QUASI-SELF-COMPLEMENTARY STRUCTURE OF CIRCULARLY POLARIZED ANTENNA FOR MILITARY BAND APPLICATIONS

Abstract

The present invention relates to a quasi self-complementary structure of a circularly polarized antenna for military band applications at 4.5 GHz and VSAT communication at 7.5 GHz. The circularly polarized antenna comprises a monopole formed by a combination of a rectangular patch and a quarter-section of a circular patch. The monopole is fed through a stripline feed with dimensions of 7.3 mm × 3 mm. A horizontal slot, measuring 6 mm × 0.5 mm, is etched into the monopole at a distance of 1.5 mm from the horizontal edge of the quarter-section circular patch. The ground structure extends to a length of x1 = 14.1 mm, and another segment of ground is added, modified to have a stepped ground plane, resulting in an asymmetrical structure of length x2 = 12.4 mm and width of 1.8 mm. Additionally, a parasitic radiating strip with dimensions of 3 mm × 0.5 mm is included as a parasitic radiator on either side of the substrate. Compact antennas with dual-band response and good, near-constant gain over the operating bands are required. Circularly polarized antennas are useful for establishing communication links via unaligned transmit and receive antennas, through obstructed LOS paths, and even in adverse weather conditions, such as smoke during wartime. This work presents a unique design of a quasi self-complementary microstrip antenna that resonates at frequencies of 4.5 GHz and 7.5 GHz. The proposed design is impedance matched over the frequency ranges of 3.75-5.0 GHz and 7.0-11.72 GHz, and circularly polarized over the frequency ranges of 4.15-4.47 GHz and 7.33-7.55 GHz. The antenna exhibits a beamwidth of more than 25° in the E-plane (f = 0 cut) and more than 23° in the H-plane (f = 90 cut).

