Antennas that rely on the opening and closing of switches that are co-located with the antenna for tuning are well known in the prior art. An example of a MEMS tuned slot antenna used for frequency tuning is described in a co-pending U.S. Patent Application (See document number 1 below). The MEMS tuned slot antenna disclosed therein contains a slot that is shorted at one end and open at the other end, with a MEMS switch serving as the short across the open end, to determine the effective length of the slot. By closing different switches along the length of the slot, the frequency of the antenna can be tuned. At resonance, the slot measures one-half wavelength long from the closed end to the first closed MEMS switch. This antenna represents an improvement over previous tunable antenna designs because the current was forced through the switch due to the open end of the slot, thus eliminating any unwanted current paths through the ground plane. However, the effective size of this antenna is dependent on the wavelength, which can create problems when a compact antenna is needed. In general, to make any effective MEMS-tuned antenna, the MEMS switch should provide the only path for one part of the antenna current, because the finite inductance of the switch can be shorted by other nearby metal structures, particularly continuous ground planes.
Other types of MEMS tuned antennas include patch designs, such as those described in document numbers 7 and 8 (identified below), as well as dipole, and various others. These designs are not preferred because patches, dipoles, and many other antennas are tuned by adding small metal regions that extend the length of the primary metal region. When tuning is performed with MEMS switches, this often causes interference from the DC bias lines. Therefore, it is necessary that the tuning be accomplished by shorting a metal object to a large ground plane, which can serve as both a RF and DC ground. In this way, the DC bias lines can be printed along this ground plane in such a way that they have very high or very low RF impedance, so that they cause minimal interference or coupling to the radiation. The slot antenna discussed above is an ideal candidate, but it suffers from a large size. It also requires that the ground plane be extended on all edges except one, which is left open for tuning.
Thus, the two important properties for a MEMS-tuned antenna are that the MEMS switch should be the only path for the particular portion of the antenna current that provides the tuning, and the switch should be able to be attached to a large ground plane to avoid interference or coupling from the DC bias. Another important property for many portable electronics or other compact devices is that the antenna should be small compared to the operating wavelength. One antenna that embodies these features is known as an F antenna. It typically consists of a metal wire or strip lying adjacent to the edge of a ground plane, with two connecting posts, one post acting as a feed for the metal strip, and the other acting as a short for impedance matching purposes. Reference 9 below discloses an F antenna by using a loop section for tuning instead of tuning the antenna itself. This design is not nearly as elegant or flexible, as the antenna does not provide a wide and arbitrary tuning range.
The disclosed antenna addresses the aforementioned needs by providing a simple, compact tunable antenna that is suitable for handheld or portable applications. The antenna can be tuned over a broad frequency range, and the size of the antenna is not solely dependent on the operating wavelength of the antenna such as is the case with typical prior art antennas.
2. Description of Related Art                1. D. Sievenpiper, “RF MEMS-Tuned Slot Antenna and a Method of Making Same”, U.S. Patent Application Ser. No. 60/343,888 and U.S. patent application Ser. No. 10/192,986, which is related to 60/343,888. These applications describe a tunable slot antenna. The presently disclosed technology is different in that the presently disclosed technology allows an antenna to be much smaller than the operating wavelength which can be important for certain handheld and/or portable applications.        2. I. Korisch, “Planar Dual Frequency Band Antenna”, U.S. Pat. No. 5,926,139 describes a basic planar RF antenna and includes meander line type structures for setting the resonant frequency.        3. S. Moren, C. Rowell, “Trap Microstrip PIFA”, U.S. Pat. No. 6,380,895. This patent describes another type of planar RF antenna, and also includes meander line structures for setting the resonant frequency.        4. N. Johansson, “Antenna Device and Method for Portable Radio Equipment”, U.S. Pat. No. 6,016,125. This patent describes an antenna that is tunable or reconfigurable by adjusting the position of a whip portion, which contacts an impedance matching inductor. This could be used either to adjust the position of the antenna to improve the impedance match, or presumably to tune the resonant frequency of the antenna. However, this antenna requires physical control of the antenna position by a user, and the antenna is largely stationary.        5. Y. J. Chen, H. J. Li, R. B. Wu, “Multi-Resonance Horizontal U-Shaped Antenna”, U.S. Pat. No. 5,644,319. This patent describes a multi-resonant antenna, however the antenna is not tunable. Furthermore, the antenna requires a folded structure that increases the size of the antenna.        6. Hiroshi Okabe, Ken Take, “Tunable Slot Antenna with Capacitively Coupled Island Conductor for Precise Impedance Adjustment”, U.S. Pat. No. 6,034,655. This patent describes a slot antenna using a cavity structure. The cavity structure increases the size of the antenna significantly, and the use of a closed-end slot forbids the use of MEMS switches.        7. Robert Snyder, James Lilly, Andrew Humen, “Tunable Microstrip Patch Antenna and Control System Therefore”, U.S. Pat. No. 5,943,016 describes a method of using a patch antenna by using RF switches to connect or disconnect a series of tuning stubs. However, this antenna is extremely sensitive to the position of the bias circuits and does not have the ability to tune the polarization and the pattern.        8. Jeffrey Herd, Marat Davidovitz, Hans Steyskal, “Reconfigurable Microstrip Array Geometry which Utilizes Microelectromechanical System MEMS switches”, U.S. Pat. No. 6,198,438 describes an array of patch antennas that are connected by RF MEMS switches. This antenna can be selectively tuned by turning on or off various switches to connect the patches together. Larger or smaller clusters of patches will create antennas operating at lower or higher frequencies. However, this antenna requires a large number of switches and the antenna does not provide a way to eliminate the problem of interference between the DC feed lines and the RF part of the antenna.        9. Gerard Hayes, Robert Sadler, “Convertible Loop/Inverted F Antennas and Wireless Communicators Incorporating the Same”, U.S. Pat. No. 6,204,819 describes an F-type antenna. However, this antenna has significant drawbacks due to its complexity. The antenna requires each separate frequency of operation to be addressed by a different type of antenna (loop, F, etch). This requires a different set of design equations for different resonant frequencies and modes of operation. Furthermore, this antenna does not allow for angle diversity.        10. De Los Santos “Tunable Microwave Network Using Microelectromechanical. Switches” U.S. Pat. No. 5,808,527 describes a MEMS switch for tuning, but does not discuss integration of a switch into an antenna.        11. Lam, Tangonan, and Abrams, “Smart Antenna System Using Microelectromechanically Tunable Dipole Antennas and Photonic Bandgap Materials” U.S. Pat. No. 5,541,614 describes an antenna system using microelectromechanically tunable dipole antennas and photonic bandgap materials.