The embodiments herein relate to a patch antenna, and in particular to a wideband patch antenna fed by a helix-shape. The embodiments herein can further relate to antenna arrays or dual band structures comprising a plurality of such patch antennas.
Microstrip patch antennas are very popular for a wide variety of applications. They have several advantages such as low profile, low cost, simple fabrication and light weight that make them very suitable in fixed and mobile communication systems. A typical microstrip patch antenna comprises a patch above a ground plane and separated from the ground plane by a dielectric. A typical patch is fed by means of a coaxial feed, where the center conductor pin is physically connected to the patch. One drawback with such microstrip patch antennas is that they have a relatively narrow bandwidth and thus, are not generally suitable for applications requiring broad bandwidth. The bandwidth can be increased by increasing the substrate thickness and decreasing the substrate permittivity. Relatively large bandwidth can be obtained by suspending the patch in air and increasing the antenna thickness or in other words increasing the distance from patch antenna to ground plane. However, the increase in thickness increases the coaxial probe inductance due to increased probe length, thus, limiting the antenna bandwidth. Several methods have been disclosed that reduce or compensate for this additional probe inductance while increasing the bandwidth of relatively thick microstrip patch antennas. These methods, as well as, our claimed method, will be described next.
Method 1: (Sabbin A. “A new broadband stacked two-layer microstrip antenna”, IEEE AP-S Int. Sym. Digest, 1983, 63-66) and (Lee, R. Q., Lee, K. F., Bobinchak, J. “Two-layer electromagnetically coupled rectangular patch antenna”, Antennas and Propagation Society International Symposium, 1988, AP-S. Digest, Vol. 3, pp 948-951). A second parasitic patch on top of the driven patch electromagnetically coupled to the driven patch. The use of a parasitic patch on top or next to the driven patch increases the overall thickness and volume of the antenna, and cost.
Method 2: (Fong K. S., Pues H. F., Withers M. J. “Wideband multiple layer coaxial fed microstrip antenna element”, Electron Lett, 1985, 21, pp 497-499.) This method utilizes a capacitively coupled feed where a conductive disk, etched on a substrate, is attached to the top section of the feed and spaced a small distance below the patch. The capacitively coupled feed, although neutralizing the extra probe inductance, is a high-cost complex structure and requires high precision, thus increasing cost.
Method 3: (Pozar D. “A reciprocity method of analysis for printed slot and slot-coupled microstrip antennas”, IEEE Transactions on Antennas and Propagation, Vol. 34, December 1986, pp 1439-1446) and (Pozar D. M., Targonski, S. D. “Improved coupling for aperture coupled microstrip antennas”, Electronics Letters, Vol. 27, Issue 13, June 1991, pp 1129-1131). This approach utilizes aperture coupling using a slot and a microstrip line for feeding the patch. The slot/microstrip line approach requires an additional substrate where the slot and microstrip line are etched. This solution also increases cost and assembly time.
Method 4: (Hall P. S. “Probe Compensation in Thick Microstrip Patches” Electronic Letters, Vol. 23, No. 11, 1987, pp 606-607). A conductive disk is attached at the end of the feed just like method 2 described above. In this case, the disk and driven patch are located on the same layer forming an annular gap between them, thus forming a capacitor. This annular gap increases the probe capacitance required to reduce the extra probe inductance. However, the antenna radiation pattern exhibits cross-polar components (Garg et al., “Microstrip Antenna Design Handbook, ISBN 0-89006-513-6, 2001 Artech House, Inc. page 19). In addition, this arrangement results in a complex structure, especially when the patch and disk are suspended in air, thus, increasing cost.
Method 5: (Hall P. S., Dahele J. S., Haskins P. M. “Microstrip patch antennas on thick microstrip patches”, Antennas and Propagation Society International Symposium, 1989, AP-S. Digest, Vol. 1, June 1989, pp 458-462). A capacitor is formed by placing a small conductive disk at the end of the feed, just like methods 2 and 4. The conductive disk is placed on top of the patch and separated from the top surface of the patch by a small gap, thus, creating the required capacitance. This extra capacitance compensates for the additional probe inductance, thus increasing the antenna bandwidth. However, this approach also results in a complex structure and high cost.
