choice for unmanned aerial vehicles conducting communication and radio monitoring missions. [ABSTRACT FROM AUTHOR]

This paper investigates a wideband omnidirectional antenna that possesses 124.7% of fractional bandwidth. The antenna's two crescent‐shaped radiating patches and their ultrawideband matching structures are printed on two laminates that are crossly placed vertically over the ground plane. Simulation shows that the structure on a single laminate radiates an "8"‐shaped pattern on the horizontal plane. The cross formation of the two printed laminates generates an omnidirectional radiation pattern by superimposing two "8"‐shaped patterns perpendicularly. The proposed antenna is fabricated and measured for demonstration. The measured results agree well with the simulation, indicating the omnidirectional antenna works from 1.38 to 5.95 GHz with a return loss of −10 dB. Simulation and measurement also show the gain variation on the xoy plane is 2 dB in the band of interest, which is small if the ultrawide bandwidth is considered. The above features make the proposed antenna a promising candidate for applications in the digital cellular system (DCS1800), long‐term evolution (LTE2300/2600), wireless local area network (WLAN2400), and IEEE 802.11p. [ABSTRACT FROM AUTHOR]

This letter investigates the coupling between two parallel‐placed polytetrafluoroethylene (PTFE) dielectric waveguides over the frequency band from 90 to 110 GHz. The coupling is considered under different working frequencies, coupling spacings, coupling lengths, and polarizations. Simulations and measurements are carried out, and the results indicate the coupling can be significantly reduced by increasing the operating frequency and coupling spacing and adopting vertical polarization (VP) for a specific coupling length. In particular, the simulated and measured results with a 9‐mm coupling distance under two polarization states can be kept under −30 dB for 50‐ and 100‐mm coupling lengths, leading to negligible interference. In the practical arrangement of PTFE dielectric waveguides working at the other frequencies, one can follow the same rule to reduce the coupling between waveguides and thus avoid interference. [ABSTRACT FROM AUTHOR]

Radar detection and imaging systems need ultra‐wideband, high‐gain antennas to improve depth resolution and detection range. This paper proposes an ultra‐wideband high gain differential patch antenna having a peak gain of 12.9 dBi and an average gain of 11.3 dBi in the bandwidth from 7.2 and 26 GHz. Two sleeves are added around the feeding probes to attain ultra‐wideband impedance matching that is explained with an equivalent circuit. A hexagonal cavity with two M‐shaped cutouts improves the gain in the broadside direction by redistributing the current and reshaping the antenna's radiation patterns at different frequencies. The antenna's measured S‐parameter validates the design and confirms it has an ultra‐wide bandwidth. The ultra‐wide bandwidth along with the high gain values make the antenna a promising candidate for high‐resolution radar systems. [ABSTRACT FROM AUTHOR]

Electromagnetic metasurfaces have been intensively used as ultra-compact and easy-to-integrate platforms for versatile wave manipulations from optical to terahertz (THz) and millimeter wave (MMW) ranges. In this paper, the less investigated effects of the interlayer coupling of multiple metasurfaces cascaded in parallel are intensively exploited and leveraged for scalable broadband spectral regulations. The hybridized resonant modes of cascaded metasurfaces with interlayer couplings are well interpreted and simply modeled by the transmission line lumped equivalent circuits, which are used in return to guide the design of the tunable spectral response. In particular, the interlayer gaps and other parameters of double or triple metasurfaces are deliberately leveraged to tune the inter-couplings for as-required spectral properties, i.e., the bandwidth scaling and central frequency shift. As a proof of concept, the scalable broadband transmissive spectra are demonstrated in the millimeter wave (MMW) range by cascading multilayers of metasurfaces sandwiched together in parallel with low-loss dielectrics (Rogers 3003). Finally, both the numerical and experimental results confirm the effectiveness of our cascaded model of multiple metasurfaces for broadband spectral tuning from a narrow band centered at 50 GHz to a broadened range of 40~55 GHz with ideal side steepness, respectively. [ABSTRACT FROM AUTHOR]

This article applies the generalized multimode S‐parameters to multiport antennas and radio frequency (RF) components for CAD applications. Consider the special case of impedance matching which could significantly simplify the multimode representation on port and remove the requirement on calculating port impedance. N single‐ended ports of a network are regarded as one generalized multimode port, supporting N orthonormal modes represented by N orthonormal basis vectors, respectively. By doing this, in antenna and RF component designs, one can only focus on specific modes interested, while ignore others. A multiport network's characteristics could be represented and understood in a much simpler manner because fewer but more intuitive physical quantities are considered. Therefore, this can significantly simplify the analysis, design, and optimization of multiport networks. For better illustration, several examples, including branch‐line coupler, mixed‐mode case, sequential rotation feeding antenna, and monopulse feed are analyzed and designed. Complicated analyses in network design using the single‐ended S‐parameters, including matching, amplitude and phase unbalance, could be converted into much simpler ones from the generalized multimode perspective. [ABSTRACT FROM AUTHOR]

