Circular microstrip antenna with surface-wave reduction for an MFCW altimeter

Abstract

The present utility model relates to an innovative structural arrangement of a pair of planar microstrip antennas which are designed according to the reduced surface wave technique (RSW-MA) and operate separately as radiofrequency (RF) signal transmitters and receivers, said antennas being characterised by high isolation between them, with a physical separation of 34 cm between their centres, and forming part of the radiating structure of a radar altimeter employing the multifrequency continuous wave (MFCW) technique. The invention provides a differentiated product in the consumer market in terms of construction and use, offering technical advantages including (i) the reduction of mutual coupling between antennas where physical limitations are critical, and (ii) the adaptability to different practical applications, particularly in diverse aerospace structures, thus representing an evolutionary solution, the practical result of which provides an innovative and effective state of construction, use and performance.

Description

[0001] Circular microstrip antenna with reduced surface waveform for MFCW altimeter. Field of application.

[0002] 001 This descriptive report refers to a Utility Model Patent application for a device used in the emission and / or reception of electromagnetic waves, more precisely a pair of planar microstrip antennas for application in a C-band Radar Altimeter, which is extremely practical and efficient, meeting the conditions to be worthy of the protection now sought.

[0003] 002 The REDUCED SURFACE WAVE CIRCULAR MICROSTRIP ANTENNA FOR MFCW ALTIMETER, through improvements and the technology employed, aims to develop an innovative constructive arrangement in a pair of planar microstrip antennas, built according to the reduced surface wave (RSW-MA) technique, operating distinctly as a transmitter and receiver of radio frequency (RF) signals, with high isolation characteristics for a physical separation between centers of 340 mm, being part of the radiating structure of a radar altimeter using the multifrequency CW (MFCW) technique, which was not possible until now, standing out in the consumer market due to presenting a differentiated product in relation to its construction and use, with technical advantages in terms of performance regarding the reduction of mutual coupling between antennas, which can be applied to various aerospace structures.Composed of different radio communication systems in a limited physical space, and adapting to different practical situations, this is an evolutionary solution whose practical result points to a differentiated and effective condition of construction and use.

[0004] STATE OF THE ART

[0005] 003 With the sole and exclusive purpose of providing veracity to the preamble of this description, the applicant will now provide, in the following paragraphs, a brief but consistent explanation of the state of the art regarding the problems and drawbacks of microstrip antennas, where a person skilled in the art will be able to recognize their shortcomings, and subsequently clearly identify in the invention now claimed the novelty requirement, mandatory under the Industrial Property Law.

[0006] 004 Antennas act as radiating structures used in the emission / reception of electromagnetic waves, being widely used in radio, telecommunications, satellite, and wireless network applications, such as Wi-Fi. Patch-type microstrip antennas are planar structures formed by a metallic radiating element deposited on a dielectric substrate with a metal layer called the ground plane on the other side. These structures can assume various radiating element shapes and are developed to meet the needs of their applications, such as performance, constructive simplicity, interface, robustness to operational environmental conditions, and production costs.

[0007] 005 A technique used to achieve greater efficiency in minimizing surface radiation from these planar antennas and consequently reducing mutual coupling between adjacent antennas is the Reduced Surface Wave - Microstrip Antenna (RSW-MA) technique, studied and disseminated in the technical literature, especially in the Handbook of Antenna Technologies, Zhi Ning Chen (Editors) Springer 2016, vol. 3, Reduced Surface Wave Microstrip Antennas – David R. Jackson.

[0008] 006 Although its use in similar products is not known, since the antennas are often inserted into a sealed plastic housing, the use of this technique is extensive to any other application, whether as an individual antenna or in a pair, where a reduction in mutual coupling between them is required.

