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【简答题】
In order to increase the power radiated,the length of the dipole has to be comparable to wavelength.The most widely used dipole is approximately half a wavelength long.The pattern of a half-wave dipole is similar to that of a very short dipole-it is a figure of eight. The half-power beam width in the figure of eight is about 78°,which is narrower than that of the very short dipole.If the length of the dipole is increased to one wavelength in the figure of eight.the half-power beam width is nOW about 47°.
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【简答题】冷冻油的作用有____、____、____及降低压缩机噪声的作用。
【单选题】下面哪一项不是两独立样本 t 检验的前提条件:( )。
A.
样本来自的总体应服从或近似服从正态分布
B.
两样本相互独立
C.
从一个总体抽取一个样本对从另一总体抽取样本没有任何影响
D.
两个样本的方差必须相等
【简答题】冷冻油的作用有（）；（）；（）及降低压缩机噪声。