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 is obtained by suspending the patch in air and increasing the antenna thickness: distance from patch 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 are described below.
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, 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, 1986, Pp 1439-1446) and (Pozar D. M., Targonski, S. D. “Improved coupling for aperture coupled microstrip antennas”, Electronics Letters, 27, 13, 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. 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, 1, June 1989, pp 458462). 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: the two probes may interfere with each other due to their close proximity. Another disadvantage is that the long horizontal probe requires means of mechanical support. This increases the cost and design complexity of the structure.
Thus, for the reasons mentioned above, a need exists of a simple, compact, and low-cost probe resulting in wide frequency bandwidth.
According to the present invention there is provided an antenna comprising a patch which may be of pure metallic form or may be etched on a dielectric and is disposed by a dielectric a distance above a ground plane, and a helix-shaped or meandering probe disposed between said patch and said ground plane, said probe is normal to said ground plane, said antenna further comprising means for connecting said probe to means for transmitting a signal to or from said antenna, and said helical probe is adapted to be electromagnetically coupled to said patch. The patch may be rectangular, elliptical, triangular, or any other geometric shape.
In one preferred embodiment of the invention, an antenna is presented comprising a rectangular patch suspended in air above a ground plane by a distance h, and a helical probe disposed between said patch and said ground plane, said helical probe is normal to said ground plane and said patch, and spaced from one edge of the patch by a distance d, said antenna further comprising means for connecting said helix probe to means for transmitting a signal to or from said antenna. The helix may consist of several turns or a fractional turn depending of its diameter. Generally, smaller diameters result in more turns.
In another embodiment of the invention, an antenna is presented comprising a rectangular patch suspended in air above a ground plane by a distance h, and a meandering-wire probe disposed between said patch and said ground plane, said meandering-wire probe is normal to said ground plane and said patch, and spaced from the top of the patch by a distance d, said antenna further comprising means for connecting said meandering-wire probe to means for transmitting a signal to or from said antenna.
All probes according to the present invention, do not exhibit the additional inductance problem, resulting in wideband patch antenna structures. For example, the capacitance between neighboring helix turns cancels the additional inductance. For example, in the case of helix probe, the capacitance between neighboring wires neutralizes the extra inductance caused by the increase of wire length. The same effect is observed in meandering-wire structures.
The antenna may be a single antenna with one patch and one probe according to the present invention. However, viewed from another aspect, a plurality of antennas according to the present invention can form an antenna array comprising a plurality of patches disposed above a ground plane, each said patch having a respective probe disposed between said patch and said ground plane, said antenna array further comprising a transmission network connecting said probes to each other and to means for transmitting a signal to or from said 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 invention. 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 preferred particular example of such structure comprises two rectangular patches and two respective probes, said both patches and probes are of different dimensions. The said 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.
Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.
It should be noted that the embodiments described herein should not limit the scope of the invention. The description above is intended by way of example only and is not intended to limit the present invention in any way except as set forth in the following claims. For example, the probes according to the present invention can be connected to a transmission line or coaxial cable in addition to a coaxial connector. The meandering-shape probes can be etched on substrates like FR-4 and can be connected to the center conductor of a coaxial cable or connector, or can be connected to a microstrip transmission line. 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.