The present disclosure relates to antennas and more specifically, to a cavity-backed spiral antenna for satellite applications that has perturbation elements within the cavity to improve performance.
Since the development of the first CubeSat in 1999, there have been great advancements in the design, construction, and deployment of amateur nano-satellites due to their simplicity and low cost. However, from 2000 to 2012, 47% of CubeSat failure was attributed to loss of contact [1], revealing the need for further development in communication and antenna systems.
CubeSat antenna development faces several challenges, including restrictions on size, mass, and transmitting power, while demanding wide bandwidth and circular polarization. Planar antennas have been designed as alternatives to the deployable monopole antennas common for small satellites, as they eliminate the possibility of mechanical failure and allow for low profile. Frequently-used planar antennas include slot and patch antennas, but these fail to achieve sufficient circular polarization bandwidth for satellite antennas [2].
In order to meet aforementioned antenna characteristics, other antenna designs have been proposed [15-18]. A patch antenna with metasurface [15] and a dipole antenna with artificial magnetic conductor (AMC) [16] are low profile and provide broadband. However, the patch antenna showed a narrow bandwidth of low axial ratio (AR) below 3 dB (3-dB AR bandwidth) at 1.45 GHz (23.4%) and 0.72 GHz (44.7%). A cavity-backed slot antenna showed a broader 3-dB AR bandwidth of 3 GHz (54.5%) [17], but the antenna gain fluctuated from 6 dBic to 9.9 dBic in the operating frequency. A cavity-backed spiral antenna array showed a similar 3-dB AR bandwidth and relatively stable antenna gain [18]. However, the antenna has a large area of 1.29 λL×1.5 λL, where λL is the free space wavelength at the lowest frequency.
The spiral antenna is a good candidate for CubeSat satellite applications because it is frequency independent and characteristically circularly polarized [3]. A need, therefore, exists for an improved spiral antenna design to meet the requirements for communication in a CubeSat satellite application.
Accordingly, disclosed herein is an antenna design to achieve broad bandwidth, high antenna gain, and circular polarization with low-profile. The antenna design embraces a cavity-backed Archimedean spiral antenna with quadruple conical perturbations (CBASA-QCP), which consists of an Archimedean spiral antenna (ASA) backed by a cavity having four conical perturbation elements (i.e., cones). The ASA typically has two radiator arms with a typical number of turns (n)=5, a typical radiator width (w)=0.7 mm, and a typical spacing (s)=1 mm. The ASA can be placed on a ROGERS DUROID™ 5880 substrate, which has a diameter of 40 millimeters (mm), a thickness of 0.787 mm, and a relative dielectric constant (εr) of 2.2. The backing cavity typically consists of four conical perturbations with a possible cone height (hcone)=2 mm and cone diameter (Dcone)=8 mm for a cavity height (hcav)=6 mm. This antenna design allows for a wide −10-dB impedance bandwidth (e.g., of greater than 124.3% over 7-30 GHz), 3-dB axial ratio bandwidth (e.g., of 107.2% over 8-26.5 GHz), 3-dB gain bandwidth (e.g., of 72% over 7-15 GHz and of 28.6% over 19.7-26.5 GHz), and a high peak realized gain (RG) (e.g., of 10.7 dBic) at boresight.
Thus, in one aspect, the present disclosure embraces an antenna, consisting of an antenna element having two conductive arms disposed on a circular substrate wherein each conductive arm begins at one side of a feed port and traces a spiral for a plurality of revolutions. The antenna also includes a cylindrical cavity comprising a cavity base and a cavity wall, wherein the cavity wall defines an opening that substantially matches a diameter of the circular substrate. The cylindrical cavity is positioned behind the antenna element so that the substrate covers the opening formed by the cavity wall. One or more perturbation elements are disposed or formed on the cavity base. Each perturbation element has an element base that is flush with the cavity base and an element top at a height above the cavity base. The height of each element top is between the cavity base and the circular substrate.
In an example embodiment of the antenna, the spiral is an Archimedean spiral.
In another example embodiment of the antenna, each perturbation element is a cone, a cylinder, or a cone with a flat top. In these embodiments, aspects of the antenna's performance may be affected by a shape, a size, a number and/or an arrangement of the perturbation elements. For example, the performance affected may include a bandwidth, an axial ratio, or a gain measured at the feed point of the antenna.
In another example embodiment of the antenna, each conductive arm may be a conducting polymer, metal, or metal alloy.
The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the disclosure, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.
The design of the antenna requires a knowledge of the link and power budges for communication in the satellite (e.g., CubeSat) environment. Accordingly, a link scenario is first determined.
