在第2章中，天线方向图的这种傅立叶变换特性允许使用线性系统的概念来理解天线对横向距离分辨率和避免空间混叠所需的脉冲重复频率的影响。
This Fourier transform property of antennapatterns will, in Chap. 2, allow the use of linear system concepts tounderstand the effects of the antenna on cross-range resolution and the pulserepetition frequencies needed to avoid spatial aliasing.
式（1.5）的一个重要特殊情况发生在孔径电流辐射为常数，即A(y) = A0时。
An important special case of Eq. (1.5)occurs when the aperture current illumination is a constant, A(y) = A0.
此时，归一化的远场电压方向图是熟悉的sinc函数，
The normalized far-field voltage pattern isthen the familiar sinc function,
如果孔径电流辐射是可分离的，那么远场方向图可以分离为两个傅立叶变换表达式的乘积，一个表达式在方位(θ)维，一个表达式在俯仰(ϕ)维。
If the aperture current illumination isseparable, then the far-field is the product of two Fourier transforms, one inazimuth (θ) and one in elevation (ϕ).
E(θ)的幅度如图1.6所示，其中定义了天线方向图的两个重要参数。
The magnitude of E(θ) is illustrated inFig. 1.6, along with the definitions for two important figures of merit of anantenna pattern.
Figure 1.6. 均匀辐射孔径的单程方向图，图中描述了3-dB波束宽度和峰值旁瓣的定义。One-wayradiation pattern of a uniformly illuminated aperture. The 3-dB beamwidth andpeak sidelobe definitions are illustrated.
天线的角分辨率由其主瓣的宽度决定，并且传统上一般以3dB波束宽度来表示。
The angular resolution of the antenna isdetermined by the width of its mainlobe, and is conventionally expressed interms of the 3-dB beamwidth.
因此，3-dB波束宽度等于0.89倍波长除以天线孔径长度。
Thus, the 3-dB beamwidth is 0.89 divided bythe aperture size in wavelengths.
注意，从上式可以看出，天线孔径越大或波长越短，则波束宽度越窄。
Note that a smaller beamwidth requires alarger aperture or a shorter wavelength.
对于笔形天线波束来说，典型的波束宽度为十分之几度到几度，而且这类波束在方位和俯仰两个维度上都要求尽量设计为窄波束。
Typical beamwidths range from as little asa few tenths of a degree to several degrees for a pencil beam antenna where thebeam is made as narrow as possible in both azimuth and elevation.
为了方便广域搜索，一些天线被精心设计成具有几十度的垂直宽波束；这些设计被称为扇形波束天线。
Some antennas are deliberately designed tohave broad vertical beamwidths of several tens of degrees for convenience inwide area search; these designs are called fan beam antennas.
天线方向图的峰值旁瓣会影响到多个相邻散射体的同时检测。
The peak sidelobe of the pattern affectshow echoes from one scatterer affect the detection of neighboring scatterers.
对于均匀辐射加权的方向图，一般的峰值旁瓣电平低于主瓣峰值13.2dB。
For the uniform illumination pattern, thepeak sidelobe is 13.2 dB below the mainlobe peak.
这在雷达系统中常常被认为太高，不满足系统设计要求。
This is often considered too high in radarsystems.
天线旁瓣可以通过使用非均匀孔径分布(Skolnik，2001)来减小，有时也称为天线tapering或shading。
Antenna sidelobes can be reduced by use ofa nonuniform aperture distribution (Skolnik, 2001), sometimes referred to astapering or shading the antenna.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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