WITH THE PLETHORA of wireless standards being applied to today's handheld devices, the antennas integrated in those products must often operate in 10 or more frequency bands. For the antenna designer, this translates into the challenge of having to cover a single very wide frequency band or multiple frequency bands while maintaining small size and high efficiency. A novel solution may be to use antennas that have a reconfigurable operating frequency with frequency-invariant radiation characteristics. A method for electrically tuning the frequency of a planar inverted-F antenna (PIFA) has been presented by Mikko Komulainen and Heli Jantunen from the Microelectronics and Materials Physics Laboratories and the Electronics Materials, Packaging, and Reliability Techniques (EMPART) Research Group at the University of Oulu, Finland together with Markus Berg and Erkki T. Salonen from that university's Telecommunication Laboratory. They were joined in this work by Charles Free from the Advanced Technology Institute at the University of Surrey, UK.
These individuals integrated a tuning circuit into the antenna element, thereby leaving it free of direct-current (DC) wires. The tuning circuit comprised an RF switch and discrete passive components. It was integrated using reactive passive components, which form separate paths for the RF signal and DC control voltage. The DC voltage is carried to the switch simultaneously with the RF signal. To demonstrate the proposed tuning method, the researchers used a dual-band PIFA that can operate in four frequency bands. The antenna's rectangular patch is slit to form two separate paths for resonating currents, thus enabling dual-band operation. The antenna covers the GSM 850, GSM 900, GSM 1800, PCS 1900, and UMTS frequency ranges with more than 40-percent total efficiency.
Obviously, this kind of antenna will not simultaneously cover all of the frequency bands. Yet it may provide several dynamically selectable narrow frequency bands. Within those bands, it can exhibit higher efficiency than conventional antenna solutions. This approach can be used to downsize antennas while maintaining the operating bandwidth or to raise an antenna's bandwidth without increasing its physical size. Such tuning methods also can compensate for proximity effects. The frequency-tuning concept can be extended to more complex PIFA structures and other types of antennas. See "A Frequency Tuning Method for a Planar Inverted-F Antenna," IEEE Transactions on Antennas and Propagation, April 2008, p. 944.
