This paper presents an AMC (artificial magnetic conductor)-based wideband circularly polarized printed monopole antenna for unidirectional radiation. The antenna includes an AMC reflector, a coplanar waveguide (CPW) feed structure to excite the antenna, a ground plane with a rectangular slot on the left side of feedline, and an asymmetrical ground plane on its right side. The induced surface currents on CWP feedline, rectangularly slotted, and asymmetrical ground planes cause circularly polarized radiations. The AMC reflector consisting periodic metallic square patches is used instead of the conventional PEC (perfect electric conductor) reflector, the distance between the antenna and reflector is reduced from 0.25λ0 to 0.18λ0 with performance improvement. By incorporating AMC layer with the monopole antenna, the gain of antenna is increased from 3.3 dBic to 8.7 dBic while the axial ratio bandwidth (ARBW) of antenna is increased from 27.27% to 51.67%. The simulated and measured results show that the proposed antenna has an overlapping 10-dB |S11| and 3-dB ARBW of 51.67% (3.0–5.09 GHz). The overall dimensions of monopole antenna backed by AMC reflector is 1.20λ0 × 1.20λ0 × 0.21λ0 and covers 5G sub-6 GHz new radio bands (n77/n78/n79) for wireless communication systems.
The race to 5th-generation (5G) communication has brought a significant paradigm shift in terms of diverse frequency allocation with massive bandwidths, low-latency, device densities, and very high number of transceiver antennas. The 3GPP (3rd Generation Partnership Project) has recently developed the 5G new radio (NR) air interface of 5G networks. This new radio access technology has very low latency (<1 ms) and high data rate, 100 times faster than 4G-LTE. The 5G NR is subdivided into frequency range 1 (FR1) and frequency range 2 (FR2). The FR1 uses frequency bands in sub-6 GHz, while the FR2 uses frequency bands in the millimeter (mm) wave range [
Countries | 1–3 GHz band | 3–4 GHz band | 4–5 GHz band |
---|---|---|---|
USA |
2.50/2.6 GHz | 3.45–4.2 GHz | |
Korea |
2.3–2.39 GHz | 3.4–4.0 GHz | |
Canada |
3.475–3.65 GHz | ||
EU |
3.4–3.8 GHz | ||
UK |
3.4–3.8 GHz | ||
Germany |
3.4–3.8 GHz | ||
France |
3.46–3.8 GHz | ||
Italy |
3.6–3.8 GHz | ||
Japan |
3.6–4.1 GHz | 4.5–4.9 GHz | |
China |
2.50/2.6 GHz | 3.3–3.6 GHz | 4.5–5.0 GHz |
Australia |
3.4–3.7 GHz | ||
India |
2.50/2.6 GHz | 3.3–3.6 GHz |
The division of electromagnetic spectrum into different sub-bands has increased the demand of antennas operating in these bands. The electromagnetic waves in these bands suffer from more severe free-space loss and blockage, which substantially degrades the signal-to-interference-plus noise ratio (SINR) [
The booming development of various wireless communication systems have posed a great challenge to have innovative antenna technologies like diversity antennas, reconfigurable antennas, metamaterial-based antennas, and antennas for software defined radio. Recently, circularly polarized (CP) microstrip antennas have got a great attention and it has been part of different wireless communication systems, such as for satellite communication, 3G/4G communication, GPS (Global Positioning System), RFID (Radio-frequency identification) tag, and RFID readers owing to its resilience to multipath interference and polarization mismatch [
Therefore, researchers came up with the new idea of using high impedance surfaces (HISs) or artificial magnetic conductors (AMCs), such as electromagnetic band-gap (EBG) structures [
CPW feed structure can be found widely in literature for its usage in many wideband circularly polarized antennas to improve ARBW [
There is very little research on circularly polarized antennas with an AMC surface for 5G sub-6 GHz communications due to the latest allocation of frequency bands (n77, n78, n78) under 5G NR. The inspiration behind the design of this antenna is to have a CPW-fed circularly polarized antenna based on AMC which could cover n77/n78/n79 bands of 5G NR communication system. In this paper, a wideband circularly polarized monopole antenna backed by AMC is designed with high antenna gain for unidirectional radiation. The organization of this paper is such that, the Section 2 discusses the configuration of the proposed antenna with its geometry, CP mechanism, parametric study, and AMC characterization in detail. The Section 3 briefly discusses performance of antenna over different reflectors. The Section 4 shows the proposed antenna performance with its simulated and measured results. Finally, the conclusion is drawn in Section 5.
The schematic diagram of proposed wideband circularly polarized monopole antenna is depicted in
This monopole antenna structure is backed by an AMC surface which is acting as a reflector surface and it is placed at a distance of “
Parameter | Value (mm) | Parameter | Value | Parameter | Value |
---|---|---|---|---|---|
11 | 14 | 3.5 | |||
17.8 | 12.08 | 17.5 | |||
1.6 | 38.3 | 30 | |||
14 | 0.813 | 87.2 | |||
13.5 | 10.8 | 0.8 |
Firstly, it is necessary to discuss how the CP radiations can be generated before going to analyze the CP mechanism in the present case of monopole antenna. For the CP radiation, current distribution forming a loop gives CP [
The second scenario can be visualized by
This section studies the performance effect on monopole antenna by analyzing the length of monopole (
It is well known that one of the components of travelling E-field along vertical direction in strip line feed causes to generate CP radiation. Therefore, the changing the length of
In this study, the AMC having periodic square unit cells (8 × 8) is used as a metallic reflector to redirect the radiations in a boresight of monopole antenna. The proposed structure contains periodical lattice having a periodicity
This unit cell model given in the
We have used build in genetic algorithm (GA) of CST MWS tool to optimize the unit cell parameters. CST uses built in MATLAB control functions to get the optimum dimensions for the unit cell. It is well known that these optimizers based on GA are robust and use the concepts of evolution [
The antennas which are backed by AMC give extra resonances due to travelling surface waves on AMC structure. In [
This section of paper discusses the performance analysis of monopole antenna in free space and when it is backed by finite-size PEC and AMC ground planes.
