Timed Arrays

Wideband and Time Varying Antenna Arrays

Randy L. Haupt

• Publisher: Wiley-IEEE Press
• Year: 2015
• Print ISBN: 9781118860144
• eBook ISBN: 9781119071914
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Ingénierie de l'électricité et des télécommunications

Introduces timed arrays and design approaches to meet the new high performance standards

The author concentrates on any aspect of an antenna array that must be viewed from a time perspective. The first chapters briefly introduce antenna arrays and explain the difference between phased and timed arrays. Since timed arrays are designed for realistic time-varying signals and scenarios, the book also reviews wideband signals, baseband and passband RF signals, polarization and signal bandwidth. Other topics covered include time domain, mutual coupling, wideband elements, and dispersion. The author also presents a number of analog and digital beamforming networks for creating and manipulating beams. The book concludes with an overview of the methods to integrate time delay into the array design and of several other adaptive arrays that prove useful in many different systems.

• Examines RF signal concepts such as polarization and signal bandwidth and their applications to timed antenna arrays
• Covers arrays of point source, elements in timed antenna arrays, active electronically scanned array technology, and time delay in corporate fed arrays
• Includes complete design examples for placing time delay in arrays

Timed Arrays: Wideband and Time Varying Antenna Arrays is written for practicing engineers and scientists in wireless communication, radar, and remote sensing as well as graduate students and professors interested in advanced antenna topics.

1
TIMED AND PHASED ARRAY ANTENNAS

This chapter briefly introduces antenna arrays and the difference between phased and timed arrays. Not long after the invention of antenna arrays, researchers experimented with moving the main beam by modifying the phase of the signals fed at the elements [1]. Manual beamsteering eventually led to the invention of the phased array where the main beam was electronically steered to a desired direction by applying a pre-calculated phase to all the elements [2]. Phase is a narrow band concept, though. Today’s applications of antenna arrays require high data rates and wide bandwidths. The term “timed arrays” applies to several classes of antenna arrays that are becoming more important with the development of new technologies that must be designed, analyzed, and tested in the time domain, rather than the steady-state, time harmonic forms used with phased arrays. One author defined timed arrays as [3]“…timed-domain equivalent of phased arrays, where each radiating element is excited by pulsed instead of narrowband signals.” This book adds adaptive arrays, reconfigurable arrays, and other time-dependent arrays to that definition of timed arrays.

1.1 LARGE ANTENNAS

Large antennas collect more electromagnetic radiation than small antennas. They have the potential to detect very faint signals—the reason they are popular for radio astronomy and radar. Consequently, an antenna has gain that magnifies the received or transmitted signal in the direction it is pointing. An approximate relationship between the physical size or aperture area (A_p) and gain is given by

(1.1)

where λ is the wavelength. Note that this formula assumes the antenna only transmits over a hemisphere. “Large antenna” may indicate its physical size but more often refers to its electrical size: area /λ². For a given aperture size, higher frequency means higher gain.

Antenna arrays are frequently used in communications and radar systems. The power received by a communications antenna is given by the Friis transmission formula:

(1.2)

And the power received by a monostatic radar antenna is given by the radar range equation:

(1.3)

where P_t = power transmitted (W), σ = radar cross section (m²), R = distance (m²), λ = wavelength (m), G_t = gain of transmitting antenna, and G_r = gain of receiving antenna.

Both (1.2) and (1.3) point out that a bigger antenna increases the received power through a higher gain.

Large antennas come in two forms: reflectors and arrays. Figure 1.1 has examples of a large reflector (Green Bank Telescope, GBT) used for radio astronomy and a large phased array (PAVE PAWS) used for radar. The Green Bank reflector is a 100 m diameter offset-fed reflector that operates from 0.1 to 116 GHz [4]. The GBT mechanically steers to access 85% of the celestial sphere. The PAVE PAWS radar has two apertures with each covering 240° in azimuth and from 3–85° in elevation [5]. It operates at 420–450 MHz. Each face is 22.1 m in diameter and has 1792 active T/R modules and 885 passive elements resulting in a gain of 37.9 dB with a 2.2° beamwidth. The nominal peak power per face is 600 kW, with a power of 150 kW.

FIGURE 1.1 (a) Green Bank Telescope reflector [4]. (b) PAVE PAWS radar [5].

(Courtesy of BMDO)

(Courtesy of NRAO)

Reflectors and arrays compete to fulfill the role of a large antenna in systems that detect weak signals The reflector is cheap compared to the array, and that is why it is the antenna of choice for commercial activities, such as satellite television. Moving a large reflector in order to locate or track a signal requires gimbals, servomotors, and other mechanical parts that result in a reliability and maintenance problem and leads to significant lifecycle costs [6]. Mechanical steering is too slow for some systems, such as radars on airplanes that need high-speed electronic steering offered by arrays.

An array makes many performance promises for a price. Some potential advantages of an array over a reflector antenna include the following [7]:

1. Fast, wide-angle scanning without moving the antenna
2. Adaptive beamforming
3. Graceful degradation in performance over time
4. Distributed aperture
5. Multiple beams
6. Potential for low radar cross section

Reflectors have the following advantages:

1. High G/T
2. Wide bandwidth
3. Relatively low cost

The missio...

Table of contents

• COVER
• TITLE PAGE
• TABLE OF CONTENTS
• LIST OF FIGURES
• PREFACE
• 1 TIMED AND PHASED ARRAY ANTENNAS
• 2 RF SIGNALS
• 3 ARRAYS OF POINT SOURCES
• 4 ELEMENTS IN TIMED ARRAYS
• 5 ARRAY BEAMFORMING
• 6 ACTIVE ELECTRONICALLY SCANNED ARRAY TECHNOLOGY
• 7 TIME DELAY IN A CORPORATE-FED ARRAY
• 8 ADAPTIVE ARRAYS
• LIST OF SYMBOLS AND ABBREVIATIONS
• INDEX
• END USER LICENSE AGREEMENT