[0009] 007 The major problem faced in a radiating system for radar altimeters is the mutual coupling between the transmitting and receiving antennas, as this coupled signal impairs the reception of desired signals that are of a lower level compared to the coupled signal, causing a desensitization effect on the receiving system. On the other hand, reducing the mutual coupling between the antennas invariably requires greater physical separation between them, which, in the specific case of the desired application, was not possible to achieve. The use of the RSW-MA technique in the antenna construction allowed for high isolation between the antennas with a physical separation compatible with the planned mechanical structure for fixing the assembly.

[0010] 008 The state of the art reveals that US patent US8971796B2, titled REPEATERS FOR WIRELESS COMMUNICATION SYSTEM, by Andrew LLC (US), published on 08 / 05 / 2014, describes the architecture of an electronic wireless signal repeater module, containing radiating elements that utilize the RSW-MA technique in its construction. The subject matter of this patent is extensive and complex, being specifically directed to Wireless Repeaters, unlike the Radar Altimeter application discussed in this document.

[0011] 009 With the results of observations in tests of the Radar Altimeter using conventional planar patch antennas and knowledge of the fundamentals of the RSW-MA technique, the facts led to the search for a solution to improve the isolation between the antennas of the radiating system and, in this sense, the RSW-MA technique was applied in a new physical construction of these antennas, which is presented in this patent.

[0012] PATENT OBJECTIVE

[0013] 010 In order to overcome the problems presented, the inventor hereby applies for a Utility Model Patent, entitled CIRCULAR MICROSTRIP ANTENNA WITH SURFACE WAVE REDUCTION FOR MFCW RADAR ALTIMETER, which fundamentally aims at developing a constructive configuration of a pair of planar microstrip antennas built according to the RSW-MA technique, operating distinctly as a transmitter and receiver of radio frequency (RF) signals, with high isolation characteristics for a physical separation between centers < 340 mm.

[0014] 010 Its architecture is that of an MFCW (Multiple Frequency Continuous Wave) radar, where four signals are emitted at fixed frequencies (tones) in the C-band, and the signal reflected from a fixed or non-fixed surface is received, filtered, and beaten. Through an algorithm based on the calculation of phase differences between pairs of transmitted tones, the average height is estimated. This topology allows height measurements to be performed with a link of up to 600m.

[0015] 010 Standing out in the consumer market by offering a differentiated product in terms of its construction and use, while also being a low-cost, safe, and efficient product.

[0016] 011 With many years in the market, and always seeking innovations and practical solutions to differentiate its products from those of its competitors, the inventor developed this simple and efficient product, capable of overcoming all the limitations and inconveniences reported in the State of the Art in an extremely effective way.

[0017] 013 It is known that radar altimeters employing FMCW and MFCW techniques require high isolation between the transmitting and receiving antennas, which can be achieved by increasing the physical separation between the antennas. However, in the application in question, this practice was impossible due to the mechanical construction characteristics of the product, which limited the maximum physical separation between the antennas to 340 mm. Therefore, the mechanical criticality required the study of antenna synthesis techniques that would reduce the mutual coupling between antennas, which was not possible to achieve with the use of conventional microstrip patch antennas.

[0018] 014 Thus, a search was conducted in the technical literature for informative material on the synthesis of high-isolation antennas. Techniques for reducing antenna coupling using metamaterials were identified and analyzed, but their use would require a long period of experimentation and technological and constructive validation. In this research and discussion with experts in the field of possible, feasible, and low-risk solutions, the potential application of RSW-MA synthesis in planar microstrip antennas was identified.

[0019] 015 Reduced surface wave microstrip (RSW-MA) antennas are a class of antennas that excite significantly less surface wave fields than conventional microstrip antennas. Furthermore, they produce a reduced amount of lateral (horizontal) radiation propagating outward from the antenna along the ground plane. This results in less diffraction from the edges of the ground plane or support structure, leading to an improved front-to-back ratio of the antenna's radiation pattern. In turn, when these RSW-MA antennas are mounted close together, mutual coupling is significantly better than with conventional patch antennas, due to the reduction in radiated surface field and lateral radiation.