The monopole antenna alone on free space will be considered as reference model for the performance analysis. It can be used as bidirectional as well directional antenna. The bi-directional characteristics of this antenna are illustrated by radiation patterns (
The monopole antenna backed by finite PEC ground plan is simulated instead of AMC ground plane. According to image theory that any radiator placed in proximity and parallel to PEC will have a negative image thus cancel each other. This happens when the radiated field from the image current cancels the radiated field emitted by antenna current itself. It causes sudden increase in the stored electromagnetic energy of the antenna near-field due to the presence of PEC ground plane [
The bidirectional monopole antenna (discussed in Subsection 3.1) is converted into unidirectional by using the AMC surface which gives a 0° reflection phase unlike the PEC which causes a phase reversal. The properties of this surface are discussed in subsection of Section 2. By keeping the radiator at a foam height of
The limitations of antenna ARBW and gain when it radiates in free space, the drawback of PEC as a ground plane and the proposed AMC structure to enhance performance of monopole antenna all are graphically illustrated and compared its results in
Antenna type | 3-dB ARBW (%) | Peak gain (dBic) | Distance between reflector and antenna |
---|---|---|---|
Antenna alone | 27.27% (3.8–5.0) | 3.35 | N/A |
Antenna with PEC | 30.13% (3.1–4.2) | 8.29 | 0.25λ0 |
Antenna with AMC | 51.67% (3.0–5.09) | 8.7 | 0.18λ0 |
The prototype of the proposed AMC based wideband CP monopole antenna is fabricated and measured to validate the proposed design.
The measured and simulated reflection coefficient |S11| is shown in
To investigate the number of resonances, we calculate the modal significance and characteristic angle of characteristics modes for the proposed antenna using free source CMA (characteristic mode analysis) [
The simulated and measured broadside gain of the monopole antenna with AMC reflector is shown in
The characteristics of radiation pattern of the antenna is investigated with the integrated AMC surface behind the antenna, which redirects all the electromagnetic energy to the broadside direction. The simulated and measured radiation patterns of proposed antenna are analyzed in both xoz- and yoz-principal planes at three different frequencies 3.3, 3.7, and 4.1 GHz. The antenna is oriented with respect to the coordinate system given in
The equiangular spiral antenna [
Ref. | Antenna geometry and design | Antenna volume (λ03) | Polarization | 3-dB ARBW (%) | Peak gain (dBic) | |
---|---|---|---|---|---|---|
[ |
Microstrip patch antenna using AMC reflector | 0.78 × 0.80 × 0.096 | 6 | CP | 20.4 | 6.6 |
[ |
AMC reflector backed aperture antenna | 0.72 × 0.60 × 0.19 | 6.0 | CP | 33.2 | 6.33 |
[ |
Crossed dipole antenna based on AMC reflector | 1.38 × 1.38 × 0.12 | 1.78 | CP | 44.7 | 6 |
[ |
CWP-Fed slot antenna backed by FSS surface | 1.40 × 1.24 × 0.33 | 4.2 | CP | 31.14 | 4.87 |
[ |
Dipole antenna based on AMC reflector | 1.22 × 1.22 × 0.1 | 3.69 | CP | 5.6 | - |
[ |
Equiangular spiral antenna backed by an EBG reflector | 3.62 × 3.62 × 0.20 | 6.5 | CP | 76.9 | 7.5 |
[ |
Slot- aperture hybrid antenna with by reflector | 1.375 × 1.375 × 0.275 | 5.5 | CP | 22.0 | 8 |
[ |
Monopole radiators loaded with metasurface reflector | 0.72 × 0.72 × 0.16 | 2.6 | CP | 26.45 | 7.02 |
[ |
Circularly ring-shaped-dipole antenna loaded with meta-columns | 0.83 × 0.83 × 0.18 | 4.15 | CP | 54 | 6 |
[ |
CWP-fed antenna based on AMC reflector | 0.37 × 0.37 × 0.18 | 2.5 | CP | 15.92 | 5.76 |
[ |
Reflector backed CWP-fed antenna | 0.58 × 0.65 × 0.32 | 4 | CP | 81 | 6.7 |
An AMC-based CP wideband circularly polarized monopole antenna with unidirectional radiation characteristics has been studied and presented for 5G sub-6 GHz band. A CPW feeding with asymmetrical ground planes is used for generating the CP radiation. The rectangular etched ground plane on the left side of microstrip line makes a loop path for the current flow which generates one sense of CP radiation. The additional CP is produced by exciting two orthogonal modes which are generated by the vertical/horizontal current flowing on the feedline and the shortened ground plane. An AMC surface is used as a back reflector to achieve wide ARBW, high gain and unidirectional CP radiation. The proposed antenna is studied under three performance scenarios i.e., antenna in free space, antenna over PEC ground plane, and antenna over AMC ground plane. It has been discussed that antenna with AMC gives the best overall results including wide AR bandwidth, high gain, and unidirectional CP radiation. The distance between monopole antenna and back reflector is minimized from 0.25λ0 to 0.18λ0 by integrating AMC surface as compared to the conventional metallic reflector.
The author acknowledges the research environment provided by OIP Lab, Chungbuk National University, Korea.