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A periodic mirror-reflected circular-polarized (CP) leaky wave antenna (LWA) with dual beam-scanning in dual polarization types is reported in this paper. The unit cell of the proposed antenna consists of two oblique stubs that are orthogonally separated (90°) to create a circular polarization. In order to achieve better CP (axial ratio (AR) < 3dB) and beam-scanning characteristics for the periodic CP antenna, a new method of adding a groove at the junction of stubs within the unit cell is introduced. To realize dual beam-scanning in dual circular polarization, a metal via is added at the end of the antenna to generate a mirror reflection. A mirror-reflected structure is fabricated and measured. The measured results show that the right-handed CP (RHCP) scanning range of the incident beam is -31°~-8° and 5°~-14°, and the left-handed CP (LHCP) scanning range of the corresponding reflected beam is 31°~6° and -6°~-15°. The measured results are in good agreement with the simulated ones.

In this paper, a gain-enhanced patch antenna with a periodic microstrip rampart line is proposed. The periodic microstrip rampart line can modify the effective dielectric constant of the dielectric plate, and help to improve the gain of the traditional microstrip patch antenna. Based on the antenna design principle, a formula for calculating the gain-enhanced frequency point is proposed. A proposed high gain microstrip antenna is designed, fabricated and tested. The experimental results demonstrate that the gain of the proposed antenna reaches 10.0 dBi, which is 2.55 dB or 79.9% higher than that of the traditional patch antenna at the same operating frequency of 5.8 GHz. The measured and simulated results show a high consistency.

A tapered continuous-element leaky-wave antenna is presented, which has the merits of suppressed higher order harmonics and reduced sidelobe level (SLL). Two important design strategies are utilized for assuring good radiation pattern purity. First, the phase difference between the main radiating edge in each unit cell is disturbed to avoid higher order mode superpositioning in-phase. Second, an extra element with a relatively large phase constant is introduced for canceling the field of the first sidelobe location. The measurement results show a continuous scanning beam with an equalized gain is achieved at the frequency band of 5.6–8.8 GHz, the corresponding normalized SLL is decreased to about –20 dB and the radiation of HOH in the high frequencies is null at the backward direction.

A patch antenna coupling of a periodic leak-wave structure with tri-frequency capability is proposed. The periodic offset double-sided parallel stripline (DSPSL) leaky wave structure (LWS) is embedded in the direction of the whole ground of the microstrip patch substrate, then the capability of tri-frequency could be obtained by the different Floquet mode of the propagation constant. According to the design principle, a unified formula was derived to calculate the tri-frequency points at the same time. Though applying of antenna design principle and the simple tri-frequency formula, a tri-frequency patch antenna was designed and tested. Experiment results show that the antenna operated at 1.71, 2.05, and 2.72 GHz. The measured and calculated results show high consistency.

In this article, we present a novel periodic microstrip leaky-wave antenna (MLWA) design based on different unit cells. The proposed antenna consists of three different kinds of unit cells developed in our previous works, in which the wavenumber of fundamental mode is basically consistent. Big period cascaded by the three different unit cells is introduced for producing a relatively narrow radiation beam. Almost same radiation angle of the antenna is obtained regardless of the permutation of unit cells, which provides more design freedom on antenna structure. Slotting and pins loading technique are applied in unit cells for canceling the reactance of the big period, thus a continuous beam scanning with small gain fluctuation around broadside direction could be achieved. The prototype is fabricated and measured, demonstrating the feasibility of this novel periodic antenna design and well agreement between simulated and measured results. Measurement results show the main beam steers continuously from −46° to 31° with the operating frequency changes from 6 to 9 GHz. The peak gain reaches up to 12.8 dBi at the frequency of 7 GHz.

A novel folded slot antenna is proposed for the design of high-gain and compact endfire arrays. This letter shows that a planar slot antenna retains bidirectional radiation characteristics when folded into a compact structure. A folded slot antenna was designed in order to obtain a high endfire gain of 6 dBi. The proposed slot antenna's high gain makes it a good candidate for designing endfire arrays with vertical polarization. A series-fed technique is proposed in order to design two-element endfire and backfire arrays. Subsequently, an eight-element endfire array is proposed by combining endfire and backfire two-element arrays. The eight-element array is capable of producing a high endfire gain of 13.2 dBi with a size of 1.8λ0 × 1.4λ0, which is less than half the size of the existing endfire antennas with similar vertically polarized gain.