[0020] 016 The use of the RSW-MA technique to improve antenna isolation arose from a design need, which required research in the literature and discussions with electromagnetism and antenna specialists from research centers. These specialists used various techniques found in the literature for their research, created prototypes in laboratories, and applied them in various sectors. Application in defense systems requires critical analysis of its operation, dimensions, and specific characteristics that may hinder other already installed systems. The technique is established in the literature, but in other sectors and with other needs. Its use in the defense sector, with highly complex environments and means to obtain significant results, is the differentiating factor for this application.

[0021] 017 The workaround solution applied to the problem of high isolation between antennas with very limited physical separation was the use of the SAR-RSW (Short Annular Ring - Reduced Surface Wave) technique for the synthesis of microstrip antennas on a microwave dielectric substrate. The chosen geometry was a circular patch with an annular ring short-circuited by metallized holes to the antenna ground plane. According to the literature, the isolation between them would be greater than 70 dB for a spacing of 4 wavelengths (4 Å).

[0022] 018 This solution was successfully implemented in projects in the specific frequency bands: 2.42 to 2.44 GHz and 4.42 to 4.44 GHz.

[0023] 019 The central technical problem was the need to obtain isolation between two antennas < 60 dB, typically required for applications in radar altimeters, with a physical separation of 340 mm from each other.

[0024] 020 The technical requirement stemmed from the physical and mechanical constraints of the specific project, demanding high isolation between the antennas, limited to a physical separation of 340 mm. Thus, it was necessary to meet this physical constraint and obtain high isolation between the antennas for the desired system performance.

[0025] 021 The RSW~MA technique is a commonly adopted approach to improve the efficiency and performance of antenna arrays with specific usage characteristics. In the case of this project application, the objective is to obtain the highest possible isolation between the antennas, for the defined physical separation.

[0026] 022 One of the advantages of applying RSW-MA planar antenna synthesis is its adaptability to different practical situations. Since the technique is already well-established in the literature, the degree of complexity and criticality of each application needs to be evaluated.

[0027] 023 Another advantage of applying the RSW-MA technique to reduce mutual coupling between antennas is that it can be extended to various aerospace structures where physical limitations are critical, for example, CUBESAT systems.

[0028] 024 The synthesis of planar antennas using the RSW-MA technique has proven to be a practical and effective solution when it is desired to reduce mutual coupling between antennas in situations where the narrow physical separation between them is a constraint to be overcome. The application of this technique in the specific design of an antenna array for a Radar Altimeter resulted in success in obtaining an isolation between antennas greater than 70 dB, for a physical separation of 340 mm between them. This technique can be extended to other applications where different radiating systems are physically integrated very closely in a mechanical mounting structure.

[0029] 025 According to the present invention, it basically comprises a set of planar antennas for application in a C-band Radar Altimeter, operating in the frequency range of 4.42 GHz to 4.44 GHz, with a bandwidth of 0.5%, using microstrip technology on a microwave dielectric substrate and synthesis with reduced surface wave, short-circuited annular ring, referenced in the literature as SAR-RSW (Shorted Annular Ring - Reduced Surface Wave), which results in a suitable mechanical solution for mounting on aerospace mechanical structures, due to its low aerodynamic profile and reduced weight.

[0030] 026 It is important to understand that the present Patent presented here is illustrated in a demonstrative and non-limiting manner regarding the application of the details, dimensions, capacities and steps described. It is capable of other modalities and of being practiced or executed in a variety of ways, without departing, logically, from the main inventive scope of the object whose protection is claimed.

