James T. Aberle

Reconfigurable antennas for wireless devices

New technologies in communications electronics, such as software-defined radio (SDR) and RF switches implemented using micro-electromechanical systems (MEMS), present new challenges and opportunities for antenna design. In sharp contrast to digital technology where Moores law reigns, a fundamental law of physics constrains the ability to realize electrically small antennas that are both efficient and broadband. As a result, covering several frequency bands concurrently with a single antenna having enough efficiency and bandwidth is a major challenge. One possible solution to this problem is to use reconfigurable antennas that tune to different frequency bands. Such an antenna would not cover all bands simultaneously, but provides narrower instantaneous bandwidths that are dynamically selectable at higher efficiency than conventional antennas. Such tunable-antenna technology is an enabler for software-definable radios, the RF front ends of which must be reprogrammable on the fly. This paper discusses the practical implementation issues, limitations, and measured results of small, narrowband, tunable antennas within portable handsets. Many of the concepts discussed in this paper will likely become practical and cost effective in the near future because of recent advances in RF MEMS switches.

Rigorous analysis of probe-fed printed annular ring antennas

This paper presents calculated and measured results for the input impedance and radiation performance of probe-fed printed annular ring antennas. Geometries featuring stacked rings as well as shorting posts are considered. A numerical model is presented that is based on a full-wave spectral-domain moment-method solution. In this solution, a specialized attachment mode-expansion function is used to model the connection between the probe feed and the printed ring. Measured results are presented and compare well with the theory. Annular rings are found to have certain advantages over circular and rectangular microstrip antennas.

Infinite phased arrays of cavity-backed patches

An analysis of the radiation properties of infinite phased arrays of probe-fed circular microstrip patches backed by circular cavities using a rigorous Greens function/Galerkins method is presented. The effect of substrate thickness on both scan volume and bandwidth performance is considered. Results are compared to those of infinite arrays of conventional probe-fed circular patch antennas. >

Two-Port Representation of an Antenna With Application to Non-Foster Matching Networks

A new theoretical tool is introduced for treating an antenna as a two-port network. This tool is very useful for allowing the antenna and its matching network to be analyzed and optimized in a circuit simulator. In this paper, we apply the idea to the situation where the matching network of the antenna incorporates non-Foster (active) matching elements, and so overall circuit stability becomes an overriding consideration. Non-Foster matching networks are particularly interesting because they offer the potential to transform the antenna into an active component that is no longer limited by the gain-bandwidth-size constraints of passive antennas, and whose performance can be improved as semiconductor technology advances.

Automatic antenna tuning unit for software-defined and cognitive radio

In this paper, we propose a new architecture for a closed-loop controlled antenna tuning unit (ATU) system (only transmitting path is shown). In contrast to previous work, no analog-to-digital converters (ADCs) are used. Using the ATU, the narrow instantaneous bandwidth of an electrically small antenna (ESA) is automatically tuned over a much wider frequency range by the ATU. This matching scheme ensures that the narrowband antenna is automatically matched to any desired frequency under all environmental conditions using circuits with practical component values and tolerances. It is imperative that the entire tuning process be completed rapidly using efficient algorithms and fast hardware, and for these algorithms and hardware to consume as little power as possible.

Rigorous and versatile solutions for probe-fed microstrip patch antennas and arrays

A rigorous spectral domain Greens function/Galerkin approach has been proposed by the authors (1988). It accurately models the effect of connecting a vertical probe feed to the patch. This approach is quite versatile and has been applied to a number of probe-fed patch geometries. A general description of this technique is given, and its application to a variety of microstrip patch geometries is discussed. Data generated by the numerical models are compared to measured data for both radiation and scattering cases. In particular, the measured input reflection coefficient of a probe-fed rectangular patch is compared to calculations made using both the rigorous solution described and the idealized probe feed model.<<ETX>>

Single and stacked circular microstrip patch antennas backed by a circular cavity

In this communication, a full wave moment method solution for a probe-fed circular microstrip patch antenna, backed by a circular cavity both in single and in stacked configurations, is used to analyze the effect of a parasitic patch on antenna bandwidth. In addition, the effect of recessing a single patch into the antenna cavity is studied. In both cases, theoretical results for the scattering and radiation characteristics of the antenna are presented. >

Analysis of infinite arrays of one- and two-probe-fed circular patches

An analysis is presented of the radiation properties of infinite phased arrays of one- or two-probe-fed circular microstrip patches, using a rigorous Greens-function/moment-method approach. Two ways of treating the connection of a vertical feed probe to the patch are considered: an idealized probe feed model, which is useful for antennas printed on thin substrates or to predict general performance trends, and a more rigorous treatment that overcomes the deficiencies of the simpler model. This rigorous treatment seems to have some advantages over other published analyses, including applicability to a wide range of printed radiator problems, but requires a huge increase in the amount of computer time required for the calculation. Comparisons of both feed models with measured data from waveguide simulators are used to validate the theory for both one- and two-probe-fed elements. >

Evaluation of input impedance and radar cross section of probe-fed microstrip patch elements using an accurate feed model

Input impedance and radar cross section results for probe-fed single rectangular and circular microstrip patch elements obtained using an accurate and efficient numerical model are presented. The model uses a full-wave Greens function/Galerkin solution in which the connection of a vertical probe feed to the patch is rigorously included in the analysis by use of a special basis function called the attachment mode, which is derived from the corresponding cavity model solution. Comparisons with measured input impedance and monostatic radar cross section data demonstrate the efficacy of the theory. This model accurately predicts the performance of probe-fed patches printed on thick and/or high dielectric constant substrates and patch elements with more than one probe feed, cases where other less rigorous models fail. >

Improving the performance of microstrip-patch antennas

This paper describes a cavity-backed patch-antenna geometry, which features multiple dielectric layers and shorting posts. These features are exploited to design antennas which retain many of the desirable characteristics of conventional microstrip antennas, yet overcome some of their inherent disadvantages.