2190 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 Tunable Miniaturized Patch Antennas With Self-Biased Multilayer Magnetic Films Guo-Min Yang, X. Xing, A. Daigle, M. Liu, O. Obi, S. Stoute, K. Naishadham, and Nian X. Sun Abstract—Magneto-dielectric substrates with thin magnetic films show great potential in realizing electrically small tunable antennas with enhanced bandwidth, improved directivity, and high efficiency. This communication introduces self-biased NiCo-ferrite magnetic films as a practical mean to tune a patch antenna by loading single layer and multilayer self-biased ferrite films. The central resonant frequency of the unloaded patch antenna is measured at 2.1 GHz with a bandwidth of 18 MHz. However, with ferrite loading of the alumina substrate, this frequency is shown to be tunable within a range of 12 MHz–40 MHz, and the antenna efficiency is increased from 41% of the non-magnetic antenna to 56%, 65%, and 74% for the three magnetic antennas. The omnidi- rectional radiation pattern is significantly enhanced with the dBic gain beamwidth increased from 140 to 155 , 156 and 160 , respectively for the three ferrite loaded antennas. In addition, the gains of the three magnetic antennas are enhanced by 0.32, 0.77, and 1.1 dB, respectively, over the unloaded antenna. Index Terms—Magnetic films, self-biased films, tunable magnetic an- tennas. I. INTRODUCTION Achieving relative permeability larger than 1 in antenna substrates can lead to antenna miniaturization, enhanced bandwidth, tunable center frequency, polarization diversity, and beam steering [1]–[3]. Bulk ferrite materials [4], composites of ferrite particles in polymer matrix, metamaterials with embedded metallic circuits, etc., have been used as antenna substrates for achieving . However, these bulk ferrite materials or ferrite composites are too lossy to be used at MHz under self-bias condition, i.e., no bias magnetic field is needed, and large biasing magnetic fields are needed for these magnetic antennas to operate at higher frequencies. In order to be practically feasible in miniature antenna applications, such as handheld wireless communication devices, it is important for antenna substrates to be comprised of self-biased magnetic materials. However, it has been challenging to achieve self-biased magnetic materials for antenna substrate applications at MHz. Magnetic thin films provide a unique opportunity for achieving self- biased magnetic patch antenna substrates with [5]–[7] and op- erating GHz. The strong demagnetization field for magnetic thin films, , and large in-plane anisotropy field allow for a self-biased magnetization with high ferromagnetic res- onance (FMR) frequencies up to several GHz, a necessary condition for operations in the cellular and WLAN bands. Most recently, we have proposed to use novel magnetodielectric composite substrates for antennas with low-loss magnetic film mate- rials and low-loss high permittivity dielectric materials. In our previous Manuscript received March 26, 2008; revised October 21, 2008. First pub- lishedMay 02, 2009; current version published July 09, 2009. This work was sponsored by the NSF and ONR. G. M. Yang, X. Xing, A. Daigle, M. Liu, O. Obi, S. Stoute, and N. X. Sun are with the Center for Microwave Magnetic Materials and Integrated Circuits, Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115 USA (e-mail: Nian@ece.neu.edu). K. Naishadham is with the Electromagnetics and Antennas Division, Georgia Institute of Technology, Atlanta, GA 30332 USA. Color versions of one or more of the figures in this communication are avail- able online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2009.2021972 work, new designs of electronically tunable patch antennas with mag- netic metallic magnetic films ((Fe Co ) B ) were investigated [5], which showed that the bandwidth was increased by 50% over the non-magnetic antennas. Most recently, new self-biased ferrite film of NiCo-ferrite films (Ni Co Fe O ) were investigated and adopted [6]–[8], these NiCo-ferrite films are magnetically isotropic in the film plane, which also have a low loss tangent under self-bias condition, i.e., no external bias field is needed for proper operation. We have demonstrated self-biased loop antennas at 1.7 GHz with a wide range of tunable resonant frequency with high quality self-biased ferrite films [7], which shows great potential in the applications of antenna miniaturization for mobile handheld wireless communication devices. In this communication, we report on a patch antenna miniaturized using single layer and multilayer self-biased NiCo-ferrite thin films on alumina substrate, thus essentially creating a magneto-dielectric sub- strate for practical applications. Three different magnetic patch an- tennas are fabricated by loading the antenna with multilayer ferrite thin films adjacent to the patch. These antennas show enhanced band- width and significantly enhanced antenna efficiency. The dBic gain beamwidth is increased from 140 to 155 , 156 , and 160 , for the three magnetic antennas, showing significantly improved omnidirec- tional performance. II. DESIGN OF PATCH ANTENNAS Microwave ferrite ceramics show relatively high permeability and high permittivity , as well as low loss at RF/microwave fre- quencies. These characteristics are highly desirable for the miniatur- ization of many different RF/microwave devices, including antennas [9]. Two major loss mechanisms account for the limited operation fre- quency, the ferro/ferrimagnetic resonance (FMR) and domain wall mo- tion. The operating frequencies of bulk microwave magnetic materials are limited to be MHz due to the excessive magnetic loss tangents associated with various loss mechanisms, with FMR being the major loss mechanism. The FMR frequency is therefore the upper frequency limit for antenna substrates for achieving . In other words, above the FMR frequency, prohibitive magnetic losses preclude the device op- eration. For a uniformly magnetized sphere, the FMR frequency is linearly proportional to the net magnetic field , with the gyromagnetic con- stant, being close to 2.8 MHz/Oe. Large bias fields of the order of 1000 Oe are needed for the FMR frequency to reach GHz range, and allow device operation in that range. The relative permeability of the magnetic sphere can be described by (1) which is inversely proportional to the net magnetic field . We can therefore readily reach the Snoek limit [10] (2) i.e., the product of the FMR frequency and the relative permeability is a constant that is determined by the saturation magnetization of the magnetic medium. The permeability of the ferrite in the film plane is still with being the net in-plane field; while the FMR frequency is increased to be (3) 0018-926X/$25.00 © 2009 IEEE