[0031] 027 To complement the present description in order to obtain a better understanding of the characteristics of the present Patent and in accordance with a preferred practical embodiment thereof, a set of drawings is attached to the description, where, in an exemplary manner, the following is represented:

[0032] Figure 1 – Constructive geometry of SAR-RSW synthesis in a planar microstrip antenna;

[0033] Figure 2 – Perspective of the constructive geometry of the SAR-RSW synthesis in a planar microstrip antenna;

[0034] Figure 3 - Mock-up of the mechanical mounting structure with the planar antennas mounted;

[0035] Figure 4 - Magnitude of the S parameters measured from the RSW antennas integrated onto the mechanical structure of the mockup. PATENT DESCRIPTION

[0036] 028 In accordance with the attached drawings, the CIRCULAR MICROSTRIP ANTENNA WITH REDUCED SURFACE WAVE FOR MFCW ALTIMETER, the subject of this present Patent application, comprises a microstrip antenna (1) composed of a dielectric substrate (2) with a ground plane (3) on the lower face (4) and with a metallic circular patch-type radiating element (5) on the upper face. It is fed through a coaxial connector (6) and, in the region inside the patch (2), there is a ring of metallized vias (7) that connect the patch to the ground plane (3) and act as a solid conductive wall.

[0037] Figure 1 illustrates the constructive geometry of a SAR-RSW synthesis in a microstrip antenna (1) on a dielectric substrate (2). The position occupied by the edge of the patch (5) is responsible for minimizing the excitation of surface waves and side (horizontal) waves that are produced in microstrip antennas with canonical geometry. On the other hand, the location of the ring of metallized vias (7) ensures that the antenna resonates at the desired frequency. The distance between their centers (8) will be < 340 mm in the mechanical fixing structure, which is equivalent to about 4 wavelengths at the central frequency of the operating band.

[0038] 029 According to the literature, for RSW-MA antennas with a circular patch and a ring of metallized paths installed on a large, flat metallic surface, the isolation between them would be greater than 70 dB, for a physical separation of 4 wavelengths. It is emphasized, however, that the antennas will be mounted in open cavities on the surface of the mechanical structure, as shown in Figure 2.

[0039] 029 The electrical requirements for the antennas, defined for the application in question, are listed in Table 1.

[0040] Table 1 - Antenna array requirements defined for the intended application

[0041] Requirement Value

[0042] Frequency range 4.42 GHz to 4.44 GHz Return loss (Ref. 50 Q) >9.5dB (VSWR < 2.0) Isolation (Ref. 50 Q) >70 dB Linear polarization Gain (10± 1) dBi -3 dB beamwidth in the E plane >50° -3 dB beamwidth in the H plane >50° Secondary lobe level in the E and H planes <-■20 dB Cross-polarization rejection in the E and H planes > 15dB

[0043]

[0044] 030 The planar antenna dielectric substrate has a relative permittivity εr = 2.50 ± 0.04 and a loss tangent tg δ = 0.0015. The defined thickness of the substrate plate was h = 0.787 mm (= 31 mils), due to its electrically thinness (kh ≤ 0.3, where k is the propagation constant in the dielectric at the antenna's upper operating frequency), and with copper layers of 1 oz / ft. 2 (≈ 35 μm) on both faces.

[0045] 030 The chosen thickness is such that it reduces the lateral waves that propagate horizontally along the interface of the radiating element with the substrate and ensures the desired operating frequency bandwidth (20 MHz). Conversely, greater control of the relative permittivity of the dielectric is necessary, because if it deviates too much from its nominal value, the antenna performance will certainly fall short of expectations in the 4.42 GHz to 4.44 GHz range.

[0046] 030 The FEM (finite element method) was analyzed for several sets of dimensions a, b, and rp, which were constructed starting from the values ​​established in the first stage. For each simulated set, the magnitudes of the S11 and S21 parameters of the array formed by the transmitting and receiving antennas of the radar altimeter were evaluated along the 4.42 GHz to 4.44 GHz range. The intention was to select the one that, according to the project requirements, guaranteed a return loss above 9.5 dB and an isolation greater than 70 dB across the entire frequency range.

[0047] 030 The final construction characteristics of the antennas are listed in Table 2.

