此外，在X波段以上的频率，电磁波的大气衰减变得更加显著。
In addition, at frequencies above X band,atmospheric attenuation of electromagnetic waves becomes significant.
因此，X波段以上频率的雷达通常工作在“大气窗口”频率，在该窗口中的大气衰减相对较低。
Consequently, radar in these bands usuallyoperates at an “atmospheric window” frequency where attenuation isrelatively low.
图1.3给出了一组大气条件下，在最常见雷达频率范围内单向传播的大气衰减曲线。
Figure 1.3 illustrates the atmosphericattenuation for one-way propagation over the most common radar frequency rangesunder one set of atmospheric conditions.
Figure 1.3. 电磁波单向传播的大气衰减One-way atmospheric attenuation of electromagnetic waves. (Source:EW and Radar Systems Engineering Handbook, Naval Air Warfare Center,Weapons Division, http://ewhdbks.mugu.navy.mil/)
大多数Ka波段雷达工作在35GHz附近，而大多数W波段的系统工作在95GHz附近，因为在这些波长处的大气衰减相对较小。
Most Ka band radars operate near 35 GHz andmost W band systems operate near 95 GHz because of the relatively lowatmospheric attenuation at these wavelengths.
由于较低的大气衰减和较高的可发射功率，在长距离监视应用中，倾向于选择较低的雷达频率。
Lower radar frequencies tend to bepreferred for longer range surveillance applications because of the lowatmospheric attenuation and high available powers.
在给定天线尺寸的条件下，工作频率越高，可实现的天线波束宽度越小、衰减较大、可用功率越低，所以对于更高分辨率、更短距离的实际应用而言，相对较高的电磁波频率往往是优选的。
Higher frequencies tend to be preferred forfiner resolution, shorter range applications due to the smaller achievableantenna beamwidths for a given antenna size, higher attenuation, and loweravailable powers.
天气条件对雷达信号的传播也具有非常重要的影响。
Weather conditions can also have asignificant effect on radar signal propagation.
图1.4显示了从毛毛细雨到热带暴雨的雨量范围中附加的单向损耗是根据射频频率变化的函数。
Figure 1.4 illustrates the additionalone-way loss as a function of RF frequency for rain rates ranging from adrizzle to a tropical downpour.
图1.4 不同降水速率对电磁波单向大气衰减的影响Effect ofdifferent rates of precipitation on one-way atmospheric attenuation ofelectromagnetic waves. (Source: EW and Radar Systems Engineering Handbook, Naval Air Warfare Center,Weapons Division, http://ewhdbks.mugu.navy.mil/)
X波段频率（通常为10GHz）及以下的频率只会受到非常严重的暴雨的显著影响，而毫米波频率甚至在轻微雨量下也会产生严重的衰减。
X-band frequencies (typically 10 GHz) andbelow are affected significantly only by very severe rainfall, while millimeterwave frequencies suffer severe losses for even light-to medium rain rates.
雷达发射机的峰值功率从毫瓦量级到超过10兆瓦。
Radar transmitters operate at peak powersranging from milliwatts to in excess of 10 MW.
AN/FPS-108 COBRA DANE是目前已有的大功率雷达发射机之一，其峰值功率达到15.4MW。
One of the more powerful existing transmittersis found in the AN/FPS-108 COBRA DANE radar, which has a peak power of 15.4 MW(Brookner, 1988).
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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