Patent Application
20210203074
The present invention relates to a low profile dual-band antenna, and more particularly to a low profile helical quadrifilar antenna utilizing a ground plane and a patch that is coupled between each of the filar ends.
Global navigation satellite systems (GNSS), location and/or elevation indication systems originally designed for the military, are now used worldwide also by both industry and the general public. Examples of growing GNSS uses include: automobile and truck navigation, smartphones, autonomous driving, aviation, smart agriculture, asset/package tracking, surveying, construction, sports equipment, workforce management, high speed trains and unmanned aerial vehicles.
As the importance of reliable GNSS availability has increased, there are a growing number of GNSS systems worldwide such as GPS (U.S.), GLONASS (Russia), Galileo (Europe), and Beidou (China).
Global GNSS Signals are right-hand circular polarized operating on these frequencies (MHz):
By receiving multiple bands at the same time, GNSS equipment can provide accuracy cross checks and failsafe redundancy to provide additional precision for critical GNSS applications and Real Time Kinematic (RTK).
Therefore, an antenna covering all the frequency bands is highly desirable. The design challenge is that the overall band from 1164-1610 MHz has a 32% Bandwidth. As frequencies between 1300 and 1525 MHz are not used for GNSS, many design efforts are focused on dual-band approaches. E.g. the whole spectrum is separated into a low band and a high band.
Low band: 1164-1300 MHz, 11% Bandwidth.
High band: 1525-1610 MHz, 5.4% Bandwidth.
Many forms of circularly polarized antennas are known. Patch antenna or microstrip antenna are cost and size efficient for use in the High band but typically require thick/heavy dielectric substrate when configured for the Low band, which increases costs and may limit bandwidth.
Another antenna configuration is the quadrifilar helix antenna (QHA), also known as the Kilgus coil, comprising a collection of cooperating phase quadrature fed helical filar elements. QHA offer advantages of being light in weight, good circular polarization quality and wide bandwidth.
In precision GNSS application such as survey and reference station etc., the stability of phase center is a critical design parameter for cm or mm level accuracy measurement. The phase center of an antenna is located at the center of curvature of the radiated equal phase surface for a given component of the far field pattern. Phase center variation will result in positional errors. There are many factors contributing to the degree of unwanted variation of phase center. For instance, reducing phase center variation may be accomplished by reducing the cross-polarization levels or improving the circularity of survey antennas.
As described in U.S. Pat. No. 5,515,057, a multiple feed characteristic can improve X-Y phase center stability. Multiple feed antennas may also provide better circular polarization than a single feed. For multi-feeding point, the main drawback is that the feeding network loss will increase, resulting in poor gain-to-noise-temperature (G/T). Therefore, the feeding points are normally not in excess of 4 in practical application. QHA is a naturally quad-feed antenna.
Prior QHA typically have a tall cylindrical profile characteristic, for example as demonstrated by U.S. Pat. No. 10,199,733. Where low profile QHA configurations have been developed, these are typically only single band solutions.
In many applications, especially precision GNSS survey antennas, dual band antennas may be collocated or “piggybacked” to realize a self-diplexing antenna. In this case, each antenna can be optimized and the requirement for an input diplexing device is eliminated, which may lower the overall noise characteristic of the resulting antenna array.
Prior attempts to combine high and low band antennas together by stacking or embedding patches or microstrip antennas are well known. For example, it is known that a very thick lower element may be used for the low band (which is heavy) when bandwidth is required. In some cases, high dielectric constant ceramic material is used to shrink the size in a tradeoff with reduced bandwidth.
There are stacked QHA antennas as well. However, these have poor performance due to interaction between the QHA.
U.S. Pat. No. 6,720,935 discloses a Single and Dual-band Microstrip Antenna (MSA) “Patch”/Helix Antenna Array utilizing a high profile n/4 wavelength open end QHA. It is known that an open end leads to the highest voltage point so it is sensitive to adjacent objects. Therefore, the disclosed antenna has to use a separate ground plane with enough distance to decouple the two antennas, increasing the height of the resulting antenna.
Therefore, there is a need for antenna solutions which improve electrical performance in a compact configuration.
It is an object of the invention to realize a low profile and dual-band quadrifilar antenna.
It is an object of the invention to provide an above-mentioned antenna to cover all existing GNSS frequency.
It is an object of the invention to provide an antenna with a hybrid working mode.
Another object of the invention is to reduce overall cost by avoiding use of expensive substrate materials.
It is another object of the invention to provide a platform for multi-feed stacked antenna integration.
Still another object of the invention is to minimize the mutual coupling in a multi-feed stacked antenna.
These objects are fulfilled in the present invention with a low profile dual frequency band and circularly polarized quadrifilar antenna with:
1. A ground plane,
2. A ½ wavelength QHA antenna element on the ground plane, and
3. A top layer MSA patch directly connected with the ends of the QHA filar. The patch is also a current path for the QHA. The patch size is selected for high band TM11 mode.
4. The QHA is driven from the bottom, via four feeds of equal amplitude but having 0°, 90°, 180° and 270° phase differentials. Changing the impedance will shift the frequency and ratio of fh/fl.
5. Four slots on the top layer may be applied to cut the unwanted current flow, improving high band and low band efficiency.
6. The highest current point and lowest voltage point at the physical center provides a natural grounding so feed lines can be carried through without adverse impact of the antenna performance, thus it can be a platform for an MSA upon QHA stacked antenna configuration.