A novel bandwidth enhancement method using loading multiple parasitic structures to realize broadband miniaturized antenna is proposed. Based on the proposed strategies, one miniaturized wideband dual-polarized dipole antenna covering LTE/CDMA/GSM bands (690–960 MHz) for base-station application is designed. The antenna consists of two folded bowtie-shaped dipoles fed with a pair of crossed baluns, and is further loaded on a ceramic block with high permittivity ($\varepsilon$) for miniaturization. A parasitical square loop is loaded for bandwidth enhancement at a lower frequency. In addition, four split rings with a small square loop are firstly adopted for antenna resonance tuning, the impedance bandwidth is thus improved effectively. The antenna size is only $0.18\,\times \,0.18\,\times \, 0.22\,\,\lambda _{0}$ at the center frequency. The measured result of the fabricated prototype exhibits a relatively large operation bandwidth of 32.73% (VSWR < 1.6) with stable radiation patterns and relatively flat broadside gains. Moreover, to verify the feasibility of the proposed miniaturized antenna element in the base-station application, an array of $2\,\times \, 4$ is further designed. The measured results of the array indicate isolation over 20 dB with an impedance bandwidth of 32%. Due to the good performance and compact feature, the proposed miniaturized broadband antenna element can find large application potential in multiband wireless communication base stations. [ABSTRACT FROM AUTHOR]

A novel coupling-cancellation method by loading planar path is proposed and applied in the millimeter-wave phased array for wide-angle scanning. By employing the improved decoupling stubs with weak radiation first, an extra introduced wideband equal-magnitude and anti-phase path is generated to cancel the inherent coupling path. The principle of the proposed method is investigated by the equivalent circuits and the full-wave simulation. Based on this method, a high-isolation 1-D phased array using low-temperature-cofired ceramic (LTCC) process is designed and fabricated. The planar paths are placed symmetrically between the subarrays and its radiation polarization is coincident with the antenna elements, resulting in high isolation and enhanced boresight radiation null-fill of subarrays. The measured isolation is below 20 dB within the relative bandwidth of 16.7% and the maximum scanning angle is ±60°. Moreover, the proposed method can be expectedly extended to large-scale arrays. [ABSTRACT FROM AUTHOR]

This article presents 3-D printed metal-only dual-band millimeter-wave circularly polarized (CP) reflectarrays (RAs). The RAs’ unit cell (UC) is made of an anisotropic metal brick mounted on a full metal ground. In-plane rotating the metal brick introduces geometric Pancharatnam–Berry (PB) phase shifting, and vertically adjusting the position of the UC provides propagation phase shifting. The geometric PB and propagation phase shifting offer two different degrees of freedom, enabling independent wavefront shaping of CP wave over dual-band. Specifically, the low-band CP beam shaping solely depends on the propagation phase. In contrast, the high-band CP beam shaping depends on the geometric PB and the propagation phases. The coupling issue between dual-band is minimal because of the unique dual-band phase-shifting methods. In addition, 3-D printed dual-band left-hand CP (LHCP) horn antennas are designed, printed, and used as the feed. To demonstrate, two RA antennas with different dual-band CP beam directions are fabricated and measured. [ABSTRACT FROM AUTHOR]

This communication proposes an additively manufactured metal-only millimeter-wave dual circularly polarized (CP) reflectarray with independent control of polarizations. A metal-only right-hand CP (RHCP) antenna is designed and used as the reflectarray’s phase-shifting unit cell (UC). The RHCP UC is immune to the LHCP incidence, and the UC directly reflects the LHCP incidence into free space. Thus, the phase shifting of the LHCP wave solely depends on the rotation angle of the UC. For the RHCP incidence, it is received by the RHCP UC first and induced into the metal waveguide. Then, the energy is reflected by the ground of the metal waveguide and reradiated into free space with RHCP. Thus, the phase shifting of the RHCP wave is determined by the rotation angle of the UC and the length of the metal waveguide. By properly selecting the rotation angle of the UC and the length of the metal waveguide, RHCP and LHCP can be independently manipulated. To demonstrate, two reflectarray antennas with different LHCP and RHCP beam directions are fabricated using additive manufacturing and experimentally verified. [ABSTRACT FROM AUTHOR]

Novel designs of paper-based bandstop frequency-selective surfaces (FSSs) are presented in this article. The characteristics of the paper-based honeycomb structure are investigated first. One folded dipole resonator with first-order bandstop response is analyzed and further printed on the sidewalls of the honeycomb structure accordingly. A second-order response with two cascade FSSs is proposed with controllable bandwidth by the coupling coefficient of the resonators with the aid of the equivalent circuit model. Two paper-based FSS samples were fabricated and measured to demonstrate the validity of the proposed design strategies. Benefiting from the merits of lightweight and strong mechanical properties, the proposed paper-based FSS is a potential candidate for application in radar systems and aircraft. [ABSTRACT FROM AUTHOR]

