Juhua Liu

A design approach is proposed for a compact pattern reconfigurable antenna based on short-open circuited transmission line. The designed antenna comprises a center-fed transmission line with two terminations and two dipoles. The currents on the dipoles that are deployed on the line with short and open circuited terminations have similar magnitudes but a large phase difference (about −140°), so the antenna radiates at endfire with high gain and high front-to-back ratio (FTBR). When the left termination is short-circuited (open-circuited) and the right one is open-circuited (short-circuited), the phase difference is about −140° (+140°) and the antenna can provide an endfire (backfire) beam at θ = +90° (−90°). By switching the states of the terminations with 2 PIN diodes, the endfire/backfire pattern reconfiguration can be achieved. With a compact size of 0.39λ₀ × 0.20λ₀ (λ₀ is the wavelength in free space at operating frequency), a prototype of the antenna is designed at 2.4 GHz, which reaches peak gains of 5.59/5.55 dBi and peak FTBR of 23.02 dB with a fractional bandwidth of 12.2% in different states.

In this letter, to meet the small wind load area requirement in a highway tunnel scenario, a compact leakywave antennas (LWA) end-fire array is designed and machined. The LWA utilizes an offset double-sided parallel strip-line (DSPSL) structure as leaky waveguides, and two short vias at the end are used for impedance matching instead of port loading. The effect of the via parameters are analyzed. Moreover, the average reference signal received power (RSRP) at N41 band (2.496GHz-2.69GHz) in a realistic highway tunnel at Fujian are measured by a handheld terminal device. The coverage performances between traditional leaky cables (LC) and different polarized LWA are compared under the highway tunnel environment. The path loss, shadow fading and secondorder statistics channel parameters are extracted and analyzed. It shows that the high gain LWA owns a higher RSRP compared to LC. Dual-antenna polarization diversity reduces small-scale fading. Distributed antenna diversity shows RSRP is more sensitive to distributed distance, and the coverage enhancement of dual-polarization distributed diversity is not significant.

In this communication, a novel technique is proposed to enhance the front-to-back ratio (FTBR) for an endfire traveling wave antenna by using shorting posts. Without matching load at the terminal, the proposed technique has the advantage of low cost and avoids heating caused by load, especially for high power application. An endfire traveling wave antenna is developed to achieve high gain with a simple structure, comprising an air-filled offset parallel transmission line with closely loaded identical dipoles. However, the initial design suffers from the substantial reflected wave that degrades the FTBR. To enhance the FTBR of the antenna, two shorting posts are introduced. With multiple reflections induced by the shorting posts and the opened end, the magnitude of the total reflected wave is substantially suppressed and the FTBR is enhanced consequently. The final design, with simple structure and lost cost, is fabricated and measured. With a compact size of 4.76 λ₀× 0.43 λ₀×0.03 λ₀, the antenna achieves a peak gain of 15.45 dBi and a high FTBR of over 15 dB in a fractional band of about 12%.

A compact magnetic-electric circularly polarized (MECP) element is proposed and employed in the design of a pattern reconfigurable antenna that provides full coverage in the azimuth plane and wide coverage in the elevation plane. The MECP element, working in the zeroth-order resonant (ZOR) mode, inherently has a compact size of 0.17λ₀ × 0.32λ₀ × 0.06λ₀ (λ₀ denotes the free space wavelength at 920 MHz). The MECP element achieves a fan beam with wide azimuthal and elevational 3-dB axial ratio (AR) beamwidths of 192° and 110°, respectively. By integrating four such sequentially rotated MECP elements, a pattern reconfigurable antenna is designed and fabricated. The antenna has the advantage of compact size of 0.58λ₀ × 0.58λ₀ × 0.04λ₀. By switching the excited elements through the feeding ports, the antenna provides four reconfigurable beams for full coverage in the azimuth plane (one effective beamwidth covering an angle larger than 90) and wide coverage in the elevation plane (effective beamwidths covering angles within the range of  ≤ 75).

A novel compact quasi-magnetic-electric circularly polarized (CP) antenna with reconfigurable patterns is proposed. The antenna, featuring a compact structure and a simple feeding mechanism, can generate four endfire CP radiation beams covering the entire azimuth plane. The antenna is composed of four quasi-magnetic-electric elements arranged in rotational symmetry. Each element consists of two layers of staggered trapezoidal shorted metals with a compact size of 0.51 λ₀ × 0.16 λ₀ × 0.05 λ₀ (λ₀ denotes the free space wavelength at 900 MHz) and is designed to provide an endfire CP beam with a single feed. By integrating four such elements and switching the excited elements through the feeding ports, the antenna can obtain four endfire CP radiation patterns in different directions. The prototype antenna has a compact size of 0.54 λ₀ × 0.54 λ₀ × 0.05 λ₀. The operational bandwidth of the fabricated antenna, constrained by |S₁₁| < −10 dB and the 3-dB axial ratio, spans from 867 to 928 MHz. The proposed CP pattern reconfigurable antenna can be a promising candidate for various applications, including radio frequency identification (RFID), Internet-of-Things (IoT), and indoor localization.

