Dimitris E. Anagnostou

A Printed Log-Periodic Koch-Dipole Array (LPKDA)

Koch-shaped dipoles are introduced for the first time in a wideband antenna design and evolve the traditional Euclidean log-periodic dipole array into the log-periodic Koch-dipole array (LPKDA). Antenna size can be reduced while maintaining its overall performance characteristics. Observations and characteristics of both antennas are discussed. Advantages and disadvantages of the proposed LPKDA are validated through a fabricated proof-of-concept prototype that exhibited approximately 12% size reduction with minimal degradation in the impedance and pattern bandwidths. This is the first application of Koch prefractal elements in a miniaturized wideband antenna design.

A Direct-Write Printed Antenna on Paper-Based Organic Substrate for Flexible Displays and WLAN Applications

This paper presents the design, fabrication and measurements of a direct-write printed low-cost and flexible inverted-F antenna on an ultra-low-cost paper-based organic substrate for wireless local area network (WLAN) and flexible display applications. Innovations include the study and utilization of paper as a high-frequency substrate for the first time in the gigahertz (GHz) range, the fabrication technology for the direct-write printing of the antenna as a flexible RF electronic device, and the investigation of antenna flexibility in conjunction with flexible displays. Although paper substrates exhibit relatively high dielectric losses (tanδ ~ 0.065 at 2.45 GHz), the maximum realized gain of the fabricated antenna is measured to be + 1.2 dBi giving a total efficiency ~ 82%. Simulated results of the antennas return loss and radiation patterns agree well with the measurements, and can lead to a whole new class of flexible low-cost electronic devices of the future.

Applications of neural networks in wireless communications

In recent years, the art of using neural networks (NNs) for wireless-communication engineering has been gaining momentum. Although it has been used for a variety of purposes and in different ways, the basic purpose of applying neural networks is to change from the lengthy analysis and design cycles required to develop high-performance systems to very short product-development times. This article overviews the current state of research in this area. Different applications of neural-network techniques for wireless communication front ends are briefly reviewed, stressing the purpose and the way neural networks have been implemented, followed by a description of future avenues of research in this field.

Dual Band-Reject UWB Antenna With Sharp Rejection of Narrow and Closely-Spaced Bands

An ultrawideband (UWB) antenna that rejects extremely sharply the two narrow and closely-spaced U.S. WLAN 802.11a bands is presented. The antenna is designed on a single surface (it is uniplanar) and uses only linear sections for easy scaling and fine-tuning. Distributed-element and lumped-element equivalent circuit models of this dual band-reject UWB antenna are presented and used to support the explanation of the physical principles of operation of the dual band-rejection mechanism thoroughly. The circuits are evaluated by comparing with the response of the presented UWB antenna that has very high selectivity and achieves dual-frequency rejection of the WLAN 5.25 GHz and 5.775 GHz bands, while it receives signal from the intermediate band between 5.35-5.725 GHz. The rejection is achieved using double open-circuited stubs, which is uncommon and are chosen based on their narrowband performance. The antenna was fabricated on a single side of a thin, flexible, LCP substrate. The measured achieved rejection is the best reported for a dual-band reject antenna with so closely-spaced rejected bands. The measured group delay of the antenna validates its suitability for UWB links. Such antennas improve both UWB and WLAN communication links at practically zero cost.

Bandwidth Enhancement of the Resonant Cavity Antenna by Using Two Dielectric Superstrates

We propose a novel approach to enhance the bandwidth of the high directivity of the resonant cavity antenna (RCA) by applying two dielectric layers as superstrates. The approach is based on creating two cavities corresponding to two operating frequency bands that combine to form a single wide band of operation. The RCA design is discussed in a methodological manner to determine the thicknesses of the superstrates, the separation distance between them, and the separation distance to the ground plane. We show that the proposed technique is capable of enhancing the bandwidth from 9% of the single superstrate RCA to 17.9% of the two superstrate RCA, with only 0.1-dB reduction of the maximum directivity (17.5 dBi). The presented design method can be replicated for any RCA with any directivity level and type of primary feeding. The performance of the analytically designed antenna is validated by simulation using commercial numerical electromagnetics software.