A low-profile and shared aperture dual-polarized (DP) omnidirectional antenna by reusing the structure of an annular quasi-dipole array is presented. The proposed antenna consists of a center-fed circular patch antenna, an annular quasi-dipole array, a wideband one-to-four power divider integrated with the center-fed circular patch antenna, and two annular strips. The center-fed circular patch antenna is utilized as the driven element for the vertically polarized (VP) radiation. The annular quasi-dipole array fed by the one-to-four power divider produces horizontally polarized (HP) radiation. Simultaneously, it also acts as the VP director to improve VP gain and bandwidth. Besides, the two annular strips enhance the HP omni-directivity and gain. In this way, the VP and HP share the same aperture to improve the profile utilization rate effectively. Due to the symmetry of the basic antenna structure, stable DP donut patterns are obtained within the entire operating bandwidth. For demonstration, a 4.1–5.0 GHz prototype is designed, fabricated, and measured. Under a low profile of $0.13\lambda _{0}$ , the prototype realizes a −10 dB overlapped bandwidth of 20%, high port isolation of above 35 dB, and stable DP donut patterns with horizontal gain larger than 1.8 dBi. [ABSTRACT FROM AUTHOR]

A planar bidirectional circularly polarized (CP) antenna using orthogonal magnetic dipoles (OMDs) without an extra phase shift line is proposed in this work. It consists of a rectangular microstrip magnetic dipole antenna (MMDA) and a loop radiator with two arms directly connected to the open edge of MMDA. The working principle is revealed by the equivalent sources model to provide a clear physical insight at first. The theoretical investigations imply that by controlling the width and the feed locations of the loop antenna on the aperture, the phase and magnitude for CP radiation can be realized in both endfire and backfire directions without introducing an additional phase shift line structure. For demonstration, one antenna prototype is designed and fabricated. The experimental results indicate that an overlapping bandwidth of 5.91% from 2.30 to 2.44 GHz is attained. Compared with the counterpart using OMDs, the proposed antenna exhibits a low design complexity and improved gains of 3.56/1.83 dBic in the endfire/backfire directions. [ABSTRACT FROM AUTHOR]

In this study, a three‐dimensional bandpass frequency selective surface (FSS) with a new fabrication technology is proposed and designed based on slotlines. The proposed FSS consists of silver paste circuits printed on aramid paper, and is vertically and horizontally arranged to form a periodic array in honeycomb structure. To better understand the design principle of the proposed FSS, the equivalent circuit model (ECM), single‐polarized structure, and dual‐polarized structure are presented and analyzed, respectively. The proposed FSS exhibits a passband at 4.1 GHz with a fractional bandwidth of 71.5% at x‐polarization and 66.2% at y‐polarization with reflectivity less than −10 dB. Moreover, the reflection and transmission coefficients have relative stable properties up to 30° for the oblique incidence. Then, the proposed structure is fabricated and measured to verify the design strategies. The measured and simulated results have reasonable agreement under a large variation of incident angles. [ABSTRACT FROM AUTHOR]

This communication presents a new multiband multiarray (MBMA) scattering suppression antenna configuration for a dual-band and dual-polarized antenna array with reduced radiation distortion. The proposed antenna array configuration consists of a low-band (LB) antenna element working in the 690–960 MHz band and four high-band (HB) antenna elements working in the 1.7–2.7 GHz band. The LB antenna introduces U-shape structures to the radiation element to neutralize HB scattering currents. On the other hand, the HB antenna involves an LC resonant structure between its radiation arm and feeding balun to suppress LB scattering currents. An antenna array using the proposed LB and HB elements is designed to achieve MBMA functions. Experimental results show that the radiation distortions in both LB and HB bands are improved significantly. The VSWR of less than 1.5, the isolation of more than 25 dB, and the symmetrical radiation pattern are obtained in both bands. [ABSTRACT FROM AUTHOR]

This communication presents a dual-band aperture-shared high gain antenna for millimeter-wave (mm-wave) multi-beam and sub-6 GHz applications by integrating transmitarray into Fresnel zone plate (FZP) lens. The transmistarray consists of two sets of unit cells (UCs), which can provide high transmission magnitude and [ $-\pi $ , $\pi $ ] phase tuning at the mm-wave frequency band. Meanwhile, they show distinct transmission features at the sub-6 GHz band (one set of UC allows the electromagnetic (EM) wave to pass while the other prohibits the EM wave from passing). Then, by properly arranging the two kinds of UCs into the transparent and opaque region of the FZP lens respectively, the sub-6 GHz FZP lens and the mm-waveband transmitarray are integrated into the same aperture seamlessly. A waveguide-integrated patch antenna is adopted as the feed. The patch antenna operates at the sub-6 GHz band and three open waveguide antennas are used for the mm-wave multi-beam radiation. The proposed antenna achieves high gain at both bands with a reused radiating aperture. Mm-wave multi-beam radiation is obtained without an extra feeding network. For demonstration purposes, the central frequencies of the sub-6 GHz and the mm-wave bands are selected at 6 and 27 GHz, respectively. An antenna prototype is fabricated and experimentally verified. [ABSTRACT FROM AUTHOR]

