Patent Application
20080316138
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1. Technical Field
The present invention relates generally to communications and radio wave antennas, and more particularly to balance-fed antennas.
2. Background Art
In numerous communication networks today it is required to establish communications between stations where at least one is mobile. Important requirements for antennas in such applications typically include having very wide beam coverage (ideally an omnidirectional pattern), compact structure, specific polarization type, and efficiency over a specific bandwidth. Cellular telephone handsets, satellite radio receivers, and global positional system (GPS) equipment are common examples of devices which impose such requirements. In fact, the latter usually needs an antenna meeting more strict conditions, e.g., right-hand circular polarization and a very wide beam coverage pattern encompassing nearly the entire upper hemisphere. This is needed to allow a GPS receiver to maintain signal lock with and to track as many visible satellites as possible, while also providing useful signal-to-noise and front-to-back ratios (that is, the radiation pattern has a substantially lower gain in the direction opposite to the direction of maximum gain). Another important requirement is enough isolation between an antenna and the platform to which it is attached, to minimize antenna detuning due to the presence of the platform.
One widely used option today for such applications is the patch antenna. However, these can require tradeoffs that are undesirable or unacceptable, especially in small or mobile applications. Generally, a patch antenna has a usefully low profile but this may be offset by the need for a large ground plane. A patch antenna therefore often cannot provide satisfactory performance where space is very limited. Patch antennas also do not provide good circular polarization over a very wide angular region and they tend to have poor gain at low angles of elevation, thus making them a poor choice for GPS applications. And patch antennas also do not provide a good front-to-back ratio or reasonable isolation from their environment.
Another candidate is the bifilar or quadrifilar helical antenna (BFH or QFH), particularly in printed forms. Some of the advantages of the helical antenna, particularly the QFH, are its relatively compact size (compared to other known useable antennas such as crossed dipoles), its relatively small diameter, good quality of circular polarization (suitable for satellite communication), and its having a cardioid pattern, i.e., a main forward lobe which extends over a generally hemispherical region together with a good front-to-back ratio. The size of helical antennas can also be reduced by dielectric loading or by shaping the printed linear elements.
In order to obtain good electrical performance and radiation patterns, helical antennas need to be balance-fed, i.e., two antenna feed points are subjected to signals of equal amplitude but having an 180 degree phase difference. Since the external port of such antennas are normally an unbalanced type, such as a coaxial line, a balance-to-unbalance converter (balun) is needed. Balance-feeding helical antennas also helps provide or improve isolation from the environment, particularly from antenna platforms. Normal practice is to use a balun at the bottom of the antenna, where it attaches to the platform. Balums for helical antennas are usually of either sleeve type or a PCB structure, both of which increase the total size of the antenna. Using sleeve type baluns at the bottom of helical antennas, particularly for printed helixes on a core made of material with a high dielectric constant, also adds substantially to the price and complexity of manufacturing. Another disadvantage of sleeve baluns is that they do not provide any impedance transformation, hence requiring an extra impedance matching network for such antennas.
Finally, in many communication networks antenna cost is a major concern. The cost of a suitable GPS antenna may be a trivial portion of the overall cost of an airline navigation system, but a cost-is-no-object approach is just not practical for antennas used in the communication networks that are becoming ubiquitous in our day-to-day lives. For example, in general consumer GPS, cellular telephone, and satellite radio, whether an antenna costs $0.20, $2.00, or $20.00 can be determinative of how a product is accepted in the marketplace.
Like most articles of manufacture, the cost of an antenna has two major components: the cost of the materials and the cost of fabricating those materials. It can therefore be productive here to view overall antenna suitability as having three major contributing factors. The first is antenna design, meaning whether the design provide an antenna with adequate or better performance. A number of concerns related to this have been discussed above, and will be touched on further throughout this disclosure. The second factor is the materials-cost for an antenna design. This is considered least herein, since the materials typically differ little between different designs and because antenna designers tend to be very well schooled with respect to material-costs. The third factor is the fabrication-cost of an antenna design. Some considerations here are which manufacturing technique is cheapest in terms of the machines used, the numbers and complexities of steps that these must perform, and the tolerances that equipment must be calibrated to and maintained at to achieve a desired yield. This last factor is one where much of the prior art is wanting.
Accordingly, it is an object of the present invention to provide improved balance-fed communication antennas.