The purpose of the link scenario is to accurately depict all visible encounters between the satellite and ground station for the CubeSat's orbit around the earth. Using NASA's open-source General Mission Analysis Tool (GMAT) [4], a circular Low Earth Orbit (LEO) was simulated at altitude of 400 km with a semi-major axis of 6771 km, an eccentricity of approximately 0, an inclination of 51.3°, and a longitude of ascending node of 170.1347°, as shown in
where G is the gravitational constant, M is the mass of Earth, and rs is the distance from the center of the Earth to the satellite. The standard mass of the CubeSat (3 kg) was used. From these calculations, vorbit is 7.676 km/s and T is 5541 seconds, meaning the CubeSat completes 15.59 orbits per day.
The satellite communication link requires Line-of-Sight (LoS) communication. Therefore, visibility calculations were performed using the following trigonometric equations [6]:
d=r
s√{square root over (1+(re/rs)2−2(re/rs)cos(γ))},
γ≤cos−1(re/rs),
where d is the distance between the satellite and ground station, γ is the angle between the satellite sub-point and ground station, re is the radius of the earth. With a minimum elevation of 20°, γ is limited to a maximum of 6.55°, which allows us to determine the maximum d using above equations. Using the simulation of the CubeSat's orbit, the range d was found over a period of 24 hours and the minimum d was determined. Therefore, LoS communication is constrained to d values between 433 km and 1,000 km.
A satellite link budget accounts for propagation losses in addition to losses caused by polarization mismatch [7, 9]. The basic link budget can be calculated using the Friis equation:
where Pr is the receiving antenna power, Pt is the transmitting antenna power, Gt is the transmitting antenna gain, Gr is the receiving antenna gain, d is the distance between the receiving and transmitting antennas, λ is the wavelength of the target frequency, Γt is the reflection coefficient of the transmitting antenna, Γr is the reflection coefficient of the receiving
antenna, and at and ar are the polarization vectors of the transmitting and receiving antennas, respectively [3]. The calculated link budget is summarized in Table I.
The parameters for the ground station antenna can be estimated from a commercially available ground station antenna [8] and used to calculate the link budget.
According to the link budget estimation at the maximum communication distance of 1,000 kilometer (km), the antenna must have a gain of 7.02 decibels relative to isotropic (dBi) and 4.94 dBi at 8 GHz and 11.2 GHz, respectively. Note that additional losses will be introduced by polarization mismatch, atmospheric effects during propagation, and insertion loss from the feeding, which were not taken into consideration in the link budget calculation. However, the link budget has been calculated for upper limit of the range of path distances, 433 to 1000 km, which allows for a loss margin of 7 dB.
From the link and power budget calculations, the CubeSat antenna must achieve a minimum gain of 7 dBi and 5 dBi at 8 GHz and 11.2 GHz, respectively, to cover a wide communication distance up to 1,000 km with high efficiency for efficient power management. In addition, a circularly polarized antenna is favorable for satellite wireless communication because circular polarization eliminates the adverse effects of using a linearly polarized antenna, which include a 3 decibel (dB) loss from Faraday rotation and additional losses from polarization mismatch [2, 9]. Also, wide bandwidth is favorable for the CubeSat applications. To meet these requirements, an Archimedean spiral antenna (ASA) backed by a copper cavity containing conical perturbations is disclosed. A balun is also disclosed for converting unbalanced to balanced input signals and transforming impedance from a coaxial transmission line to the a feed port of the spiral antenna element.
In what follows, example embodiments of the disclosure (and their simulation and measured performance) will be described. Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described can be configured without departing from the scope and spirit of the disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the disclosure may be practiced other than as specifically described.
The ASA has a planar structure and characteristically wide bandwidth with respect to both circular polarization and impedance [3, 10]. An ASA consists of two conductive radiator arms (i.e., arms) with a number of turns (n) of 5, an arm width (w) of 0.7 millimeters (mm), and a spacing (s) of 1 mm. The conductive arms are disposed on a circular substrate of ROGERS DUROID™ 5880. As shown in
As shown in
A tapered microstrip balun is used to feed ASA (see
In order to alleviate issues for the ASA discussed in the previous section, a backing cavity was designed to improve realized gain (RG) and reflect the back radiation. Three backing cavities were designed and simulated with the optimized ASA. As shown in
A conventional cavity-backed Archimedean spiral antenna (CBASA) with no perturbation (NP) elements is shown in
All three variations of cavities (i.e., NP, SCP, and QCP) have good frequency independent characteristics when placed behind the ASA in simulation.
To address the above issues, the QCP cavity design was optimized by varying hcone. It is clearly observed that the RG00 drop shifted to a lower frequency as hcone increased from 3 mm to 11 mm, as shown in
The ASA and balun may be milled on Rogers Duroid 5880 using an LPKF S62™ Milling Machine. The backing cavities can be fabricated from copper C101 oxygen free stock. The balun extends into a parallel plate transmission line, which passes through a 2.5 mm hole in the bottom of the cavity to connect to the fabricated spiral antenna.
