天线全书9

Waveguide Slot Antenna Arrays 9-2
CHAPTER NINE
9.1 INTRODUCTION
Geometric simplicity, efficiency, polarization purity, conformal installation, and ability to radiate broadside beams and vertically polarized E-plane beams at very near grazing angle above a ground plane make slot-antenna arrays ideal solutions for many radar, communications, and navigation applications. Especially today with the desire to make antennas for aircraft as low profile as possible to reduce drag and conserve fuel, slot antenna arrays can be positioned above wings and on top of the fuselage while having the capability to look toward the horizon. Classical slot arrays are depicted as narrow nonconductive slits etched or milled into the host metallic ground plane. They are characterized by the methods used to feed or excite the slots. Without what is behind the slot opening being seen, slot radiators appear similar on the surface of the ground plane. Narrow conformal slots tend to be narrowband (< 5 percent f0) and have high cross-polarization isolation when operating near their resonant frequency. Wider slots can exceed an octave bandwidth given a well-matched feed. However, polarization purity is usually not as good as with the narrower slots. Conformal slot arrays are generally limited in bandwidth because the array lattice spacing has to be large enough to accommodate the waveguide and feed structures behind the slots without creating grating lobes. Conformal slot elements can be fed in a variety of ways: (1) tapping into a transmission line such as a waveguide, (2) coupling to a resonant cavity, and (3) feeding them directly with voltage sources across the slots. Each method has some impact on the radiation performance and operating bandwidth of the slot radiators and hence the array. This chapter focuses on the first approach, which is the transmission-line method whereby slots are cut along a waveguide to couple energy in a controlled manner to slots that radiate. Waveguidefed array systems are either traveling-wave or standing-wave approaches. Because waveguides are dispersive transmission lines, the array excitation along the waveguide has a differential phase relationship between elements that changes with frequency, causing the array beam to scan. For fixed beam (nonscanning) arrays, the waveguide is converted into a resonant, standing-wave structure. Before the advent of broadband MMIC T/R module technology, waveguide-fed slot arrays were most common in microwave radar applications. Today corporate feed networks offer a greater flexibility to excite slot arrays over a broader bandwidth. They are most effective when T/R modules, comprised of amplifiers, attenuators, and phase shifters or time delay devices, are connected in series with each array element. The slot elements can be phased independently to scan a beam anywhere in a hemisphere above their host ground plane. However, such large phased arrays that have independent control of every radiator are still prohibitively expensive. Therefore array architectures are employed that utilize hybrid scan approaches where electronic scanning is used in one plane, and either mechanical or frequency scanning is used in the other plane. Thus waveguide slot arrays can still provide a very cost-effective solution to fulfilling many high performance array needs.
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Source: ANTENNA ENGINEERING HANDBOOK
Waveguide Slot Antenna Arrays
Roland A. Gilbert
BAE Systems, Inc.
Chapter 9
CONTENTS
9.1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 WAVEGUIDE SLOT RADIATORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 SLOTTED WAVEGUIDE ARRAY DESIGNS . . . . . . . . . . . . . . . . . . . . . 9-2 9-2 9-5
9.2 WAVEGUIDE SLOT RADIATORS
The radiating elements of a waveguide slot array are an integral part of the feed system, which is the waveguide itself. This simplifies the design since baluns or matching networks are not required. A familiarization with the modal fields within a waveguide is necessary to understand where to place slots so that they are properly excited. Narrow slots that are parallel to waveguide wall currents do not radiate. However, when a slot is cut into a waveguide wall and it interrupts the flow of current, forcing it to go around the slot, power is coupled from the waveguide modal field through the opening to free space. To have good control of the excitation of a linear slot array, it is recommended that the waveguide