In this research, the novel concept of a resonance-based reflector (RBR) was proposed, and a ring-shaped RBR was utilized to design a unidirectional antenna with low-profile and broadband characteristics. Research found the ring operates as two half-wavelength (λ /2) resonators. Then, the resonance effect transforms the reflection phase of the ring RBR, and achieves a reflection phase of 0◦ < φ < 180◦ in a wide frequency range above the resonance.
Then, the in-phase reflection characteristic (− 90◦ < φ < 90◦) can be obtained in the wide frequency band by placing an antenna above the RBR with a distance smaller than λ /4. Two unidirectional antennas, named Case 1 and Case 2, were designed with the ring-shaped RBRs and bowtie antennas (RBR-BAs). The impedance bandwidths of Case 1 and the Case 2 are 2.04–5.12 GHz (86.3%) and 1.97–5.01 GHz (87.1%), respectively.
The front-to-back ratio (FBR, an important parameter to measure the unidirectional radiation) of Case 1 ranges from 5–9.9 dB for frequencies 2.04–2.42 GHz, and the FBR of Case 2 ranges from 5–16 dB for frequencies 2.16–3.15 GHz. The proposed concept of RBR is desirable in wideband unidirectional antenna design, and the designing antennas can be used at the front end of wireless systems—such as indoors communication, remote sensing, and wireless sensor systems—for signal receiving or transmitting.
ANALYSIS AND DESIGN OF RBR
As shown in Figure 1, a reflector can be considered as surface impedance Zs. Then, the reflection phase φ at the surface of the reflector can be defined by Equation (1). Note that η is the characteristic impedance of the free space above the reflector. For Equation (1), the terms “ Im ” and “ ln ” are mathematical symbols. The “ Im ” term means “taking the imaginary part of a complex number”, and the “ ln ” term means “taking the natural logarithms”.
Then, if the ring RBR is designed to other frequencies, we can firstly determine its perimeter as a wavelength λ. From the current distribution shown in Figure 4a, the currents originate from the bottom and flow along two half-rings to the top, therefore, the ring operates as two λ/2 resonators. Note that the effective length of the resonators is longer than a half-ring, as the two half-rings are connected.
ANTENNA DESIGN AND DISCUSSION
The proposed ring-shaped RBR was used to design a unidirectional antenna, as shown in Figure 8. The schematic view of the reference antenna (RA) and the ring-shaped RBR are demonstrated in Figure 8a,b, respectively. The RA has an identical structure to the RAs of our previous proposed antennas. The RA is a bowtie antenna surrounded by a metal ring. Therefore, it is a UWB antenna with an omnidirectional pattern. From simulations, the antenna parameters were obtained.
The antennas were manufactured and measured as shown in Figure 9a,b. Similar, a wide-band balun was used to transform the 75 Ω input impedances of the antennas to 50 Ω of a subminiature version A (SMA) connector for measurement. The |S11|was measured by an Agilent N5230A network analyzer, and the radiation patterns were measured in a microwave chamber with NSI2000 system. The Agilent N5230A network analyzer is an S-parameter measuring instrument from Agilent Ltd. (Santa Clara, CA, USA) and the NSI2000 system is a field measuring system from NSI Ltd.
A novel reflector of a resonance-based reflector (RBR) was proposed in this research. The concept and theory of the RBR were discussed. A ring-shaped RBR was investigated and used to design unidirectional antennas. A novel broadband unidirectional antenna that utilizes the RBR was proposed in this paper. Compared to the PEC and the AMC reflectors, the proposed RBR is capable of realizing an antenna with advantages of low profile, compact size, ultrawideband and unidirectional pattern, simultaneously.
Two cases of RBR-BA were designed, and their measured results agreed with simulation results. This research indicates that the proposed RBR is a good choice for antenna design with low-profile, compact size, ultrawideband and unidirectional pattern characteristics, which makes it a good candidates for wireless applications such as indoors communication, remote sensing, and wireless sensor systems.
Source: Guilin University
Authors: Lin Peng | Ji-yang Xie | Kai Sun | Xing Jiang | Si-min Li