[0048] Table 2 - Construction characteristics of the planar microstrip antenna with RSW synthesis.

[0049] Parameter Value Radius of metallized via ring (a) 11.539 mm Radius (external) of patch (b) 21.220 mm Radial position of probe tip (r p) 15,850 mm Diameter of the connector insulation at ground plane 3.80 mm Diameter of the hole for probe passage 1.30 mm Diameter of the metallized wires 1.0 mm Number of metallized wires 32 Sides of the laminate 70.0 mm

[0050]

[0051] 031 The isolation between the antennas and return loss at their access ports were measured in the frequency range of 4.42 GHz to 4.44 GHz with a vector network analyzer in an open outdoor environment, free of nearby obstacles, considering the mockup shown in Figure 3.

[0052] 032 The magnitude curves of the measured parameters S11, S22, and S21 are shown in Figure 3. As can be seen, the isolation between the antennas is greater than 70 dB in the 4.42 GHz to 4.44 GHz range, and the return loss at each port is better than 14 dB (VSWR = 1.5) in this range.

[0053] 033 Radiation pattern and antenna gain tests were performed in an anechoic chamber at the Institute for Industrial Development and Coordination ~ IFI / DCTA. The measured radiation pattern of the antennas met the beamwidths of -3 dB in both the E and H planes, as well as the measured average gain.

[0054] Table 3 presents a summary of the measured results from the SAR-RSW planar antennas.

[0055] Parameters Specification Measurement Meets Frequency range 4.42 GHz to 4.44 GHz 4.42 to 4.44 GHz YES Return loss (Ref.

[0056] > 9.5 dB > 14 dB YES 50 Q)

[0057] Isolation (Ref. 50 Q) > 70 dB > 73 dB YES Linear Polarization ““ YES Gain (10 ± 1) dBi 9.6 dBi YES

[0058]

[0059] Beamwidth > 50° 58.9° YES 3 dB in the E plane

[0060] Beamwidth > 50° 65.1° YES 3 dB in H-plane

[0061] Lobe level

[0062] Secondary signals in planes < -20 dB < 22 dB YES E and H

[0063] Rejection of

[0064] Cross-polarization > 15 dB > 18 dB YES in the E and H planes

[0065]

[0066] 034 Although one embodiment of the invention has been shown and described, other embodiments may be apparent to those skilled in the art. Therefore, it will be understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and are not intended to limit the scope of the invention, which is defined by the claims that follow.

Claims

CLAIM 1) Circular antennas made of reduced surface wave microstrip for MFCW radar altimeter, constructed according to the reduced surface wave (RSW-MA) technique, operating distinctly as a transmitter and receiver of radio frequency (RF) signals, characterized by high isolation between them, with a physical separation between centers of less than 340 mm, using the multifrequency CW (MFCW) technique, operating in the frequency range of 4.42 GHz to 4.44 GHz (0.5% bandwidth), synthesized in a microstrip topology on a microwave dielectric substrate, with a ground plane on the lower face and the radiating element on the upper face, with the architecture of an MFCW (Multiple Frequency Continuous Wave) radar, where an emission of 4 signals at fixed frequencies (tones) in C-band is made and the reception, filtering and beating of the signal reflected on the surface is performed through an algorithm based on the calculation of phase differences between From pairs of transmitted tones, the pitch estimate is made.allowing height measurements to be taken with a linkage of around 600m. 2) Circular antennas made of reduced surface wave microstrip for MFCW radar altimeter, constructed according to the reduced surface wave (RSW-MA) technique, operating distinctly as a transmitter and receiver of radio frequency (RF) signals, according to claim 1, characterized by a pair of planar antennas, having a constructive geometry of a metallic circular patch and an inner annular ring short-circuited to the ground plane, through metallized holes that extend along the entire edge of the ring's boundary circle, a technique called SAR-RSW (Shorted Annular Ring - Reduced Surface Wave), and fed through a coaxial connector.

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