This letter presents a dual-band antenna with a large frequency ratio of 11.7 (2.4 GHz/28 GHz) by integrating an mm-wave Fabry–Perot cavity (FPC) antenna into a sub-6 GHz patch antenna. The patch with periodic slots functions as both the 2.4 GHz radiator and the partially reflective surface (PRS) of the 28 GHz FPC antenna. By properly tuning the length of the periodic slot, the PRS's reflection can be easily adjusted. As the periodic slot's length and width operating at 28 GHz are much smaller than the wavelength at 2.4 GHz, periodic slot operating at 28 GHz have little impact on the radiation of the patch. Furthermore, because of the Fabry–Perot resonance, the antenna can have a peak gain reaching 15 dBi at 28 GHz band with an easy feeding structure. For demonstration, a prototype is fabricated and experimentally verified. Note that the frequency ratio is not limited to the proposed design (11.7 for demonstration). It can be easily adjusted based on the same principle. [ABSTRACT FROM AUTHOR]

This communication proposes a dual-band antenna hybridizing a folded reflectarray antenna (FRA) and a folded transmitarray antenna (FTA). First, a polarization-rotation transmitarray is designed based on a new Fabry–Perot (FP)-liked phasing element. Unlike its counterparts, this new FP-liked phasing element twists the polarization of electromagnetic (EM) waves and provides sufficient phase tuning range only in the operating band of FTA. In the meantime, in the band of FRA, the transmitarray acts as the polarizing grid, which reflects one polarization while allowing the orthogonally polarized one to pass without affecting its polarization and phase. Second, a polarization-rotation-ground-integrated (PRGI) reflectarray is designed. The reflectarray collimates the EM wave in the band of FRA and twists its polarization by 90°, forming the folded reflectarray. Meanwhile, it also twists the polarization of the EM wave in the band of FTA. By hybridizing the FTA and FRA sharing the same aperture, the proposed design achieves dual-band high-gain radiation. To the best of our knowledge, this is the first time that the FRA and FTA are combined to form a shared aperture antenna, which inherits the advantages of both FRA and FTA, such as high gain, low profile, and reduced blockage effect of the feed. An antenna prototype is fabricated and measured to verify the idea. [ABSTRACT FROM AUTHOR]

Low-profile shared-structure dual-polarized (DP) Yagi–Uda antennas are proposed, which are formed by three kinds of basic components, namely DP driven element, DP director, and DP reflector. An open-cavity dipole is utilized as the DP driven element to generate DP endfire radiations. The quarter-wave resonance mode for vertical polarization (VP) and half-wave dipole resonance mode for horizontal polarization (HP) are excited by differential feeding structures in the DP driven element. The open-cavity element is adopted as the DP director or reflector to enhance endfire gain. For the proposed design, two polarization radiators are integrated into one structure, realizing structure sharing, and thus a low profile. Three-element (Ant. 1) and seven-element (Ant. 2) antennas with a profile of 0.1 λ0 are designed, fabricated, and measured. The measured overlapped bandwidth of the Ant. 1 is from 4.11 to 4.83 GHz (16%) with ports isolation higher than 30 dB, and the measured VP and HP gains are from 5 to 6 dBi and 5 to 7 dBi, respectively. The Ant. 2 further increases the DP endfire gain by 2–3 dBi at the overlapped bandwidth of 16.3%. [ABSTRACT FROM AUTHOR]

This article presents an aperture-shared dual-band large frequency-ratio high gain antenna for sub-6 GHz and millimeter-wave (mm-wave) bands applications. Initially, the partially reflective surface (PRS) of the Fabry–Pérot cavity (FPC) antenna operating at the sub-6 GHz band is realized by using single-layered periodic grid patches while the opaque region of the mm-wave bandwaveband Fresnel zone plate (FZP) lens antenna is implemented by using periodic double-screen dipoles. Then, the PRS and the FZP lens are hybridized together and upgraded into a kind of composite metasurface, which simultaneously functions as the PRS of the sub-6 GHz FPC antenna and the mm-wave bandwaveband FZP lens with little dual-band mutual interference. Thus, the FPC antenna and the FZP lens can share the same aperture with high aperture reuse efficiency. Because the principles are based on the FPC resonance and the collimating FZP lens, high gains are achieved at both bands without a feeding network. Meanwhile, a dual-band large frequency-ratio antenna is designed as the feed. A prototype working at 3 and 28 GHz bands is designed, fabricated, and measured to verify the idea. [ABSTRACT FROM AUTHOR]

This communication proposes a new all-dielectric broadband dual-band reflectarray with a large frequency ratio using low-cost 3-D printing. In contrast to conventional reflectarrays using metallic resonant cells or dielectric slabs as phasing elements with full metal ground, the proposed design uses air as the phasing element and a stepped dielectric mirror structure as the ground. In this way, the metal ground is removed, which makes the design an all-dielectric one. Taking advantage of the dielectric mirror that only exhibits a bandgap in the predesigned band while allowing electromagnetic (EM) waves to pass through it at the frequency out of the bandgap region, a dual-band reflectarray is obtained. By properly selecting the bandgap frequency of the dielectric mirror, the dual-band frequency ratio is scalable and can be very large. Furthermore, instead of using a metallic or dielectric resonator based on resonance, air layers with linear phase response are adopted as the phasing element. Thus, the reflectarray shows broadband and stable performance over the dual-band. Compared with the state-of-the-art works using printed circuit boards (PCBs) or microfabrication, the proposed design is low cost and lightweight, and can be rapidly prototyped. For proof-of-concept, a prototype operating at $K$ -band and $V$ -band with a frequency ratio of 2.7 is printed and measured. [ABSTRACT FROM AUTHOR]