Specification

Description:Field of the Invention
The present invention relates to the technical field of antennas, more particular to a quasi self-complementary structure of a circularly polarized antenna for military band applications at 4.5 GHz and VSAT communication at 7.5 GHz.
Background of the Invention
The 4.5 GHz band allows for TDD communication over short distances (1-5 km) for bandwidth intensive applications data peaking at 20 Gbps or more. For mission-critical applications such as tracking, control and guidance of drones and UAVs (unmanned aerial vehicles). Such applications need compact size antennas with good gain characteristics and relatively broader beamwidths operating over a large frequency band. Also for telemetry applications, it is required to transmit data of images to ground stations and terminals. These applications require compact antennas having high gain and narrow beamwidths to interference from unwanted sources and at the same time operation over a large frequency band. This intends to propose a single antenna which may be extended to MIMO antennas for increasing the gain further. It intends to serve as a replacement for currently used relatively big-sized reflector antennas. The higher band may be used for establishing communication through VSAT terminals for military communication to send data and messages securely and quickly to troops at any location.
The present design is a quasi-self-complementary structure of a monopole printed on both sides of the substrate as shown in Fig. 1. It's not exactly self-complementary as seen by the modification in ground plane creating a stepped ground and also the parasitic vertical radiator strips on front and rear side of the substrate.
Self complementary antenna was first proposed by Mushiake in 1948.
A circular polarized meta-surface has been proposed for circular polarization in the Ka-band, the proposed meta-surface has been demonstrated for a cylindrical DRA. The resonance frequency of the proposed unit cell can be tuned in the range 37-41 GHz.
A dual-band circular polarized quasi-self-complementary antenna (QSCA) has been investigated. The axial ratio bandwidth at the low-frequency band is very narrow and only 40 MHz, whereas no mention of the minimum axial ratio. The design proposed in the present disclosure has good polarization purity in each of the bands as indicated by the axial ratio and cross-polar discrimination values.
A semi-circular slot antenna with quasi self-complementary structure for MIMO antenna has been investigated for UWB operation. Though the given antenna achieves wide bandwidth, the proposed structure is only linearly polarized, whereas the antenna in the present disclosure is circularly polarized.
A quarter section circular-disc shaped QSCA structure is proposed for UWB operation with band-notch characteristics in the frequency range from 4.925 GHz to 5.936 GHz in order to avoid interference with HIPERLAN and WLAN.
In US patent no.US 8,638,269 B2: A quasi-self-complementary non-planar structure of spiral antenna is proposed with low-profile and compact size. The given antenna in patent has frequency-invariant phase center. It is claimed to have both linear and circular polarization but there is no mention of value of axial ratio achieved or results regarding same that indicate circular polarization. The proposed antenna in the present disclosure is planar & achieves circular polarization response.
In US patent no. US 2010/0207836A1: A low profile and compact non-planar ultra-wide band antenna. It is also mentioned regarding both linear and circular polarization, but there is no mention of axial ratio achieved or results regarding same that indicate circular polarization. The proposed antenna in the present disclosure is planar & achieves circular polarization response.
Drawings
Fig.1 illustrates the design of antenna
Fig 2. Construction of monopole as a combination of rectangular and quarter-section of a circular patch
Fig 3. Reflection coefficient for the proposed antenna
Fig. 4 Axial ratio curve of the proposed antenna
FIG. 5 is a series of plots indicating typical co-polar and cross-polar radiation pattern plots at cuts (a) phi 0° and (b) phi 90° for frequency 4.4 GHz
FIG. 6 is a series of plots indicating typical co-polar and cross-polar radiation pattern plots at cuts (a) phi 0° and (b) phi 90° for frequency 7.4 GHz
FIG. 7 is a plot of antenna peak gain as a function of frequency;
FIG. 8 is a plot of axial ratio as a function of angle and 3 dB axial ratio beamwidth in E-plane (cut phi=0°) and H-plane (cut phi=90°) at frequencies (a) 4.4 GHz and (b) 7.4 GHz
FIG. 9 is a graph depicting plot of radiation efficiency for the proposed antenna as a function of frequency
FIG. 10 is a plot of surface current distribution for the proposed antenna at frequency 4.4 GHz
Detailed Description of the Invention
The following description includes the preferred best mode of one embodiment of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore, the present description should be seen as illustrative and not limiting. While the invention is susceptible to various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.
In any embodiment described herein, the open-ended terms "comprising," "comprises," and the like (which are synonymous with "including," "having" and "characterized by") may be replaced by the respective partially closed phrases "consisting essentially of," consists essentially of," and the like or the respective closed phrases "consisting of," "consists of, the like. As used herein, the singular forms "a", "an", and "the" designate both the singular and the plural, unless expressly stated to designate the singular only.
As indicated in Fig. 1 the proposed antenna is in the form of a monopole designed by a combination of rectangular and quarter-section of a circular patch as shown in fig 2. The design uses horizontal slit on the periphery of the monopole antenna structure, whereas the self-complementary structure uses a horizontal stub. The horizontal slit and the horizontal stub of the complementary structure are spaced at a relative vertical distance 3 mm apart. The monopole is excited using a stripline feed and a QSCA structure is etched on the back. The antenna structure is not strictly complementary due to the extension of the ground plane on the rear side of the substrate. Moreover, the parasitic radiating rectangular strip on the front side of proposed antenna corresponds to another similar parasitic radiating rectangular strip on the back side.
The proposed antenna structure is designed as a self-complementary structure, starting from an initial design as mentioned in fig 2, which is a combination of rectangular monopole of dimensions 11.7mm?9.94mm. The monopole is fed through a stripline feed having dimensions 7.3mm?3 mm. A horizontal slot having dimensions 6mm?0.5mm at a distance 1.5 mm from the horizontal edge of the quarter section circular patch is etched in the monopole at the periphery as indicated in fig 2. Thereafter, a complementary structure is designed in the form of a slot antenna for the monopole. The ground structure is extended upto length x1=14.1 and another segment of ground is added of dimensions modified to have a stepped ground plane resulting in an asymmetrical structure of length x2=12.4 mm and width 1.8 mm. Further a parasitic radiating strip having dimensions 3 mm?0.5 mm is included as a parasitic radiator to either side of the substrate.
The reflection coefficient in fig. 3 indicates dual band response, with reflection coefficient values better than -10 dB in the frequency range 3.75-5.0 GHz and 7.0-11.72 GHz. The impedance matching as indicated by reflection coefficient in the two bands is better than -13 dB and -10 dB respectively for each of the two bands.
The proposed antenna is dual band circular polarized in the frequency bands at 4.15-4.47 GHz and 7.33-7.55 GHz as indicated in fig. 4.
Fig 5 indicates the proposed antenna radiates bi-directional radiation pattern in the two frequency bands with left hand circular polarized radiation (CP) in the positive z-direction, whereas right hand circular polarized radiation (CP) in the negative z-direction. The best value of cross-polar discrimination at 4.4 GHz is more than 20 dB in each of the principal planes (fig 5).
Fig 6 indicates the best value of cross-polar discrimination is more than 25 dB at 7.4 GHz in each of the principal planes.
The peak value of gain is 4.04 and 2.60 dBic in the two frequency bands respectively as indicated in fig 7.
The graphs in fig. 8 indicate the axial ratio beamwidth at frequency 4.4 GHz in E-plane is 26°, whereas in H-plane it is 23°. The axial ratio beamwidth is 25° in E-plane, whereas it is 35° in H-plane at frequency 7.4 GHz.
The antenna radiation efficiency is better than 70% throughout the entire frequency range as indicated in fig 9.
The surface current distribution in fig 10 illustrates that the antenna is LHCP in broadside direction.
The present invention antenna has:
• A planar quasi-self-complementary antenna design
• An operating bandwidth of 1.25 and 4.72 GHz in the two bands respectively.
• A compact, low profile antenna of overall dimensions 0.475?max× 0.475?max× 0.02?max. Here ?max is the maximum operating wavelength corresponding to the lowest operating frequency for the proposed antenna.
• Dual polarized characteristics with circular (LHCP/RHCP) polarization at lower band and linear polarization in upper band.
• The input reflection coefficient at antenna port is better than -13 dB and -10 dB in the two bands respectively, whereas the cross-polarization is better than 20 dB and 25 dB respectively.
• The antenna has a peak value of gain 4.04 dBic in the lower band and 2.6 dBic in the higher band.
• Wide Axial ratio bandwidth of 320 MHz and 220 MHz in lower and upper bands respectively.
• An axial ratio beamwidth of 26º and 25º at phi 0 cuts at center frequency of lower band whereas it is 23 º and 35 º at phi 90 cut at center frequency of lower band in the principal planes.
• The antenna presents minimum axial ratio of 0.235 dB and 0.683 dB in the two bands.
Advantages of the present antenna:
• The proposed structure is designed using a low-cost substrate that is easily available. However, this is only indicative and can be designed with other substrate materials over other frequency ranges as well as per the desired application.
• The design is compact having dimensions only ( 0.475?max× 0.475?max× 0.02?max), planar, low-profile, planar, easy to fabricate and mount on desired aircraft/spacecraft/drone/UAV.
• The design exhibits circular polarization characteristics with bidirectional radiation pattern and hence is robust to non-alignment of transmitter and receiver, bad weather, absence of LOS path, multipath fading, etc. scenarios. The antenna is circular polarized over wide range of frequencies & having higher CP bandwidth.
• The antenna radiates circular polarized radiation over a beamwidth of 26º and 25º at phi 0 cuts at center frequency of lower band whereas it is 23º and 35º at phi 90 cut at center frequency of lower band in the principal planes.
• The low frequency band of the antenna may be useful for different military applications such as tracking, control and guidance of drones and UAVs (unmanned aerial vehicles). It is also useful for telemetry applications if extended as an array to improve the gain, it is required to transmit satellite images over short distances
• The upper band of antenna at 7.5 GHz can be used for antennas can be used for establishing communication through VSATs (very small aperture satellite terminals). Single antenna is used for both lower and upper frequency bands respectively.
• The structure is well suited to be extended in the form of a MIMO antenna and can maintain good isolation amongst the different ports of the multiple element design.