The tapered microstrip balun feeds the CBASA and converts unbalanced input signals to balanced input signals. The balun also transforms the impedance from 50 ohms (Ω) to 150Ω. When assembled, the tapered microstrip balun passes through a 2.5 mm hole in the bottom of the QCP cavity to connect to the ASA.
To test the balun, a double ended balun was fabricated which transforms the impedance from 50 ohms (Ω) to 150 Ω then back to 50 Ω. A Vector Network Analyzer (VNA: AGILENT™ N5320A) was used to measure the S11 and S21. The fabricated balun showed a low reflection coefficient (Γ) and insertion loss (IL) at X-band. However, measured behaviors of the Γ and IL were slightly degraded at high frequencies (f>10 GHz), which was different from the simulated results. This small discrepancy occurred due to fabrication errors.
The Γ of the fabricated CBASA-QCP is shown in
In
Further improvement in −10-dB impedance bandwidth, 3-dB AR bandwidth, and 3-dB gain bandwidth was achieved by optimizing Hcav, Hcone, and Dcone.
Antenna performance of the optimized CBASA-QCP was compared with the reported wideband circularly polarized antennas [15-18] and is summarized in Table II.
The disclosed CBASA-QCP has a −10-dB impedance bandwidth of 124.3%, 3-dB AR bandwidth of 107.2%, and 3-dB gain bandwidth of 72% and 28.6% with high peak gain of 10.7 dBic. These are much higher than those of recently developed antennas [15-18]. Accordingly, the disclosed CBASA-QCP can simultaneously cover operation frequencies of X-band (8-12 GHz), Ku-band (12-18 GHz), and K-band (18-27 GHz) with a small area of 0.89 λL2, where λL is the free space wavelength at the lowest frequency. The CBASA-QCP is suitable for small satellite applications.
Simulation and measurement were performed to investigate the effect of an aluminum (Al) body of a 3U CubeSat on antenna performance. Accordingly, the optimal placement of the antenna by varying the location of the cavity (Locz: distance between the bottom of the antenna cavity and the face of the CubeSat) was determined as shown in
An antenna with circular polarization, high radiation efficiency (RE), and high gain of 7.02 dBi and 7.9 dBi at 8 GHz and 11.2 GHz, respectively is disclosed. The cavity design allows for a low axial ratio (<3 dB) and high peak realized gain (>6.7 dBic) at boresight in the frequency range from 8 GHz to 12 GHz. A uni-directional radiation pattern was achieved with the invented cavity-backed Archimedean spiral antenna with quadruple conical perturbations (CBASA-QCP). Therefore, electromagnetic interference with internal electronics can be reduced.
The antenna meets the requirements of circular polarization, high radiation efficiency (RE), and high gain (i.e., 7.02 dBi and 4.94 dBi at 8 GHz and 11.2 GHz, respectively), which matches the determined link and power budgets for a calculated link scenario.
The antenna is a cavity-backed Archimedean spiral antenna with quadruple conical perturbations (CBASA-QCP). The antenna shows an axial ratio at boresight of less than 3 dB for most of the frequency range from 8 GHz to 12 GHz. Also, the antenna has a uni-directional radiation pattern at 8 GHz. The realized gain of the antenna at boresight (RG00) is stabilized over the whole frequency range by introducing a QCP cavity. The simulated and measured RG00 of the antenna are 8.2 dBi and 6.9 dBi at 8 GHz, and the simulated RG00 of the antenna is 7.9 dBi at 11.2 GHz. Therefore, the requirements of the link budget are met. In addition, the simulated RE is stabilized at 95% from 7.5 GHz to 12 GHz. Lastly, the antenna is unaffected by mounting location on the CubeSat mock-up. Therefore, the developed CBASA-QCP is suitable for satellite-ground wireless communication, especially for small satellites.
The antenna is further improved in −10-dB impedance bandwidth (>124.3%: 7-30 GHz), 3dB-dB axial ratio bandwidth (107.2%: 8-26.5 GHz), and 3-dB gain bandwidth (72%: 7-15 GHz and 28.6%: 19.7-26.5 GHz) by optimizing the cavity with quadruple conical perturbations (QCP cavity).
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. As used in the specification, and in the appended claims, the singular forms “a,” “an,” “the” include plural referents unless the context clearly dictates otherwise. The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. The terms “optional” or “optionally” used herein mean that the subsequently described feature, event or circumstance may or may not occur, and that the description includes instances where said feature, event or circumstance occurs and instances where it does not. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, an aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
This application claims priority to and benefit of U.S. provisional patent application Ser. No. 62/613,640 filed Jan. 4, 2018, which is fully incorporated by reference and made a part hereof.
Number | Date | Country | |
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62613640 | Jan 2018 | US |