A periodic microstrip leaky wave antenna (MLWA) constructed of non-uniform aspect ratios microstrip patch periods is presented in this communication. Effective equations and simulations for the relevant antennas are used to obtain the propagation constants of the proposed antenna and design it. However, due to the presence of an open stop-band (OSB) in the broadside direction, double-side parallel-strip lines are then introduced in the antenna design. Truncating and interlacing the unit cells are developed to make them become impedance matched structures with the elimination of the OSB and the improvement of the radiation patterns. As a result, a new antenna configuration is created, and a prototype of the antenna is fabricated and measured. Continuous main beam scanning is observed from back-firing to end-firing with the operating frequency band of 4.8-18 GHz. Excellent agreements between the predicted and measured results are obtained. The measured peak gain of the prototype is 14 dBi, and the gain exceeds 6 dBi over the frequency band.

A multi-frequency patch antenna with double-sided parallel-strip line (DSPSL) periodic offset structure is proposed. The designed antenna consists of a simple rectangular patch as well as an embedded one-dimensional DSPSL periodic offset structure, which is employed to replace the traditional rectangular microstrip patch antenna. The periodic DSPSL structure is characterized leaky wave structure. After the periodic DSPSL structure is embedded, the resonance frequency of rectangular patch is offset, and the leaky wave characteristics can determine the number and position of new operating frequencies. As an application, a tri-band antenna is manufactured and these three operating frequencies can be calculated by the derived formulas studied in this communication. Experiment results suggest that the antenna can work at 4.28, 4.78, and 6.42 GHz, respectively. The results of simulated, measured and calculated show high consistency.

A gain-enhanced tri-band microstrip antenna using higher order modes was analyzed and designed in this communication. It first proved in theory that by manipulating the equivalent magnetic currents’ (EMCs) relative magnitudes, the higher order modes’ radiation patterns could be converted from conical to broadside. Their zenithal directivities would therefore greatly increase. The relationships between the EMCs’ relative magnitudes and the directivities were studied. Then, those analyses were applied to design a gain-enhanced tri-band microstrip antenna using three higher order modes. A simpler one-port differential feeding and several shorting pins and slots were used to control the antenna’s EMCs to improve its radiation patterns. They could further increase the antenna’s gains and suppress its sidelobe levels and cross-polarizations. The proposed antenna worked at 3.9, 4.95, and 5.9 GHz with consistent broadside radiation patterns and the same polarization. Their measured gains were about 10.0, 10.4, and 10.2 dBi, respectively.

A novel dual-polarized antenna, with a large bandwidth and a high front-to-back ratio, is proposed in this communication. The antenna is composed of printed dipoles and tapered baluns above a square ground plane to yield a wide bandwidth. Semicylindrical walls are loaded on the edge of the ground plane to suppress the back lobe and produce a stable gain. Backward radiation is suppressed by the diffraction effect of the semicylindrical sidewalls. Simulation shows that the back-lobe level can be reduced below −13 dB in a large frequency range without degrading the antenna bandwidth. The proposed antenna is fabricated and the measured results show that it has an impedance bandwidth of 69.6% (1.46–3.02 GHz) for voltage standing wave ratio

Novel microstrip magnetic monopole and dipole antennas are proposed to achieve a high directivity along the elevation plane as well as a horizontally polarized omnidirectional pattern in the azimuth plane. The directivity of this microstrip magnetic monopole antenna is calculated by numerical integration; the results show that the directivity increases with the antenna length when the antenna resonates in the fundamental mode. However, simulation shows that the main beam is tilted away from the azimuth plane when the antenna length exceeds $2\lambda $ due to high-order resonant mode interference. A solution is provided to suppress the second resonant mode and retain the radiation peak in the azimuth direction. The peak gain of the microstrip magnetic monopole is increased to 7.2 dBi when the antenna length is approximately $3\lambda $ . A microstrip magnetic dipole antenna with an antenna length of $6\lambda $ is also designed to obtain a higher gain of 9.7 dBi. The impedance bandwidths of the microstrip magnetic monopole and dipole antennas are 6.5% (2.37–2.52 GHz) and 7.3% (2.35–2.53 GHz), respectively. The proposed antennas produce maximum radiation in the azimuth direction and have horizontally polarized omnidirectional patterns in the azimuth plane.