Fresnel zone plate (FZP) lens antenna, consisting of a set of alternative transparent and opaque concentric rings arranged on curvilinear or flat surfaces, have been widely used in various fields for sensing and communications. Nevertheless, the state-of-art FZP lens antennas are limited to a single band due to the frequency-dependent feature, which hinders their use in multi-band applications. In this work, a shared aperture dual-band FZP metalens antenna is proposed by merging two single-band FZP metalens antenna operating at distinct frequency bands seamlessly into one. Instead of using conventional metallic conductors, double-screen metagrids are devised in this work to form the concentric rings. Because the metagrids show distinct transmission/reflection properties at different frequencies, the performance of one set of concentric rings operating at the one band will not be affected by the other operating at the different band. In addition, to compensate for the phase shift introduced by the metagrids, an additional dielectric ring layer is added atop the FZP taking advantage of additive manufacturing. Thus, the radiation performance of the dual-band FZP lens antenna is comparable to that of each single FZP metalens antenna. For proof-of-concept, an antenna prototype operating at the dual band, 75 and 120 GHz with a frequency ratio of 1.6, is fabricated using an integrated additively manufactured electronics (AME) technique. The measured peak gains of 20.3 and 21.9 dBi are achieved at 75 and 120 GHz, respectively. [ABSTRACT FROM AUTHOR]

A low-profile broadband vertically polarized microstrip magnetic dipole antenna with endfire radiation is proposed for millimeter-wave (mmWave) applications. For the bandwidth enhancement, a lower resonance is introduced by the parasitic patches firstly. Then, a top-loaded circular probe and an inset proximity-coupled feed structure are employed for adjusting the resonances from the magnetic dipole antenna and the parasitic patches. In this way, a broad impedance bandwidth is attained. After that, a 1 × 4 antenna array prototype is designed, fabricated and tested. Experimental results demonstrate that the proposed array features an impedance bandwidth wider than 19% at 28 GHz mmWave frequency band under an ultralow-profile of 0.075λ0 at the center frequency. [ABSTRACT FROM AUTHOR]

A high gain oversized annular lens antenna for 28‐GHz band channel measurement system is presented. The proposed antenna is composed of an omnidirectional feeding source and an oversized annular lens. The feeding source is a compact multi‐stepped biconical antenna with a wide impedance bandwidth. By rotating a horizontally placed two‐dimensional extended hemispherical lens around the vertical axis, the oversized annular lens is realized, which effectively increases the vertical radiation aperture and maintains high aperture efficiency to enhance the omnidirectional gain. Under the same envelope, the proposed antenna features a higher gain than the biconical antenna and the ordinary annular lens antenna. A prototype is fabricated and measured. The measured results demonstrate that the ‐10‐dB impedance bandwidth is 20.7% (24‐29.6 GHz), with stable vertically polarized (VP) omnidirectional radiation patterns and gain over 10.3 dBi within the entire band of interest. [ABSTRACT FROM AUTHOR]

This article presents an antenna array to produce a circularly polarized (CP) isoflux beam for CubeSat earth coverage applications. Distinguished from conventional arrays formed by identical elements, the proposed one is based on sequential rotation-fed concentric annular aperture elements. By adjusting the excitation phase, magnitude, and polarization of each annular aperture element, the radiation patterns of the array can be shaped to form isoflux beams. Radiation pattern modeling method is developed to obtain array radiation parameters. Besides, to effectively characterize the impedance matching of the radiators of the array with multiple excitations, an overall reflection coefficient is deduced from signal decomposition. A prototype operating at $C$ -band (5 GHz) is designed, fabricated, and measured. It consists of a radiator and a feeding network. The radiator is formed by two annular aperture elements, of which the inner one is realized by a circular patch antenna while the outer one is formed by eight planar inverted-F antennas (PIFAs). The feeding network drives annular aperture elements in the manner of sequential rotation. In this way, a CP isoflux beam array with a low profile ($0.078~{\lambda }_{0})$ , lightweight, and ease-of-integration for CubeSat is realized. The proposed array can also be extended to realize other shaped beams. [ABSTRACT FROM AUTHOR]

