雷达频率机械调控机构的设计
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黄河科技学院毕业设计(论文)开题报告表课题名称雷达频率机械调控机构的设计课题来源教师拟订课题类型工程设计真实课题AX指导教师学生姓名专 业机械设计制造及其自动化学 号1、 调研资料的准备 根据任务书的要求,在做本课题前,查阅了与课题相关的资料有:机械设计、机械制图、机械制造工艺学、机械原理、电子技术、机械制造工艺课程设计指导书、冶金机械设计、液压与气压传动、AUTODESK INVENTOR二维设计软件和CAD绘图相关资料等以及与设计相关的手册。二、设计的目的与要求 毕业设计是大学教学中最后一个实践性教学环节,通过该设计过程,可以检验学生所学的知识,同时培养学生处理工程中实际问题的能力,因此意义特别重大。 要求:查阅文献资料12种以上,外文资料不少于两篇。拟定设计方案。本设计零件在车床、铣床、钻床上制造加工完成,因零件形状所致,制造过程需各种专用夹具。先设计零件形状尺寸,选择零件所需材料,完成各种专用夹具,然后相应的分别在车床、铣床、钻床上粗加工和精加工,制造出符合要求的零件。最终组装成机器,完成整个机械设计三、设计的思路与预期成果 (1).了解频率调控计的结构和工作过程,对频率调控系统进行设计。(2).进行基本的几何计算和受力分析,选择合适的参数 (3).完成文献综述和外文翻译。 (4).设计图样(包括总装图和主要零部件图)。 (5).完成毕业论文,编写设计说明书。四、任务完成的阶段内容及时间安排1周4周 完成开题报告、文献翻译、文献综述及总体方案设计 5周-10周 完成总体设计、完成部分机构的装配图及部分零件图并撰写说明书 10周11周 修改论文、资格审查等 12周 毕业答辩五、完成设计(论文)所具备的条件因素 首先完成本次设计,主观上要有主动性,阅读与题目相关的资料等,同时也必须了解论文的格式,撰写的方法等等。其次客观上要具备一定的条件,如通用计算机,绘图机,设计室,AUTOCAD、SOLIDWORKS、UG、电子图板等绘图软件,有关参考书、工具书、资料等。 指导教师签名: 日期: 课题来源:(1)教师拟订;(2)学生建议;(3)企业和社会征集;(4)科研单位提供课题类型:(1)A工程设计(艺术设计);B技术开发;C软件工程;D理论研究;E调研报告 (2)X真实课题;Y模拟课题;Z虚拟课题要求(1)、(2)均要填,如AY、BX等。黄河科技学院毕业设计(文献综述) 第8页单位代码 02 学 号 080105037 分 类 号 TH 密 级 毕业设计文献综述 院(系)名称 工学院机械系 专业名称机械设计制造及其自动化 学生姓名 指导教师2012年 03 月 18 日雷达频率机械调控机构摘要:雷达频率机械调控装置是用于调节雷达频率的机械装置,是雷达的一个重要组成部分,本文主要介绍雷达频率调控装置的一些形式及其发展历程,和雷达频率调控装置中机械调控部分的原理,结构,参数的介绍和计算 关键词:雷达、频率、调控、机械、传动 前言:雷达频率调控计,顾名词义是用于调节雷达频率的装置,它要求设计的简单可控,便捷灵敏,它以机械传动装置为主体,配合磁控管的电感销的上下移动来实现雷达频率的调节,本设计则主要是设计其机械传功装置,从而实现雷达频率调控计的主要部分。而机械传动装置的应用范围非常广泛,各种机器,包括汽车,轮船,火车等上面都有许多的机械传动装置。此外,机械传动的方式有很多种,比如齿轮传动,带传动,链传动等,各种传动方式都有其自身的优缺点和适用范围,这个要视具体情况而定频率调控器的介绍与比较 变频器是近几年在兴起的一种调速节能新产品,它是电力电子技术和计算机应用技术的完美结合,因其调速精度高、操作方便,并且节约能源(输出频率小于50Hz时),现已被广泛应用在机械、化工、冶金、轻工等领域。根据实际应用的需要,弯频器频率设置的方法有不同类型,现说明几种频率设置的特点。 变频器频率设置的方法可以分两大类,第一类是利用变频器操作面板进行频率设置,第二类是利用变频器控制端子进行频率设置。第一类通过面板上的键盘或电位器进行频率给定(即调节频率)的方式,称为面板给定方式,它利用变频器操作面板进行频率设置,只需操作面板上的上升、下降键,就可以实现频率的设定。该方法不需要外部接线,方法简单,频率设置精度高,属数字量频率设置,适用于单台变频器的频率设置。第二类是利用变频器控制端子进行频率设置,又分两种方法,第一种是利用外接电位器进行频率设置;第二种是利用变频器控制端子的特写功能,用电动电位器进行频率设置。 第一种利用外接电位器进行频率设置,如图1,FR-500系列变频器的10端子提供标准的10V直流电压,2端子是频率设定输入端,5端子是模拟量输入公共端子。通过调整外接电位器R的2端输出电压,改变了变频器2端的输入电压值,也就改变了变频器的频率设定值,达到了频率设置的目的,该方法有以下优点: (1) 接线简单,只需把电位器的三端分接到变频器的电压输入端,电压输出端和公共端就可。 (2) 频率设置简单,操作方便,只需轻轻转动外接电位器的旋钮,就可以进行频率设置。 (3) 安装灵活,可以根据实际需要,将外接电位器安装到任何位置,进行远距离操作。 但是,该方法也有以下缺点: (1) 有温漂现象,由于电阻值受温度的影响,当外界温度发生变化时,电阻值了也就随之变化,频率设定值也就发生变化。 (2) 抗干扰能力低。当周围有强电磁干扰时,变频器和外接电位器的连接电缆线内会产生感应电压,使输入到变频器2端的电压值发生变化,也就使频率设定值发生变化,影响设定频率的稳定。 (3) 电位器安装距离受到一定限制。理论上讲,变频器2端的电压变化范围是0-10V,但如果外接电位器安装距离太远,连接电缆就会产生压降,变频器2端电压也就达不到10V,从而使输出频率达不到最高设定值。 因此,该变频器频率设置方法一般应用在调速精度低、周围干扰小、环境温度变化小的场合,属模拟量调节。 第二种方法是利用变频器控制端子的特定功能,通过设置变频器的内部参数,可以使端子RH、RM成为电动电位器,即当RH与公共端SD之间接通时,变频器输出频率上升当RM与SD之间接通时,变频器输出频率下降达到频率设置的目的,如图2,同第一种方法相比,该方法具有以下优点: (1) 频率设置精度高,外接电位器法属模拟量设置方法,频率变化范围为最大输出频率的0.2%以内,而用电动电位器设置频率,频率变化范围为最大输出频率的0.01%以内。 (2) 抗干扰能力强。由于这它只是开关信号输入,因此不受周围电磁场的干扰。 (3) 无温漂现象。由于取消了外接电位器,因此,不受环境温度变化的影响。 (4) 安装灵活,可以将按钮SB1,SB2安装到任何位置。 (5) 同步性能好,可以同时实现多台变频器的频率升高和降低。 总之,我们应根据实际需要,合理选择频率设置方法,以达到应用效果。 本设计是设计面板频率调控器,并且主要是设计其机械传动装置,下面对频率调控器的机械传动装置作简单的介绍频率计的机械传动装置发动机的转动轴带着工作机的轴一起转动,也就是发动机传递到工作机上。这种转动的传动可以用多种不同的方式来实现。常见的三种机械传动方式是皮带传动、摩擦传动和齿轮传动。 在皮带传动里,发动机和工作机的轴上各装一个皮带轮,轮上紧套着一圈(或并列的几圈)皮带(图1).发动机轴上的皮带轮A叫做主动轮,工作机轴上的皮带轮B叫做从动轮。主动轮转动时,依靠摩擦作用使皮带运动,皮带的运动又带动从动轮转动。在转动时,一般不允许皮带打滑,这时两个皮带轮边缘上的各点线速度相同。因此如果两个皮带轮的直径不同,他们的角速度或转速也就不同,且角速度或转速跟两皮带轮的直径成反比: n2/n1=d1/d2比值n2/n1叫做传动速度比。从上式可知,工作机轴上的皮带轮的直径越小,它的轴的转速就越大。 实际上常用的传动速度比一般不大于5.这是因为传动速度比越大,从动轮的直径就越小,它跟皮带接触的圆弧就越短,带动它的摩擦力也就越小。图1的两皮带轮转动方向相同。图2的两皮带轮转动方向相反。 在摩擦传动中,两个轮互相紧压着(图3)。当主动轮向一个方向转动时,由于两轮之间的摩擦作用,从动轮也发生转动,它的转动方向跟主动轮相反。在皮带传动和摩擦传动中,对从动轮来说摩擦力是动力,必须设法使它增大,因为要用摩擦因数比较大的材料如皮革、橡胶,填充石梯的钢丝等包在轮缘上,还要增大压力。如果所传递的功率是P,那么有P=fv和v=dn。可求出作用在轮缘上的摩擦力: f=P/dn,作用在轮缘上使轮转动的摩擦力矩: M=fd/2一般来说,摩擦传动只能在功率不大(15千瓦以下)的情况下使用,如果所传递的功率较大,两轮就会发生滑动。为了提高所传动的功率,必须保证两轮不发生滑动,因此在两轮的轮缘上作出许多齿,使一个轮的每个齿能够嵌入令一个轮的两齿之间。这样,在转动时就不断地互相啮合,不会发生滑动,这种轮叫做齿轮。齿轮传动是,两齿轮的齿距就必须相等。这样,两轮的转速就跟它们的齿数成反比。齿轮传动装置在生产技术上应用的非常广泛,它可以传递几千瓦的功率。当主动轮和从动轮所在的两轴互相平行时,采用圆柱形齿轮(图4中A和B):当两轴成90时,采用截锥形齿轮(图4中的C和E)。此外,我们还常见到用链条来传动的,这实际上也是齿轮传动的一种变形。 各种机床、汽车、拖拉机等用来调节速度用的机械变速箱,一般都是用齿轮来传动的。 雷达频率调控机构的发展前景目前雷达频率调控装置正向着便捷,准确,自动化方向发展,一般的机械调控装置已不再适应现代化雷达的发展,雷达的频率调控需要更加迅速,准确,且能根据环境的变化自动的进行调节,其中频率捷变雷达的技术已经相当成熟,现在雷达的频率调控正在向着自动化方向发展,其中基于DDS技术的自适应米波雷达自适用频率调控系统在国外已经发展成熟,而我国相对还比较落后,因此,我们应朝着雷达频率的自动调控技术方向发展,运用PLC,单片机等高技术手段来实现雷达频率的自动调控。