Method 6: (Luk K. M., Chow Y., Mak L., U.S. Pat. No. 6,593,887, Jul. 15, 2003). The inventors describe a patch antenna using an L-shaped feed probe. The L-shaped probe has a first portion normal to ground plane and patch, and a second portion parallel to ground plane and patch. The L-shaped probe is electromagnetically coupled to the patch. This arrangement is also effective in reducing the extra inductance of the probe. However, the total physical length of the probe is relatively large, approximately ¼ of the wavelength (i.e., 8.72 cm at 860 MHz). This large size can cause interference and EMI problems with RF circuits located in the vicinity of the probe. In addition, since the horizontal component of the L-shaped probe is much longer than the vertical section of the probe, it will be difficult to implement a two-feed circularly polarized patch antenna. More particularly, the two probes may interfere with each other due to their close proximity. Furthermore, the long horizontal probe requires means of mechanical support which once again increases the cost and design complexity of the structure.
Method 7: (Luk; Kwai-Man, Lai; Hau Wah, U.S. Pat. No. 7,119,746, Oct. 10, 2006).
The inventors describe a patch antenna using a meandering feed probe. The meandering probe is electromagnetically coupled to the patch. This arrangement is also effective in reducing the extra inductance of the probe. However, such structure is still physically large for lower frequency applications. Just as in method 6, this large size can cause interference and electromagnetic interference (EMI) problems with RF circuits located in the vicinity of the probe.
None of the solutions described above provide a simple, compact, and low-cost feed system resulting in wide frequency bandwidth.
According to the embodiments an antenna as contemplated herein can comprise a patch which is of pure metallic form or etched on a dielectric and is disposed by a dielectric a distance above a ground plane, and a helix-shaped or meandering probe that meanders over multiple planes or in a three dimensional space disposed between the patch and the ground plane where the probe is normal to the ground plane. The antenna can further comprise a connector, a transmission line or coaxial line center conductor that couples the probe to a transmitter to or from the antenna, and the probe is adapted to be electromagnetically coupled to the patch. The patch may be rectangular, elliptical, triangular, or any other geometric or irregular shape.
In one embodiment, an antenna can comprise a rectangular patch suspended in air above a ground plane by a distance h, and a helical or three dimensional probe disposed between the patch and the ground plane, the probe is substantially normal to the ground plane and the patch, and spaced from one edge of the patch by a distance d, the antenna further comprising a connector coupling the probe to a transmitter to or from the antenna. The probe may consist of several turns or a fractional turn depending on its diameter. Generally, smaller diameters result in more turns.
The helix probe, according to the present embodiments, does not exhibit the additional inductance problem present by other methods or structures, resulting in wideband patch antenna structures. For example, the capacitance between neighboring wires neutralizes the extra inductance caused by the increase of wire length.
The antenna may be a single antenna with one patch and one probe according to the present embodiments. However, viewed from another aspect, a plurality of antennas according to the present embodiments can form an antenna array comprising a plurality of patches disposed above a ground plane, each patch having a respective probe disposed between the patch and the ground plane, the antenna array further comprising a transmission network connecting the probes to each other and to a transmitter or a means for transmitting a signal to or from the antenna array. Such an antenna array may take several forms. One simple structure is an array that comprises two patches with their respective probes being connected by a single transmission line. The arrays can use one type or combination of the probes according to the present embodiments. Antenna arrays such as a two-by-two or four-by-four array may be formed. More complicated arrays may also be formed.
Another example is a dual band antenna structure. A particular example of such structure comprises two rectangular patches and two respective probes, both patches and probes are of different dimensions. The patches are disposed above a ground plane and spaced at different distances from the ground plane. The dual band antenna structure further comprises a transmission line connecting said probes to each other and to means for transmitting a signal to or from said antenna structure, said transmission line being parallel to said ground plane.
It will also be understood that the patch antennas may be spaced from the ground plane by any form of dielectric material (including air) or by multiple layers of differing dielectric materials.
It should be noted that the embodiments described herein should not limit the scope of the claimed embodiments. The description above is intended by way of example only and is not intended to limit the present scope in any way except as set forth in the following claims. For example, the probes according to the present embodiments can be connected to a transmission line or coaxial cable in addition to a coaxial connector. The coaxial cable shield may be soldered to the bottom side of the ground plane and the cable center conductor can connect to the probe through a hole on the ground plane. The coaxial cable shield may also be soldered to the top side of the ground plane and the cable center conductor can connect to the probe without the need for a hole on the ground plane. It shall be understood, that the patch antennas may be of pure metallic form or etched on any type of dielectric. In addition, as mentioned above, the patch antennas may be spaced from the ground plane by any form of dielectric material (including air) or by multiple layers of differing dielectric materials. The probe can be any type of helix element such as a standard helix, conical, and square helix.