This study proposes a novel miniaturized circularly polarized (CP) ultrahigh frequency (UHF) quadrifilar helix antenna for spaceborne applications. The dual‐band operation is realized using four inverted‐U shaped helical strips (IUSHSs) that are rotated and alternately arranged on the four faces of a hollow polyimide cuboid in a sequential rotation manner, which effectively reduces the size of the antenna. Furthermore, the four IUSHSs are connected by a cross‐shape strip at the top of the antenna to control the dual resonant frequencies, resulting in a small dual‐band frequency ratio. The proposed antenna is both lightweight and robust when compared with the conventional miniaturized CP antennas operating at similar bands with similar performance. In particular, its compact radiator provides effective miniaturized spaceborne solution without the need of high‐dielectric coefficient materials. A device for spaceborne application that operates at 402/505 MHz is designed, fabricated, measured, and in‐orbit tested with a weight of 651 g and an effective size of 0.161 × 0.161 × 0.228 λ3402MHz (λ402MHz is the wavelength at 402 MHz). The measured gain and axial ratio of the proposed antenna are better than 5.32 dBi and 2.18 dB, respectively, within 2 and 12 MHz bandwidth for the two bands. The test results proved that the methods used to design the proposed antenna are effective. [ABSTRACT FROM AUTHOR]

In this paper, low-profile compact microstrip magnetic dipole antennas with large beamwidth and broad bandwidth are proposed for vehicular applications. At first, by introducing a strip on the bottom layer of microstrip magnetic dipole antenna, the electric current sources are introduced for the enhancement of beamwidth together with the magnetic current source. The operation principle of an equivalent complementary source model is then revealed to provide a physical insight. As seen, the half-power beamwidth (HPBW) can be controllable and effectively enlarged by the electric length of the planar magnetic dipole antenna. Next, by using a stub crossing on the strip, the impedance bandwidth is significantly improved comparing with the unloaded counterpart. After that, the antenna size is miniaturized by etching a pair of slots to extend the length of current path. Experimental results indicate that, under a very low profile of 0.016λ0 and 0.032λ0, the antenna prototypes can still achieve an impedance bandwidth of 14.5% and 20.8%, and a maximum HPBW of 99° (E-plane)/180° (H-plane) and 112° (E-plane)/159° (H-plane). It is also demonstrated that when the antenna size is reduced to 0.442λ0×0.284λ0, the impedance bandwidth up to 14.9% and the maximum HPBW of 93° (E-plane)/231° (H-plane) can be guaranteed with a low profile about 0.032λ0. Comparing with other design approaches, the proposed designs feature a very low profile structure, and superior beamwidth and bandwidth properties, as well as simple configuration and compact size. [ABSTRACT FROM AUTHOR]

An 4G/5G antenna with small clearance for metal frame smartphone applications is proposed. The proposed antenna consists of metal frame, inverted‐T feeding lines, S‐shaped branch, and the system ground on which U‐shaped slot is etched. The metal frame is broken and there is a 2 mm open slit on the metal frame. There is a 2 mm gap between the metal frame and the substrate. The ground clearance of the proposed antenna is 34.5 × 5.5 mm. Inverted‐T feeding lines are connected to the metal rim. Using the multiple radiation patterns of S‐shaped branches and the coupling ring mode of S‐shaped branch and ground‐extending metal frames, the measured −6 dB impedance bandwidth are 1680 to 2820 MHz and 2870 to 5500 MHz bands, which can fully cover DCS1800, PCS1900, UMTS2100, LTE2400, 2600 (1710 to 2690 MHz), and 5G N77/N78/N79 (3300‐4200 MHz, 4400‐5000 MHz) bands. The measured efficiency is greater than 43% in desired bands. The proposed antenna has the advantages of small ground clearance, wide bandwidth, and no lumped elements. [ABSTRACT FROM AUTHOR]

An annular array antenna is proposed in this letter. The proposed low-profile antenna produces an enhanced gain with omnidirectional vertically polarized radiation patterns. Center-fed via-shorted circular patch antenna with omnidirectional radiation pattern can be low profile and wide bandwidth but suffers from tilted beam leading to a low gain in the azimuth plane. To improve the azimuth gain, instead of the traditional way of increasing the profile, a low-profile omnidirectional director is exploited, which is basically a passive magnetic dipole consisting of an annular side-coupling open-cavity. Several directors cooperating with the driven element form the proposed antenna, whose working principle is similar to the Yagi–Uda antenna. A prototype with a low profile of 0.11 ${\lambda _0}$ is designed and fabricated. Measured results show that the prototype can realize −10-dB impedance bandwidth of 13% (4.72–5.42 GHz) and gain of 3.72 dBi at 5.34 GHz with good omnidirectivity. Compared with that of the driven element only, the omnidirectional azimuth gain is significantly improved within the bandwidth, with the maximum gain being enhanced from −4.2 to 3.72 dBi at 5.34 GHz. The proposed antenna is designed for a portable 5G sub-6 GHz wireless channel measurement application. [ABSTRACT FROM AUTHOR]

