為什么天線都有旁瓣？（sidelobe）

是什么原因導致的旁瓣 sidelobe？ 
 
 

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天線單元間距太遠

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深層次的原因我想是由于干涉引起的

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場到達某處的相位不同 所以引起的疊加不同 從而產生了主瓣和旁瓣

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樓上所說即是干涉的最基本原理  其實微波和光的干涉原理是一樣的 而光的干涉可以看到明暗相間的條紋，天線的波瓣反映則是電磁波的干涉效果（不知道理解是否正確還請各位指教）

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又查了一些資料， 
是這樣解釋的： 
天線上的電流分布的傅立葉變換就是天線的pattern 
如果電流是高斯分布的話那么天線的pattern 就是高斯的，沒有旁瓣， 
如果電流是其他的分布，那么大部分的傅立葉變換都會有旁瓣。 
 
我學習了傅立葉變換的性質，還是不理解這一點，難道有這樣的性質：有限面積（截斷）的電流分布對應 傅立葉變換的旁瓣？ 
等待高人指導， 
謝謝拉！

網友回復:

我查的資料是wiki百科全書上的解釋，大家可以參考一下。 
 
還有一種不會出現旁瓣的可能就是，天線尺寸特別小，比如特別小的dipole（極子天線）或是其他的小的天線單元等等出來的場就沒有旁瓣的 
 
（為什么小的不出現旁瓣呢？還是不明白呀？等待大家的討論） 
 

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方便大家討論，把原文copy過來了，和干涉有關系。 
 
The nulls between sidelobes occur when the radiation patterns passes through the origin in the complex plane. Hence, adjacent sidelobes are generally 180° out of phase to each other. 
 
Because an antenna's far field radiation pattern is a Fourier Transform of its aperture distribution, most antennas will generally have sidelobes, unless the aperture distribution is a Gaussian, or if the antenna is so small, as to have no sidelobes in the visible space. Larger antennas have narrower main beams, as well as narrower sidelobes. Hence, larger antennas have more sidelobes in the visible space (as the antenna size is increased, sidelobes move from the evanescent space to the visible space). 
A typical radiation pattern of phased arrays whose inter-element spacing is greater than half a wavelength, hence the radiation pattern has grating lobes. 
 
For discrete aperture antennas (such as phased arrays) in which the element spacing is much greater than a half wavelength, the aliasing effect causes some sidelobes to become substantially larger in amplitude, and approaching the level of the main lobe; these are called grating lobes, and they are identical, or nearly identical in the example shown, copies of the main beams. Grating lobes are a special case of a sidelobe. In such a case, the sidelobes should be considered all the lobes lying between the main lobe and the first grating lobe, or between grating lobes. It is conceptually useful to distinguish between sidelobes and grating lobes because grating lobes have larger amplitudes than most, if not all, of the other side lobes. 
 
For antennas used as receivers, sidelobes make the antenna more vulnerable to noise from nuisance signals coming far away from the transmit source. For transmit antennas communicating classified information, sidelobes represent security vulnerability, as an unintended receiver may pick up the classified communication.

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有人理解這段英文的第二段話嗎？

網友回復:

電流分布的數學表達，再經傅里葉變換后的結果，并不是所有天線都有副瓣，全看天線或陣面的電流分布

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