A Self-Adapting Flexible (SELFLEX) Antenna Array for Changing Conformal Surface Applications

A phased-array test platform for studying the self-adapting capabilities of conformal antennas is developed and presented. Specifically, a four-port 2.45-GHz receiver with voltage controlled phase shifters and attenuators is designed along with four individual printed microstrip patch antennas attached to a conformal surface. Each antenna is connected to the corresponding receiver port with a flexible SMA cable. It is shown that with appropriate phase compensation, the distorted radiation pattern of the array can be recovered as the surface of the conformal array changes shape. This pattern recovery information is then used to develop a new self-adapting flexible 1 × 4 microstrip antenna array with an embedded flexible sensor system. In particular, a flexible resistive sensor is used to measure the deformation of the substrate of a conformal antenna array, while a sensor circuit is used to measure the changing resistance. The circuit then uses this information to control the individual voltage of the phase shifters of each radiating element in the array. It is shown that with appropriate phase compensation, the radiation properties of the array can be autonomously recovered as the surface of the flexible array changes shape during normal operation. Throughout this work, measurements are shown to agree with analytical solutions and simulations.

FPGA-Controlled Switch-Reconfigured Antenna

In this letter, p-i-n diodes are used as switches to connect and disconnect four patch sections to a midsection of a planar antenna. The antenna system is connected to the field programmable gate array (FPGA) board controlling the activation of these switches. The antenna with the incorporated diodes is designed, installed, and measured. The methodology for using an FPGA to optimally control and produce the desired antenna frequency operation is presented and analyzed. The analogy between the measured and simulated results is found to be satisfactory. The proposed control methodology can be used with various antenna designs to obtain different possible states in an easy, fast, and low-cost manner.

Planar Monopole Antenna With Attached Sleeves

The analysis of a new printed antenna is presented and discussed. This antenna consists of a printed monopole, with one or two sleeves on each side, fed by a coplanar waveguide (CPW) line. Switches are used to control the length of the monopole and the sleeves and to tune the resonant frequencies of the antenna. In the case of the double-sleeved antenna, the switch is used to connect or disconnect a second sleeve in the cactus antenna. Measurement results show that the cactus antenna maintains the dipole-like radiation patterns for all the different resonant frequencies

Broadband and Dual-Band Coplanar Folded-Slot Antennas (CFSAs) [Antenna Designer's Notebook]

This paper presents a unified design methodology for dual-band and broadband coplanar folded-slot antennas (CFSAs). The design is achieved by using a coplanar waveguide-fed antenna element that is asymmetric with respect to the folded slot, and by adjusting the length of the stub inside the slot. A mathematical derivation, based on a transmission-line model for the asymmetrically-fed coplanar folded-slot antenna, is used on a single-frequency coplanar folded-slot antenna to determine the condition for a second resonance. In this way, when the second resonant frequency is close to the first, broadband uniplanar antennas with ~30%, bandwidth can be designed. In addition, at the dual-band mode, a frequency ratio (f2/f1) of the order of 2.5 or more can be obtained. The effect of the ratio of the feed-shift distance to the length of the stub on the bandwidth is an important parameter, and is shown for both the broadband and dual-band designs. The presented method and tables are simple to use, provide very accurate results, and correctly predict the resonant frequencies for the dual-band coplanar folded-slot antennas. The theoretical and simulated results were verified by measurements of fabricated prototypes. The design guidelines cover a broad range of applications in the 2.4 GHz to 5.25 GHz range, with various bands and bandwidths.

Reconfigurable UWB Antenna With RF-MEMS for On-Demand WLAN Rejection

A MEMS reconfigurable ultra-wideband (UWB) antenna that rejects on-demand all WLAN signals in the entire 5.15 to 5.825 GHz range (675 MHz bandwidth) is presented. The antenna design, miniaturization procedure, and monolithic integration with the MEMS and biasing network on SiO2 Quartz substrate are described. The integration challenges are addressed and the work is presented in a way that is useful for antenna engineers. A method to vary the rejection bandwidth is also provided. The fabricated prototype is conformal and single-sided. The antenna is measured using a custom-built platform at a university laboratory. Results indicate a successful integration and minimal interference of the MEMS and biasing circuitry with the antenna, paving the road for more integrated reconfigurable antennas on SiO2 using MEMS technology. Such antennas can improve UWB, WLAN and cognitive radio communication links.