Broadband polarisation reconfigurable antenna based on crossed dipole and parasitic elements is proposed in this study, which can switch among three polarisation modes, i.e. linear polarisation, left‐hand circular polarisation and right‐hand circular polarisation. The proposed antenna consists of two crossed dipoles fed by a 3/4 square ring, two microstrip feeding elements etched at the centre of the upper and lower sides of the dielectric substrate, and three sets of microstrip parasitic elements helping broaden the axial‐ratio (AR) bandwidth. Four PIN diodes are embedded between each crossed dipole and the microstrip feeding element, which enable the polarisation reconfigurability. The working principle of polarisation reconfigurability is analysed in detail. The measured results show that the proposed antenna has a −10 dB overlapped impedance bandwidth of 63.5% (2.04–3.94GHz) and the 3 dB overlapped AR bandwidth is about 54.5% (2.4–4.2GHz), resulting in a wide overall bandwidth of 48.6% (2.40–3.94GHz). Besides, the measured peak gain of this antenna reaches up to 8.5 dBi. The proposed antenna can be a good candidate for 2.45GHz WiFi, LTE 4G, and sub‐6GHz 5G applications. [ABSTRACT FROM AUTHOR]

A novel very‐high frequency (VHF) band spaceborne phased antenna array for satellite‐automatic identification system/VHF data exchange (SAT‐AIS/VDE) applications operating at 159 MHz is proposed in this letter. Element phase shifting is achieved by controlling each element mechanical rotation angle rather than the phase shifter behind each element. Thanks to the new working scheme, the proposed array shows several advantages over its former counterpart, including low cost, high power capacity, and broad phase‐shifting bandwidth. This letter explains the basic principle, and presents a preliminary study on element rotation angle controlled phased antenna array technology, and an eight‐element‐array which is implemented on solar panel supporting plate is designed to validate the concept. [ABSTRACT FROM AUTHOR]

This article presents a new linear-to-circular polarization conversion coding unit, on which two new kinds of beam-shaping lenses are proposed. First, under periodic boundary conditions, a linear-to-circular polarization conversion coding unit is introduced, which introduces the necessary phase delay by adjusting its geometrical parameters. The phase delay ranges from 0° to 360° and is discretized into 3 bit coding units corresponding to specific delays. Second, by properly arranging the coding units, a high-gain circularly polarized (CP) lens is proposed. The lens achieves linear-to-circular polarization conversion and beam collimation in the transmission mode simultaneously with a planar configuration, which is different from counterparts that place a lens atop of a polarizer. Furthermore, the coding units are used to form Wollaston-prism-like and Rochon-prism-like planar CP beam-shaping lenses, which split the beams with different polarizations into right- and left-handed components. These beams can be controlled independently. Prototypes working at 30 GHz band are designed, fabricated, and measured to verify the idea. [ABSTRACT FROM AUTHOR]

This letter presents a dual-band, dual-polarized antenna array arrangement for low grating lobe base-station antenna applications. The presented array arrangement uses two different lower band elements to achieve a 2.5-time element spacing array scheme, in which lower band elements spacing is 2.5 times of upper band elements spacing. With this arrangement, both the lower and upper band element spacing can be reduced, leading to a better grating lobe performance. To verify the idea, an antenna array operating at two bands, namely, 690–960 and 1690–2690 MHz, is presented. For 690–960 MHz array, two different radiation elements are designed, each of them has two pairs of bent dipoles for dual-polarization operation. The smaller lower band element is embedded with one upper band element. Meanwhile, the larger one is embedded with two upper band elements. An array consisting of five lower band elements and 12 upper band elements is fabricated and tested. Measured results show that the array has a good performance with return loss larger than 15 dB, ports isolation better than 28 dB. Besides, the grating lobe value is better than –12 dB at tilt angle 12° for both upper and lower band, which is a significant improvement over its former counterparts. [ABSTRACT FROM AUTHOR]

A wideband polarization reconfigurable antenna based on optical switches and a C‐shaped radiator is proposed. The antenna can achieve three polarization modes, linear polarization (LP), left‐hand circular polarization (LHCP), and right‐hand circular polarization (RHCP). The proposed antenna consists of a simple C‐shaped patch, a modified ground plane, a feeding unit, and two parasitic stubs. Four slots are embedded in the C‐shaped patch properly to adjust the axial ratio (AR) performance for CP modes. And two silicon optical switches are inserted into each brand of the feeding unit. The measured results show that the proposed antenna has a 10‐dB impedance bandwidth of 86.8% (1.82‐4.61GHz) in LP mode, 81.8% (1.3‐3.1 GHz) and 14.1% (3.3‐3.81 GHz) for LHCP mode, and 81.8% (1.3‐3.1 GHz) and 19.2% (3.3‐4 GHz) for RHCP mode. The 3‐dB AR bandwidth is about 49.1% (2.38‐3.93 GHz) for LHCP and 51.4% (2.33‐3.94 GHz) for RHCP. The simulated and measured results show a pretty good agreement. Furthermore, in the range of operating bandwidth from 2.38 to 3.81 GHz, the proposed antenna can satisfy the 2.45 GHz WiFi, LTE 4G, and sub‐6 GHz 5G applications. [ABSTRACT FROM AUTHOR]