Briefly, one preferred embodiment of the present invention is an antenna. A dielectric core region having cylindrical shape is provided. This defines top, bottom, and side surfaces. Two laterally opposed conductive linking tracks are provided at the top surface. Two groups of conductive antenna elements are also provided, wherein each includes mutually adjacent instances of at least two of the antenna elements that connect to a respective linking track. The antenna elements extend across the top surface and at least partially down the side surface. The core region has an axial passage extending from the bottom to the top surfaces and a feed line having two conductors extends from outside of the antenna through the axial passage to the top surface. A balun is provided that has two input terminals and two output terminals, wherein the input terminals each connect respectively to a feed line conductor and the output terminals each connect respectively to a linking track.
Briefly, another preferred embodiment of the present invention is also an antenna. A dielectric core region having cylindrical shape is again provided and this again defines top, bottom, and side surfaces. Two laterally opposed conductive linking tracks are provided, only here at the bottom surface. Two groups of conductive antenna elements are again provided, with each again including mutually adjacent instances of at least two antenna elements that connect to a respective linking track. Here the antenna elements instead extend across the bottom surface and at least partially up the side surface. A balun is provided that has two input terminals and two output terminals. The output terminals each connect respectively to a linking track and a feed line having two conductors extending from outside of the antenna has each conductor connecting respectively to an input terminal of the balun.
An advantage of the present invention is that it provides an antenna that is particularly suitable for mobile and handheld applications.
Another advantage of the invention is that it provides an antenna that can have a compact structure.
Another advantage of the invention is that it provides an antenna that is efficient at the frequencies of many important and emerging applications, and an antenna that is efficient across the bandwidths needed for such applications.
Another advantage of the invention is that it provides an antenna that can have suitable signal-to-noise and front-to-back ratios for many applications.
Another advantage of the invention is that it provides an antenna that can have wide beam coverage, providing near-hemispherical radiation coverage approaching an omnidirectional pattern.
Another advantage of the invention is that it provides an antenna that employs a simple feed system able to provide desired features (e.g., antenna polarization) as applications require.
Another advantage of the invention is that it provides an antenna that can have linear or circular polarization over a wide angular range (e.g., right-hand circular polarization, beam width up to about 150 degrees, and with a suitable front-to-back ratio all as typically required for GPS and satellite radio applications).
And another advantage of the invention is that it provides an antenna suitable for simple fabrication, and therefore mass production and low cost production.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the figures of the drawings.
The purposes and advantages of the present invention will be apparent from the following detailed description in conjunction with the appended figures of drawings in which:
And
In the various figures of the drawings, like references are used to denote like or similar elements or steps.
A preferred embodiment of the present invention is a balance-fed helical antenna. As illustrated in the various drawings herein, and particularly in the view of
The exterior of the core region 12 is defined as having a top surface 18, a side surface 20, and a bottom surface 22. As discussed presently, the core region 12 may simply be air, some other gas, or vacuum and the boundaries of these “surfaces” then are set by the other elements of the antenna 10.
The antenna 10 has a pair of laterally opposed conductive linking tracks 24 at the top surface 18 that each connect to a group of conductive antenna elements 26. In
The feed line 16 passes axially through the core region 12, from the bottom surface 22 to a feeding region 30 at the top surface 18. The antenna 10 inherently has a longitudinal axis 32 and the feed line 16 can have a longitudinal axis 34 that is normally coaxial with this. As shown in
A balun 44 is provided here at the top surface 18 of the core region 12, and thus of the antenna 10, between the feed line 16 and the linking tracks 24 and antenna elements 26. The balun 44 provides a balanced feed to the antenna 10, thus permitting the overall structure to especially be quite compact. Optionally, the balun 44 can be an impedance transformer type (discussed presently)
The core region 12 is filled with or made of a dielectric material. For example, it may be of a low loss type like air, plastic, or ceramic. Of course, many other materials may also be used, with other gasses and even vacuum having already been noted. General radio frequency design principles will apply here, and the selection of a material should usually be straightforward. It should be appreciated, however, that this dielectric material can be either homogenous or inhomogeneous. For instance, an in-homogeneity can be created by providing multiple domains in the material with different dielectric constants. The dielectric material can thus be of an artificial type, say, of a material with a particularly high dielectric constant that is a blend of a true dielectric material and metal particles, inclusions, or various inserts.
[N.b., herein the terms “exterior” and “interior” are used with respect to an element's influence on the electrical characteristics of the inventive antenna 10, and not necessarily with respect to their literal physical position with respect to inactive other elements. For example, the core region 12 may actually be inside a thin layer of nonconductive material, such as foam or plastic, that acts as a protective cover or radome. To facilitate manufacture the elements of the antenna 10 also may be deposited onto a more outward base material that provides physical support yet does not substantially alter performance. Such usage of relative terminology is common in this art and, in any case, should now be clear in view of this reminder.]