Similar to stacked MSA, an MSA of the high band is stacked onto the top of the preferred embodiment where the grounded top layer provides a ground plane for the stacked element. In this case the top element should not be resonant to the high band and the lower QHA only works for the single band: low band. Furthermore, as the QHA and MSA have different electromagnetic field distributions, the coupling is alleviated.
7. In a specific example, an MSA is on top of the QHA. It provides an alternative stacked microstrip antenna configuration with the traditional lower microstrip element being replaced by a low profile QHA.
A significant benefit of the lower QHA versus the microstrip antennas is that as bandwidth limiting heavy relative dielectric is not needed, surface waves on an otherwise thick MSA are mitigated.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, where like reference numbers in the drawing figures refer to the same feature or element and may not be described in detail for every drawing figure in which they appear and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.
Referring to
The inventor has recognized that where a % wavelength Kilgus QHA configuration is applied, the RF current flow and voltage is null at the centers of the filar. Therefore, one can fill this area with conductor without changing the working principle. Further, the MSA TM11 mode equivalent circuit is parallel to that in the QHAs.
One skilled in the art appreciates that more current path will be excited when the patch size is resonant to a certain frequency. In this case, the lower band will have the half-wave shorted QHA. This is because the feeding impedance near edge reaches the highest point so MSA TM11 mode is mismatched. In other words, the lower band has minimal MSA mode contribution.
A ground plane 10 is about 154 mm in diameter. Concentric with the ground plane 10, Helical QHA filar 31, 32, 33, and 34 may be provided as approximately 4 mm wide copper strips. The feeding points 41, 42, 43 and 44 at bottom of the respective filar, through the ground plane 10, may be driven by 0°, −90°, −180° and −270° phase differentials, respectively. The top layer is a deposited copper surface or copper planar patch 30, provided parallel to the ground plane 10, that is connected to the distal ends of the filar 31-34 where each intersects with a circumferential edge of the patch 30 at a connection point 61, 63, 63 and 64.
A supporting body 20 of the antenna may be provided as dielectric polymer operative as supporting structure and/or a conductor deposition surface for the filar 31-34 as well as the patch 30. Specific to this embodiment, only, a diameter of the supporting body 20 (and thus of the filar 31-34 arrayed upon it) is 70 mm and the height is 25.8 mm. The filar 31-34 have a width of 4 mm. Alternatively, the patch 30 and filar 31-34 may be applied as portions of flexible printed circuit boards (PCB), attached, for example, via adhesive and/or double side tape or the like. For cost efficiency and/or precision, the supporting body 20 may be formed via injection molding.
For additional tuning, the distal ends of the filar 31-34 may be provided with corresponding enlarged impedance matching pads 65, 66, 67 and 68 that then couple with the patch 30.
As best shown in
Although the low band is more detuned for MSA mode as previously discussed, the circular patch 30 does provide a path for the adjacent connection point, leading to higher loss of the antenna. A slot 51, 52, 53, 54 extending radial inward from the patch 30 periphery, positioned in between each of the adjacent connection points 61-64 may be applied to reduce these harmful currents. These slots 51-54 may be provided, for example, equidistant between adjacent connection points 61-64 about 2 mm in width and 7 mm in length.
The supporting body 20 is provided with, for example, an about 3 mm wall thickness 21. Similarly, a plated through hollow 22 provides a stand 23 to structurally reinforce the center of the patch 30, and also operate as a mounting point for the antenna (and/or a feed pathway in further embodiments).
Further tuning of the filar 31-34 may be applied, as shown for example in
Where the additional hardware costs and electrical losses of diplexing the antenna feeds between the high and low bands are undesired, the antenna may be provided in a self-diplexing dual antenna/feed embodiment.
In the preferred embodiment, the filar 31-34 may be tuned toward the low band as the high band is now served by the high band patch 71 assembly. Furthermore, it is possible to avoid configurations generating resonance with respect to the high band patch 71, for example by applying a narrower thickness of approximately 1.5 mm to the width of the traces of the filar 31-34. It is found that the dual band resonant behavior will be shifted and changed to single band when moving the feed points simultaneously to make the electrical path less than ½ wavelength of the lower frequency band.
The high band patch 71 may be dual feed or multi-feed depending on desired application. For instance, a quad feed may be applied for high precision survey applications. Typically, the feed lines are probes and can be fed through the stand 23. There is no coupling on the feed line since the plated stand 23 completely shields the feeds. Feeds 41, 43 can be connected to the phasing network or to a low noise amplifier (LNA). Here only two leads are illustrated for clarity but one skilled in the art appreciates that the number of feeds is not limited to two.
It is observed that the edge provide a patch to couple the adjacent port. Quad-feed MSA probes 72, 73, 74 and 75 couple the high band patch 71 to the respective feeds. Isolation may be improved by rotating the feeds 45 degrees, with respect to the phase differentials assigned to the filar feeding points 41-44. A coupler 76, such as a mounting screw or the like may be applied to secure the assembly as well as provide an additional grounding path for the high band patch 71.
In further embodiments, the supporting body 20 and thus the filar 31-34 may be provided in other than cylindrical configurations, for example with octagonal or square cross-sections. Notably, non-circular cross-sections enable multiple antenna assemblies to be provided each in closer proximity to one another in compact antenna arrays where multiple antenna elements are provided, for example with each optimized for a specific frequency band.
The embodiments of the present invention relate generally to a novel design for a low profile, dual-band, quadrifilar helix antenna structure. While the preferred embodiments represent implementations optimized for GNSS survey application, the design may be equally applied to other applications where a more compact, dual band antenna is desirable when higher dielectric constant material loading is applied.
Where in the foregoing description reference has been made to materials, ratios, integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth.
While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant's general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.