, C , Claims:1. A Quasi-self-complementary structure of circularly polarized antenna, comprising of:
a) a monopole designed by a combination of rectangular and quarter-section of a circular patch;
b) monopole is fed through a stripline feed having dimensions 7.3mm?3 mm;
c) A horizontal slot having dimensions 6mm?0.5mm at a distance 1.5 mm from the horizontal edge of the quarter section circular patch is etched in the monopole at the periphery;
d) ground structure is extended upto length x1=14.1 and another segment of ground is added of dimensions modified to have a stepped ground plane resulting in an asymmetrical structure of length x2=12.4 mm and width 1.8 mm; and
e) a parasitic radiating strip having dimensions 3 mm?0.5 mm is included as a parasitic radiator to either side of the substrate.
2. The Quasi-self-complementary structure of circularly polarized antenna as claimed in the claim 1, wherein antenna structure is designed as a self-complementary structure which is a combination of rectangular monopole of dimensions 11.7mm?9.94mm.
3. The Quasi-self-complementary structure of circularly polarized antenna as claimed in the claim 1, wherein antenna has:
• dual band response, with reflection coefficient values better than -10 dB in the frequency range 3.75-5.0 GHz and 7.0-11.72 GHz;
• Impedance matching as indicated by reflection coefficient in the two bands is better than -13 dB and -10 dB respectively for each of the two bands.
• antenna is dual band circular polarized in the frequency bands at 4.15-4.47 GHz and 7.33-7.55 GHz;
• Antenna radiates bi-directional radiation pattern in the two frequency bands with left hand circular polarized radiation (CP) in the positive z-direction, whereas right hand circular polarized radiation (CP) in the negative z-direction. The best value of cross-polar discrimination at 4.4 GHz is more than 20 dB in each of the principal planes;
• the best value of cross-polar discrimination is more than 25 dB at 7.4 GHz in each of the principal planes;
• The peak value of gain is 4.04 and 2.60 dBic in the two frequency bands respectively;
• The axial ratio beamwidth at frequency 4.4 GHz in E-plane is 26°, whereas in H-plane it is 23°. The axial ratio beamwidth is 25° in E-plane, whereas it is 35° in H-plane at frequency 7.4 GHz.
• The antenna radiation efficiency is better than 70% throughout the entire frequency range as indicated in fig 9; and
• The antenna is LHCP in broadside direction.
4. The Quasi-self-complementary structure of circularly polarized antenna as claimed in the claim 1, wherein design is compact having dimensions only ( 0.475?max× 0.475?max× 0.02?max), planar, low-profile, planar, easy to fabricate and mount on desired aircraft/spacecraft/drone/UAV.

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