Novel designs of planar aperture antenna (PAA) arrays are proposed in this article, which achieve dual-polarized and high-gain performances in the 60 GHz unlicensed band. In order to achieve high gain, the original PAA is expanded to its two-dimensional array form, and the two linear polarizations share the same apertures. The two polarizations are realized by using orthogonal coupling slots excited by substrate-integrated waveguide (SIW) power dividers below. Two array prototypes with $8 \times 8$ and $16 \times 16$ radiation cells are designed and manufactured based on low-temperature co-fired ceramic (LTCC) technology for ease of integration. The simulated results show that the two antennas have −10 dB impedance bandwidth and 3 dB gain bandwidth of 57.5–65 and 57–64 GHz, respectively, with stable, no-squint, and symmetrical beams in band, which is verified by measurements. Compared with other reported high-gain and dual-polarized millimeter-wave antennas, the proposed antennas feature a low-profile, simple structure while keeping high aperture efficiency. The proposed antennas are very suitable for integration and are promising in 60 GHz short-range communication applications. [ABSTRACT FROM AUTHOR]

This communication presents a low-profile wideband and high-gain patch antenna array using low-temperature cofired ceramics (LTCCs) fabrication technique for 60 GHz applications. A coupled feeding scheme is adopted so that the antenna achieves wide impedance bandwidth that covers the entire 60 GHz license-free band with a height of only 0.384 mm (four tape layers). The shorting via connecting the upper patch and ground achieves the function of a virtual ac ground plane of differential feeding. This enables the antenna element to achieve good radiation performances, including stable gain (variation less than 1 dB) and a symmetrical beam with low cross-polarization over the entire frequency band. The performances of the antenna element are comparable to that of the differential-driven patch antenna while the complex differential feeding network is not required. Furthermore, the proposed antenna element with simple and easy-to-integrate geometry is successfully extended to a $4 \times 4$ antenna array using a substrate integrated waveguide (SIW)-based feeding network. The measured results show that the impedance bandwidth of the array covers the 60 GHz license-free band. The maximum gain can reach 16.7 dBi and the radiation performances are very stable over the operating frequency band. [ABSTRACT FROM AUTHOR]

A new differential frequency-reconfigurable antenna based on dipoles is presented in this letter. The basic structure of the proposed antenna consists of two pairs of vertical arms forming two dipoles, the feeding structure, mode switching structure, and p-i-n diodes. By switching “on” and “off” p-i-n diodes, the antenna can resonate at two states, and thus work centered at 3.5 and 5.5 GHz, respectively. Though the two states correspond to different resonant modes, the effective radiation parts are the same and the mode switching structure will not contribute to the far field for both states. This results in similar radiation patterns for both states. Besides, the proposed antenna is also simple in structure and wide in bandwidth. The measured −10 dB impedance bandwidths are 2.89–4.07 GHz (33%) and 5.1–6.19 GHz (19.8%) for the two states, respectively. [ABSTRACT FROM AUTHOR]

This article reports the bandwidth expansion from 1.65 to 8.9 GHz obtained with a new feeding structure featuring a sleeve shell and an L‐probe. Theory analysis and parameter study are performed to reveal the bandwidth expansion mechanism. A prototype antenna is designed and tested based on the study. The antenna's measured results prove the sleeve shell broadens the antenna's impedance bandwidth from 42% to 118%. The measured and simulated results also show the enhancements on the antenna's maximum gain and radiation efficiency in the bandwidth because of the sleeve shell. Besides, it keeps the antenna's radiation patterns intact without requiring additional accommodation space because of the new feed's structural configuration. The combination of the sleeve shell and the L‐probe may be applied to other microstrip antennas for wideband applications. [ABSTRACT FROM AUTHOR]

In this article, key parameters that characterize field distribution and beam-wave coupling in confocal circuits are studied. The kinetic theory of confocal gyro-TWT considering launching loss is deduced based on it, which is capable of predicting the linear gain rate of confocal circuits with arbitrary mirror widths. Numerical results manifest that the output power/gain versus interaction position agrees well with the nonlinear results in the small-signal regime. In addition, main oscillations in confocal circuits including the near-cutoff oscillation and backward-wave oscillation (BWO) have been examined. Analytical expressions that can predict the near-cutoff oscillation threshold of uniform circuits without considering loss are derived and kz-f plots are introduced for lossy circuits. Furthermore, an algorithm that can automatically search the BWO start-oscillation thresholds of confocal circuits with arbitrary mirror widths has been developed. [ABSTRACT FROM AUTHOR]

The improvement of the beam–wave interaction efficiency of the sheet beam extended interaction oscillator (SBEIO) is studied and presented in this article. A novel multi-parameter optimization scheme is developed, which is based on the simulated annealing global optimization algorithm, the nonlinear theory, and 3-D simulations, including the axial electric field distribution and the distances between each gap. The 3-D particle-in-cell (PIC) simulations are performed to verify the scheme. A $\textit {Ka}$ -band SBEIO with nine gaps is optimized with a 36.25-kV and 3.5-A electron beam. The final optimized efficiency obtained from the developed code is 31%, which shows a good agreement with the 32.6% results of the PIC simulation. Compared with the uniform axial electric field distribution, the optimized design increased the efficiency from 15.8% to 32.6%. [ABSTRACT FROM AUTHOR]