The terms “radiate” and “excite” can be used to refer to the inventive antenna 10 for both transmitting and receiving signals. The electrical characteristics of the antenna 10, such as its frequency response and radiation pattern, obey the reciprocity rule. Accordingly, if the antenna 10 is configured and tuned to radiate right hand circular polarization when excited, it can absorb a right hand circular polarized signal at the same frequency in the receiving mode.
Returning now again to
To design a circular polarized embodiment of the antenna 10 it would normally be necessary for all of the antenna elements 26 to radiate with equal amplitude but in different phases, e.g., to provide a progressive 90-degree phase shift between each two adjacent antenna elements 26. However, a prior art approach that can be extended to the inventive antenna 10 to provide the abovementioned condition is to differentiate the lengths of each pair of adjacent antenna elements 26 by a specific amount. The slightly different lengths of the antenna elements 26 then cause them to resonate at different frequencies, with the phase of each varying with respect to the actual frequency present. By appropriately tuning the lengths of the antenna elements 26, a fixed phase offset for each can be obtained and a predetermined total phase difference equal to the required value can be provided at a desired specific frequency, i.e., the main application frequency of the antenna 10. Such dual-resonance techniques for creating circular polarization are relatively simple and help make circular polarized embodiments of the antenna 10 cheaper to manufacture. This can also permit embodiments of the antenna 10 to create circular polarization over a very large angular region (e.g., about +/−50 degrees in both planes).
As is known in the art, double resonance methods of creating circular polarization generally produce relatively narrow bandwidths. In contrast, the inventive antenna 10 here can be designed to have a fairly low VSWR over a wider bandwidth. Thus it can have a mixed linear polarization in frequencies other than the circular polarization narrow bandwidth, and it therefore can be used for specialized applications, e.g., mobile applications, which need both circular polarization and mixed linear polarization albeit in different portions of their total bandwidths.
The adjacent antenna elements 26 preferably have similar shapes (as shown in
Many other known prior art techniques can also be applied to further improve the inventive antenna 10. For example, in order to reduce the vertical extension of the antenna 10, the antenna elements 26 can follow simple helical paths (as shown in
Another technique that can be extended to the inventive antenna 10 is to fill or make the core region 12 of a low loss plastic or ceramic material with a high dielectric constant, to improve the mechanical stability and/or reduce the size of such an antenna 10 compared to that of one with air as the dielectric. Using a material with a high dielectric constant, e.g., more than 10, helps constrain the antenna near field. The resulting antenna 10 then is highly tolerant to the proximity of people, other components and other antenna. Miniaturization of the antenna 10 also helps it to have a very sharp filtering response, hence reducing the need for additional filtering between the antenna 10 and a receiver or transmitter for many applications, e.g., GPS.
When an embodiment of the antenna 10 comprises a core region 12 of a solid dielectric, it can be made by conventional photoetching techniques. This is particularly useful for a fully dielectric loaded antenna 10 (versus a partially loaded embodiment). For example, first the cylindrical core region 12 of a dielectric material is provided. Then a metallization procedure is used to coat the top surface 18 and the side surface 20 of the core region 12. Next, portions of these metallized surfaces 18, 20 are partially removed in a predetermined pattern to produce the opposing groups of antenna elements 26.
In order to have desired performance, including radiation pattern, the balun 44 provides balanced signals to the opposing groups of antenna elements 26. This also helps to prevent common mode noise from entering a receiver through the antenna path. The balun 44 can also help to isolate the antenna 10 from a platform to which it is physically connected, thus reducing undesired coupling effects and making it much less sensitive to environmental presences (e.g., in a mobile handset from influence due to the handset being handheld). By selecting a suitable impedance transformer for the balun 44, its dimensions/discreet elements and other features can all be designed for a specific embodiment of the antenna 10. Alternatively, particularly to further improve the performance, the antenna 10 can be designed to include the effect of the balun 44 or, in the extreme case, both can be optimized/designed together.
Of course, many well-known prior art approaches can be used for designing and constructing the balun 44. For instance, the balun 44 can be embodied completely or partially in a generally multilayer printed circuit boards. Unlike well-known prior art approaches, however, the balun 44 here is preferably, but not necessarily, placed at the feeding region 30 on the top surface 18 of the core region 12.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and that the breadth and scope of the invention should not be limited by any of the above described exemplary embodiments, but should instead be defined only in accordance with the following claims and their equivalents.