A nonbalancing microstrip dipole with end-fire capability is presented. The proposed antenna consists of a simple microstrip-fed dipole with a periodic-offset double-sided parallel-strip line (DSPSL) substrate, which is used to replace the balun of a conventional end-fire antenna. The periodic-offset DSPSL substrate exhibits phase shift characteristics and excites the odd mode of the microstrip dipole. A director and a truncated ground are used to increase the directivity of the antenna. After placing the DSPSL, the bidirectional radiation pattern is converted to a unidirectional one. The measured and simulated results indicate that the antenna operates at 2.4 GHz. End-fire directional radiation patterns are obtained with this novel periodic substrate. When compared with the original design of the microstrip dipoles, the proposed microstrip dipole uses the periodic-offset DSPSL substrate to replace the balun. The front-to-back ratio is about 10 dB. The design method of the end-fire antenna is a new technique and can be easily applied in other antennas.

A backfire to endfire beam-scanning antenna based on double-side parallel-strip lines (DSPSL) is presented in this communication. The antenna consists of periodic offset DSPSL structures with triangular truncation. A series of uniform DSPSL and offset DSPSL equations were established to analyze its operating modes, transmission mode, and first higher order. The working band range in the periodic DSPSL structure is shown to assist the design of the proposed antenna. The main beam of the antenna scans from backfire to endfire as operating frequency increases owing to its periodic structure. Truncating the periodic offset DSPSL improved the current distribution and consequently broadened the impendence, enhancing the radiation patterns. Experimental results showed that the main lobe scans electronically and continuously from the backfire (-z-direction) to endfire in the yz plane toward the absolute end-firing direction (+z-direction) with two open stopbands when operating frequency increases from 1.2 to 5.9 GHz. The peak gain of the antenna is about 9.7 dBi, and the gain in the whole working band exceeds 4 dBi.

A dual-band sector ring microstrip antenna operating at the TM11 and TM02 modes is analyzed and then designed in this communication. Unlike previous analyses that use only cavity model or transmission line, a simpler analysis using both models is proposed. Accordingly, several closed equations are derived. Such equations determine the relationship among three important units of construction, namely the radii, the angle, and its operating frequencies, and with the operating factors in a dual-band sector ring patch. The errors between the proposed method and the cavity model are acceptable. The field and radiation patterns of the sector ring patch are also discussed. In addition, these analyses and equations are applied to design a dual-band sector ring microstrip antenna for WLAN. A novel coupling sector ring patch is then proposed to improve the performance of the antenna. The experimental results are in good agreement with the analyses and the simulations. The impedance bandwidths of the antenna are about 40 and 900 MHz with consistent broadside radiation patterns at 2.4 and 5.5 GHz band. Additionally, their measured average gains are about 5.1 and 6.3 dBi, respectively.

A new microstrip endfire array antenna with vertical polarization is proposed in this paper. The novel array element consists of two symmetrical rectangular patches on both sides of a substrate with two edges shorted and other edges opened. We use a cavity model to analyze the mechanism of the array element, which we find can work as a “magnetic monopole antenna.” The magnetic current on the open sides, which features quarter cycle sinusoidal variation, generates a tilted monopole-like radiation pattern. The tilted angle can be easily determined by the length-to-width ratios of the magnetic monopole antenna. A two-element endfire array and a four-element endfire array are proposed here to achieve endfire vertical polarization with a very low profile (3 mm). The two-element endfire array has a bandwidth from 2.42 to 2.48 GHz and a peak endfire gain of 5.3 dBi. With four array elements, the peak endfire gain can be enhanced to 7.7 dBi in a bandwidth from 2.40 to 2.48 GHz.

A new planar quasi-magnetic–electric circularly polarized (CP) antenna is proposed. The antenna has a simple planar structure with a small size and a single feed, and produces an omnidirectional CP radiation pattern. It is based on a quarter-wave shorted patch connected with two printed strips. The tangential electric field at the main open aperture of the shorted patch can be considered as an equivalent magnetic current, while the two strips contribute two electric currents. With a single feed, the proposed antenna exhibits an inherent phase difference of about 90° for the two kinds of currents, and therefore, a CP radiation is obtained. The antenna is designed and fabricated. Measured results show that the antenna has an impedance bandwidth of 4.3%. In the band of interest, the antenna generates a circular polarization and an omnidirectional pattern in the plane perpendicular to the magnetic and electric currents, with an average axial ratio below 1.5 dB and a gain between 2.4 and 3.3 dBi.