In this communication, we propose and analyze an endfire traveling wave antenna with high gain and high efficiency. The antenna is composed of an offset double-side parallel strip line that is loaded by a set of closely spaced T-shaped patches. The structure supports an improper leaky wave mode that is evolved from the TEM mode in the parallel transmission line, and a common and a differential proper surface wave modes that are evolved from the corrupted-conductor-like surface wave mode in a single conductor line with T-shaped patches loaded. When fed at one end, the leaky wave mode and the differential surface wave mode can be excited. With their phase constants close to the wavenumber in free space, the two modes contribute to endfire radiation wherein the surface wave mode plays the dominant role. A prototype of the antenna is fabricated and measured. The antenna with a size of 4.65×0.43×0.03λ₀³ achieves a high gain of 15.1 dBi and a peak efficiency of 95.8%, with a 3 dB gain bandwidth of 18.5%.

This paper proposes a new slot-mode substrate integrated waveguide periodic leaky wave antenna (PLWA) with glide-symmetry (GS). The scanning range of a PLWA is generally limited by two stopbands (the open stopband at the broadside and the next coming stopband in the forward quadrant) and the grating lobe (caused by the n = −2 space harmonic). To suppress the two stopbands and the grating lobe, we investigate a novel PLWA based on GS unit cells, in which each unit cell consists of two identical sub-unit cells, with each sub-unit cell containing two nonidentical slots. The two nonidentical slots are adopted to suppress the open stopband by quarter-wavelength impedance matching, while the unit cell with GS is employed to suppress the next coming stopband by three-quarter-wavelength impedance matching. Additionally, the antenna with GS avoids the next coming stopband and the grating lobe by directly suppressing the n = −2 space harmonic. With the suppression of the two stopbands and the grating lobe simultaneously, the PLWA generates a single beam that continuously scans from the near backward to forward endfire directions. A prototype of the proposed antenna based on a broadband feeding network is fabricated and measured for validation.

A novel technique is proposed for the design of an endfire/backfire reconfigurable leaky wave antenna by radiating different space harmonics. The antenna with a period of about a quarter wavelength can produce a unidirectional endfire beam through the radiation from the fundamental space harmonic. If the period is doubled, the antenna produces a unidirectional backfire beam through the main radiation from the n = −1 space harmonic, with negligible radiation from the fundamental space harmonic and with other space harmonics far away from radiation region. Using PIN diodes to tune the period in the antenna, the desired fundamental / n = −1 radiating space harmonic can be obtained, and then the endfire/backfire radiation can be reconfigured. Besides, to reduce the performance differences between the forward and backward radiation and to enhance the gain, the antenna is designed with a symmetric structure and fed at the center, in which the left-half and right-half apertures are operated in backfire and endfire states for forward radiation and vice versa. A prototype of the antenna is designed at 5.2 GHz, which reaches peak gains of 9.94 dBi and 9.89 dBi with bandwidths of 23.7% and 25.6% in the forward and backward states, respectively.

Empirical formulas are proposed for the superdirectivities of broadside linear and square arrays. The formulas are derived based on a large amount of numerical results of optimum directivity, for the superdirectivity of a broadside linear array of isotropic elements with an electrical element spacing varying from 0 to 0.5. The formulas for the linear array can also be developed for the square array. Besides the isotropic elements, the superdirectivity of the square array of different unidirectional elements are also numerically analyzed. It is found that the square array of unidirectional elements offers higher directivity than that of isotropic elements for a small number of elements, but tend to consistency as the element number increases. The numerical results are validated with the full-wave simulation of a square array of planar inverted-F antennas, considering coupling effects between elements.

A novel technique is proposed for the fixed-beam design of leaky wave antennas (LWAs) by periodically loading multiresonance elements. The single-resonance elements can generate phase shift with a negative slope in a narrow band for phase compensation, which is used to design an LWA with fixed beam. Furthermore, the fixed-beam bandwidth can be expanded by loading a set of different resonant elements. A design of fixed-beam microstrip LWA using the multiresonance technique is demonstrated and investigated. The antenna consists of a main microstrip line that is periodically loaded with a set of $L$ -shaped coupling patches of different sizes on both sides. The patches have different resonant frequencies, which act as radiators to generate leaky wave radiation and as resonant elements to compensate for dispersion in a wide operating band. A prototype of the antenna designed for radiating at -14° is fabricated and measured. Measurement results show that the antenna has only a 2.9° beam-squinting in the operating band from 14.2 to 15.7 GHz (with a fractional bandwidth of 10.1%), with a peak gain varying from 11.6 to 14.2 dBi. The bandwidth is almost four times that of the LWA with single-resonance elements loading.