Design And Analysis Of An Electrically Small Inductively Loaded Sector Antenna For Frequency Reconfiguration Using Varactor Diodes

Global wireless communications networks are facing an ever increasing demand to deliver content to the consumer. Along with increased consumption of wireless bandwidth, consumers demand that their devices interface with a plethora of different wireless bands and protocols. To meet this demand, mobile devices have a multiplicity of antennas designed to work for specific bands. However, as mobile communication demands continue to grow, the number of antennas will become even more constrained by size and cost. In order to address this issue, an inductively loaded, electrically small, frequency reconfigurable, sector antenna that covers most consumer frequency bands has been developed. This thesis details the design and characterization of the antenna element, which tunes the GSM 900, GSM 1800, 3G, WiFi, and WiMAX bands. The antenna developed is modified from a previous design for frequency reconfiguration using varactor diodes placed in inductive loads. In order to allow for frequency agility, a DC bias network is carefully designed utilizing blocking capacitors to allow for an impedance match over the bands of interest. Characterization of the antenna on different ground plane sizes and construction issues are also addressed. Finally the inductively loaded, electrically small, frequency reconfigurable, sector antenna is fabricated and measured to confirm the simulation analysis. Comments on the radiation patterns, efficiency, and gain are also provided.

Abstract: Emerging broadband applications with market pressures for miniaturized communication devices have encouraged the use of electrically small antennas (ESA) and highly integrated RF circuitry for high volume low cost mobile devices. This research work focuses on developing a novel scheme to design wideband electrical small antennas that incorporates active and passive loading as well as passive matching networks. Several antennas designed using the proposed design technique and built and measured to assess their performance and to validate the design methodology. Previously, the theory of Characteristic Modes (CM) has been used mostly for antennas analysis. However; in this chapter a design procedure is proposed for designing wide band (both the input impedance bandwidth and the far field pattern bandwidth) electrically small to mid size antennas using the CM in conjunction with the theory of matching networks developed by Carlin. In order to increase the antenna gain, the antenna input impedance mismatch loss needs to be minimized by carefully exciting the antenna either at one port or at multiple ports and/or load the antenna at different ports along the antenna body such that the Q factor in the desired frequency range is suitable for wideband matching network design. The excitation (feeding structure), the loading of the antenna and/or even small modifications to the antenna structure can be modeled and understood by studying the eigenvalues and their corresponding eigencurrents obtained from the CM of the antenna structure. A brief discussion of the theory of Characteristic Modes (CM) will be presented and reviewed before the proposed design scheme is introduced. The design method will be used to demonstrate CM applications to widen the frequency bandwidth of the input impedance of an electrically small Vee shape Antenna and to obtain vertically polarized Omni-directional patterns for such antenna over a wide bandwidth. A loading technique based on the CM to either design frequency reconfigurable antennas or broaden their bandwidth by Non-Foster loading will also be discussed as part of the design methodology. In the Appendix, a brief discussion of the fundamental limits of electrical small antennas is presented and then followed by a discussion of the fundamental limits of the impedance bandwidth of the ESA when a passive matching network is used. Matching network implemented using Non-Foster matching is also discussed in the appendix.

For decades, it was believed that the Martian subsurface contains no liquid water but only shallow ground ice. Nevertheless, over the past few years scientists have been discovering on Mars new evidence of liquid water, which is an essential ingredient for life. Post processing high-resolution pictures taken by NASA's Mars Reconnaissance Orbiter (MRO) revealed sheets of water ice located in the Martian subsurface. To obtain the dielectric spectroscopy of these potential water bodies, a demand has arisen for electrically small loop antennas that can be used on the broadband dielectric spectrometer for future Mars rover missions. The hope among the scientific community is that by sweeping the frequencies of the transmitting electromagnetic waves from 0.8 to 10MHz, the presence of water and hydrated minerals can leave some signature in the reflected wave. To provide the scientists with the "ears" to listen to the flow of water underneath the Martian surface, proposed in this work are two novel electrically small loop antenna designs for 0.8-10MHz wide-band operation: a single-turn loop antenna with a continuous mechanical frequency tuning capability (Chapter 2) and a reconfigurable multi-turn loop antenna with an electrical frequency tuning capability (Chapter 3). The usage of lumped elements, RF MEMS switch, varactor diodes, transformers, and RF balun in the realm of small antenna designs is investigated. Prototypes have been fabricated, tested and measured (Chapter 4).

As wireless devices and systems get both smaller and more ubiquitous, the demand for effective but small antennas is rapidly increasing. Small Antenna Design describes the theory behind effective small antenna design and give design techniques and examples for small antennas for different operating frequencies. Design techniques are given for the entire radio spectrum, from a very hundred kilohertz to the gigahertz range. Unlike other antenna books which are heavily mathematical and theoretical, Douglas Miron keeps mathematics to the absolute minimum required to explain design techniques. Ground planes, essential for operation of many antenna designs, are extensively discussed. Author's extensive experience as a practicing antenna design engineer gives book a strong "hands-on" emphasis Covers antenna design techniques from very low frequency (below 300 kHz) to microwave (above 1 GHz) ranges Special attention is given to antenna design for mobile/portable applications such as cell phones, WiFi, etc

In this thesis, a top-loaded, inductively-coupled, electrically small antenna is designed for high-frequency (HF) ground-wave transmission. The parameters of the antenna are optimized using a genetic algorithm together with the Numerical Electromagnetics Code. Prototypes for two top-loaded antenna designs are tested, and the measured results match well with simulations. Both antenna designs have an electrical size kr of 0.2, an efficiency greater than 65%, and a transmission performance within 1 dB of that of a commercial whip. In the second part of this thesis, the design of a miniaturized ground plane is investigated. It is first shown that when the size of the ground plane is reduced to less than one tenth of a wavelength, the transmission loss increases rapidly due to impedance mismatch. A spiral ground-plane design is then proposed that reduces the ground-plane size without a rapid increase in transmission loss. The spiral ground plane is tested under a quarter-wave monopole, and the measured results match well with simulation. Outdoor transmission test demonstrates that the transmission loss of a 0.43m-diameter spiral ground plane can achieve nearly the same transmission performance as that of a 1.2m-diameter eight-arm radial ground plane

This dissertation shows a detailed investigation on reconfigurable low profile antennas using tunable high impedance surfaces (HIS). The specific class of HIS used in this dissertation is called a frequency selective surface (FSS). This type of periodic structure is fabricated to create artificial magnetic conductors (AMCs) that exhibit properties similar to perfect magnetic conductors (PMCs). The antennas are intended for radiometric sensing applications in the biomedical field. For the particular sensing application of interest in this dissertation, the performance of the antenna sub-system is the most critical aspect of the radiometer design where characteristics such as small size, light weight, conformability, simple integration, adjustment in response to adverse environmental loading, and the ability to block external radio frequency interference to maximize the detection sensitivity are desirable. The antenna designs in this dissertation are based on broadband dipole antennas over a tunable FSS to extend the usable frequency range. The main features of these antennas are the use of an FSS that does not include via connections to ground, their low profile and potentially conformal nature, high front-to-back radiation pattern ratio, and the ability to dynamically adjust the center frequency. The reduction of interlayer wiring on the tunable FSS minimizes the fabrication complexity and facilitates the use of flexible substrates. This dissertation aims to advance the state of the art in low profile tunable planar antennas. It shows a qualitative comparison between antennas backed with different unit cell geometries. It demonstrates the feasibility to use either semiconductor or ferroelectric thin film varactor-based tunable FSS to allow adjustment in the antenna frequency in response to environment loading in the near-field.