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Karl F. Warnick

Karl Warnick

Dr. Karl F. Warnick
Professor, Department of Electrical and Computer Engineering 
Brigham Young University 
Provo, UT 84602 
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New IEEE Standard Terms and Figures of Merit for Active Antenna Arrays

Active multi-antenna systems and antenna arrays are of great interest currently for applications such as high-sensitivity astronomical aperture phased arrays and phased array feeds, multiple input multiple output (MIMO) communications systems, digitally beamformed arrays, steered beam antennas for passive remote sensing, and arrays for mobile, airborne, and maritime satellite communications. The standard definitions for gain, radiation efficiency, antenna efficiency, and noise temperature are directly applicable only to receiving antennas that can be operated as transmitters. For active receiving arrays with complex receiver chains, nonreciprocal components in the beamforming network, or digitally sampled and processed output signals, existing transmit-based antenna terms such as gain and radiation efficiency cannot be directly applied. Using the reciprocity principle to obtain an equivalence between the total power radiated by a transmitting antenna and the noise power at the output of a receiving antenna, a new set of figures of merit has been developed for active array receivers.

These figures of merit have been formulated into a set of new antenna terms, including isotropic noise response, active antenna available gain, active antenna available power, receiving efficiency, and noise matching efficiency, and additions to the existing definitions for noise temperature of an antenna and effective area. The terms were reviewed by the IEEE Antenna Definitions Working Group and the IEEE Standards Association and are included in the recently published IEEE Std 145-2013, Standard for  Definitions of Terms for Antennas. The last version of the standard was published 20 years ago, so this represents a major milestone for the worldwide antenna community. The presentation will explain the theoretical basis for the new antenna terms, show their equivalence to existing definitions in the passive case, and give example applications for which the figures of merit have impacted the development of new types of array antenna technologies.

Network Theory, Antenna Arrays, Noise, Mutual Coupling, and Array Signal Processing

Network theory provides a theoretical bridge between array antenna models and the techniques of array signal processing. The antenna community often takes a simplistic approach to array beamforming and processing algorithms that lags far behind the state-of-the-art in the signal processing community. Similarly, the signal processing community usually employ simplistic physical models and assumptions that do not accurately represent key electrical effects that occur in realistic multiport antenna systems. To bridge the gap between the antenna and signal processing communities, we present a network theory treatment of phased arrays and multiantenna systems that brings concepts such as mutual coupling, impedance matching, electronics noise, thermal noise, and antenna losses into a unified theoretical framework. In particular, the network point of view demystifies antenna noise and mutual coupling effects and provides a simple way to understand and work with the interactions between nearby elements in an antenna array. This theoretical framework provides a powerful set of modeling tools that can be used to design, optimize, and characterize antenna systems for multiple input multiple output (MIMO) communications systems, digitally beamformed arrays, and steered beam antennas for remote sensing and satellite communications.

Ultra-high Efficiency Planar Phased Arrays for Satellite Communications

Aperture phased arrays and phased array feeds (PAFs) are a promising technology for sensing and communications applications requiring electronic beamsteering and large signal collecting area, but current technologies are too costly and inefficient for widespread use in satellite communications. To meet strict efficiency and sensitivity requirements, existing satellite communications terminals typically use reflector antennas with horn feeds. Because the microwave sky is quite cool, small improvements in antenna efficiency lead to large gains in the key figure of merit for a satellite receiver, signal to noise ratio. Horn antennas inherently have a high radiation efficiency, and off-the-shelf low noise block downcoverter feeds (LNBFs) cost only a few dollars to manufacture, yet have been so carefully optimized that further improving signal quality would require cryogenic cooling. These considerations have motivated significant recent interest in research aimed at achieving low cost, high efficiency phased array feed receiver systems. To meet this combination of high performance requirements and low cost, we have used computational design optimization to develop efficient, low noise planar array feed antennas that can be fabricated using standard microwave PCB techniques. This presentation gives an overview of work on passive, fixed beam array feeds with linear and circular polarization, including the first demonstration of planar phased arrays with performance comparable to traditional horn antennas, and active beam steering feeds that adaptively track a signal source as the antenna moves. This research opens up new possibilities for phased arrays in terms of low cost, high efficiency, and performance for satellite communications applications.

Research Frontiers in Phased Array Antennas for Radio Astronomy

For nearly 75 years, the challenge of detecting extremely weak signals from deep space has been a driving force in antenna theory, receiver technology, and signal processing. The astronomical community is currently working to develop dense aperture phased arrays and phased array feeds, which offer a significantly larger field of view than conventional single-pixel telescopes and will enable new astronomical observations such as rapid sky surveys, radio transient searches, and tests of fundamental physics. Because sensitivity and stability requirements for radio telescopes far exceed those of other applications such as wireless communications, efforts to develop astronomical phased arrays have opened up new and exciting challenges for antenna design, microwave systems, and multichannel signal processing. Work at BYU and elsewhere over the last few years has uncovered many fundamental research questions. How should antenna gain and other figures of merit be defined for an active phased array? What impedance should array elements be designed for to maximize SNR? How does mutual coupling affect antenna performance? What is the best achievable efficiency with a phased array? Can phased arrays be as sensitive as a state-of-the-art horn antenna with liquid helium-cooled electronics? How can computational electromagnetics tools be combined with microwave network system models to optimize an entire system including a phased array antenna elements, receiver electronics, and signal processing? This presentation will highlight recent progress in these areas, including phased array antenna figures of merit, high efficiency antenna element design, active impedance matching, noise minimization for wideband arrays, phased array receiver characterization, measurement techniques, design optimization methods, array calibration, beamforming algorithms, polarimetric phased array antennas. Experimental results and hardware development supported by these theoretical advances will also be highlighted, including a digitally beamformed cryogenic phased array feed for the world’s largest fully steerable antenna, the Green Bank Telescope.


Karl F. Warnick (SM’04, F’13) received the B.S. degree (magna cum laude) with University Honors and the Ph.D. degree from Brigham Young University (BYU), Provo, UT, in 1994 and 1997, respectively. From 1998 to 2000, he was a Postdoctoral Research Associate and Visiting Assistant Professor in the Center for Computational Electromagnetics at the University of Illinois at Urbana-Champaign.  Since 2000, he has been a faculty member in the Department of Electrical and Computer Engineering at BYU, where he is currently a Professor.  In 2005 and 2007, he was a Visiting Professor at the Technische Universität München, Germany.  Dr. Warnick has published many scientific articles and conference papers on electromagnetic theory, numerical methods, remote sensing, antenna applications, phased arrays, biomedical devices, and inverse scattering, and is the author of the books Problem Solving in Electromagnetics, Microwave Circuits, and Antenna Design for Communications Engineering (Artech House, 2006) with Peter Russer, Numerical Analysis for Electromagnetic Integral Equations (Artech House, 2008), and Numerical Methods for Engineering: An Introduction Using MATLAB and Computational Electromagnetics Examples (Scitech, 2010).

Dr. Warnick is a Fellow of the IEEE and is a recipient of a National Science Foundation Graduate Research Fellowship, Outstanding Faculty Member award for Electrical and Computer Engineering, the BYU Young Scholar Award, the Ira A. Fulton College of Engineering and Technology Excellence in Scholarship Award, and the BYU Karl G. Maeser Research and Creative Arts Award. He has served the Antennas and Propagation Society as a member and co-chair of the Education Committee and as Senior Associate Editor of the IEEE Transactions on Antennas and Propagation and Antennas. Dr. Warnick has been a member of the Technical Program Committee for the International Symposium on Antennas and Propagation for several years and served as Technical Program Co-Chair for the Symposium in 2007.