1. Field of the Invention
The present invention relates to antennas and more specifically to planar broadband antennas incorporating an overlapping offset slot line.
2. Description of the Prior Art
Practitioners of the antenna arts have long realized that a tapered antenna feed leads to an improved broadband match. Early examples of such antennas include those of Carter [U.S. Pat. No. 2,181,870], and Brillouin [U.S. Pat. No. 2,454,766]. These concepts have been applied to planar antennas as well, notably by Nester [U.S. Pat. No. 4,500,887] who taught a tapered microstrip horn. Antenna radiating elements have been similarly tapered. For instance, Barnes [U.S. Pat. Nos. 6,091,374; 6,400,329 and 6,621,462] disclosed a tapered slot antenna and the inventor disclosed a semi-coaxial horn with a tapered horn element [U.S. Pat. No. 6,538,615].
In some cases, a tapered feed and tapered radiating element have been combined in the same antenna structure. For example, Lindenblad [U.S. Pat. No. 2,239,724], invented a wideband antenna with a tapered feed connected to a tapered bulbous radiating element. More recently the inventor implemented a planar antenna with a tapered feed structure smoothly flowing into elliptically tapered planar dipole elements [U.S. Pat. Nos. 6,512,488 and 6,642,903].
This prior art is characterized by generally monotonic variations in impedance with distance along a signal path traversing an antenna feed structure, radiating elements, and surrounding medium or space. These monotonic variations in impedance are generally considered desirable because they help to optimize a broad band match between an antenna and a transmission line. These monotonic variations may be discontinuous (as in a Klopfenstein taper) or have points of inflection (as in an Exponential taper).
Wavy shaped or corrugated antenna structures have been adopted for diffraction control or to increase impedance [Kraus, Antennas 2nd ed., New York: McGraw-Hill, pp. 657-9]. McCorkle [U.S. Pat. No. 6,590,545] discloses (FIG. 21) a planar UWB antenna with a wavy shaped slot. McCorkle suggests that a band stop transfer function might be possible by adjusting the width of the tapered clearance, however neither the drawings nor the detailed description provide any guidance to one skilled in the art as to how such adjustment gives rise to band stop behavior. In practice, the small periodic variations in tapered clearance shown by McCorkle are largely ineffective in giving rise to significant manipulation of an antenna transfer function, particularly since the disclosed variations maintain a continuous increase in width.
The inventor [U.S. Pat. No. 6,774,859] discovered that a practical means for implementing band stop or frequency notch filters in an otherwise ultra-wideband antenna is to incorporate a discrete narrow band resonant structure.
An alternate filtering technique, stepped impedance low pass filtering is also known in the art [David M. Pozar, Microwave Engineering, 2nd ed., New York: John Wiley & Sons, 1998, pp. 470-473]. This technique has not been applied to control impedance of antennas and implement desired transfer functions in antennas, however.
The extreme bandwidths of ultra-wideband antennas leave them especially vulnerable to interferers. It is a challenge to design an RF-front end to provide sufficient rejection to adjacent interferers just above an antennas operating band without adversely impacting performance in a desired band. For instance, it is desirable to have an ultra-wideband antenna responsive to the 3.1-5.0 GHz band without being responsive to interferers operating above 5.0 GHz. An electrically small UWB antenna is naturally unresponsive to signals lying below its operational band. Making such an antenna unresponsive to higher frequency signals is a greater challenge.
In view of the foregoing, there is a need for a system and method of modifying an antenna slot or notch to create the large variations in impedance necessary to implement effective distributed filters. There is a further need for a method to implement filtering or a desired transfer function with minimal modifications to an existing antenna design. Additionally, there is a need for an antenna apparatus that implements filtering capability inexpensively without requiring the added expense and board space of a lumped element filter structure in the RF front end of a radio device.