A novel dual-frequency broadband design of a stacked microstrip monopolar patch antenna is proposed for WLAN applications. The antenna is composed of a via-loaded ring on the bottom layer and a circular patch on the top layer, both of which are coupled-fed by a circular coupler. A broad lower band is generated by converging the TM 01 and TM 02 modes of the via-loaded ring, and a broad upper band is yielded by coupling the TM 03 mode of the via-loaded ring and the TM 02 mode of the circular patch. Measured results show that the proposed antenna provides a broad lower band from 2.28 to 2.55 GHz and a broad upper band from 5.15 to 5.9 GHz with a low profile of 6 mm. Monopole-like radiation patterns are obtained in the dual bands with peak gains in the lower and upper bands of about 5 and 6 dBi, respectively.

A novel directional microstrip quasi Yagi array antenna is proposed to achieve a wide bandwidth with a low profile and a compact structure. The array is composed of a driven circular sector and annular sector directors. Arrays with both a single director and a dual director have been studied. Measured results show that the proposed antenna provides a stable high gain in a wide bandwidth with a low profile of 1.5 mm ( $0.03\lambda_{0}$ at 5.8 GHz). For an array using a single annular sector director, an impedance bandwidth of 13.6% (5.10–5.85 GHz) is achieved with a peak gain of 8.2 dBi. With two annular sector directors, the bandwidth is improved to 17.6% (5.05–6.02 GHz), and the peak gain is 10 dBi.

A novel multifrequency one-quarter-rings microstrip antenna is presented in this letter. The proposed antenna consists of two one-quarter rings in different sizes, with an inverted T-shaped slot loaded. The T-shaped slot is used to suppress the higher-order mode. The relationship of the resonant frequency $f$ with different physical size parameters for the single one-quarter-ring microstrip antenna is also reported, and it contributes to the antenna design. The resonant frequencies of the multifrequency antenna are determined by the size of the one-quarter rings. The experimental results show that the proposed antenna operates at 0.9, 1.4, 1.8, and 2.4 GHz, respectively. The radiation patterns at different resonant frequencies are similar.

A novel design of a multiband monopole mobile phone antenna with circular polarization for GNSS application is presented. The proposed antenna generates four resonant frequencies with branch lines and a shorted parasitic strip to obtain a wide operating band. With the definition of 2.5:1 VSWR, the bandwidth covers several wireless communication systems, including GSM (880 ~ 960 MHz), DCS (1710 ~ 1880 MHz), PCS (1850 ~ 1990 MHz), UMTS (1920 ~ 2170 MHz), WiBro (2300 ~ 2390 MHz) and ISM (2400 ~ 2483 MHz), and also covers GNSS, including COMPASS (1559.052 ~ 1591.788 MHz), GPS (1575.42 ± 5 MHz), GLONASS (1602 ~ 1615.5 MHz). A tuning stub is added to the ground plane and the feeding strip is mounted 45 ° at the corner to achieve circular polarization for GNSS application. The 3 dB axial ratio (AR) bandwidth (AR-BW) is obtained from 1540 to 1630 MHz, covering the L1 band of GNSS, including COMPASS, GPS and GLONASS. In the 3 dB axial ratio bandwidth, right hand and left hand circularly polarizations are obtained in different broadside directions, with the peak circularly polarized gain of more than 2.7 dBic. An equivalent circuit network is used to analyze the mechanism of circular polarization. Details of the proposed antenna parameters, including return loss, radiation characteristics, and AR spectrum are presented and discussed.

A biplanar monopole with a double-sided parallel strip line (DSPSL) feed and coupling line for broadband mobile phone is presented. The proposed antenna consists of a DSPSL feed and a coupling line, suitable for broadband operation (824-960/1710-2620 MHz). It just requires a small area of 15 × 40 mm2 on the circuit board, and it is configured along the boundary of the no-ground portion so that it can save much area, on which other electronic components can be put. The antenna is formed by a short strip line at the back side and a long strip line at the front. The overlapping part of them forms the DSPSL structure, which leads to a better impedance matching result. The long strip line contributes to cover the lower band (824-960 MHz), while the short strip line generates a resonant mode to cover the upper band (1710-2620 MHz). A coupling line alongside the long strip line and the ground are also formed to make the high resonant mode more effective. Details of the antenna are presented.