参考文献1、 李运华 机电控制【M】 北京航空航天大学出版社,20032、 芮延年 机电一体化系统设计【M】 北京机械工业出版社,20043、 王中杰,余章雄,柴天佑 智能控制综述【J】 基础自动化,2006(6)4、 章浩,张西良,周士冲 机电一体化技术发展与应用【J】 农机化研究,2006(7)5、 梁俊彦,李玉翔,机电一体化技术发展与应用【J】 科技资讯,2007(9)6、 邓星鈡 机电传动控制(第三版),华中科技大学出版社,20017、 裴仁清 机电一体化原理,上海大学出版社,19988、 秦曾煌 电工学电子技术(第五版),高等教育出版社,20049、 朱龙根 机械系统设计(第二版),机械工业出版社10、 纪名刚 机械设计(第七版),高等教育出版社,200511、 袁峰 UG机械设计工程范例教程【M】,北京机械工业出版社200612、 王志、张进生、于丰业、王鹏、任秀华 基于模块化的机械产品快速设计【J】应用科技2006.33.213、 林德杰 过程控制仪表及控制系统,机械工业出版社,200414、 丁鹭飞、耿富录、陈建春 雷达原理(第四版),电子工业出版社15、 濮良贵、纪名刚 机械设计(第八版),高等教育出版社II黄河科技学院毕业设计(论文) 第 页单位代码 02 学 号 080105037 分 类 号 TH 密 级 毕业设计说明书雷达频率机械调控机构的设计 院(系)名称 工学院机械系 专业名称机械设计制造及其自动化 学生姓名杰 指导教师 2012年 5 月 10 日 黄河科技学院毕业设计说明书 第 III 页_ 雷达频率调节机构设计摘 要频率调节机构是雷达的一个重要的组成部分。在本设计中主要通过蜗轮蜗杆的传动来调节雷达频率,蜗轮蜗杆机构具有结构简单、单级传动比大、结构紧凑,传动平稳等特点。本设计介绍了雷达频率调频机构的现状,主要介绍了本设计的主要零件部分,即蜗轮蜗杆之间的传动特点和连接轴的应用与计算,说明了本机构的工作原理和相关计算。本设计的特点是原理简单,工作可靠,生产经济,便于安装与维修,符合设计要求与生产要求。本设计对蜗轮蜗杆传动比,强度,润滑等方面都进行了比较完整的阐述,比较详细地对各传动部分做了具体分析,以确保其在实际生产中稳定高效地工作。并对本机构的链接部分即螺纹链接进行了说明和计算,满足了强度,寿命等一系列生产条件。 关键词 频率 调节 蜗轮 蜗杆 传动 The design of radar frequency regulation mechanismAbstractRadar frequency adjustment mechanism is an important part of the rader. In this design mainly through the worm gear transmission to adjust the radar frequency, Worm gear mechanism has the characteristics of simple structure, single stage transmission ratio, compact structure, stable transmission. The design of the radar frequency FM mechanism present situation, introduced the main design of the main parts, namely the worm between the transmission characteristics and is connected to the shaft with applications and computing, explains the working principle of the mechanism and related calculation.。This design is characterized by simple principle, reliable work, convenient installation and production economy, repair, meets the requirements of design and production requirements The design have quite complete elaboration,to transmission ratio, strength, lubrication etc of the worm gear,and more detailed on the drive parts did concrete analysis,in order to ensure its in the actual production of stable and efficient work.And the mechanism whereby the threaded portion of the link link are described and calculated, satisfies the intensity, lifetime and a series of production conditions.Key words frequency regulation worm gear worm drive目 录1 绪论11.1 频率调节机构11.2 蜗轮蜗杆简介21.3 本课题研究的背景31.4 探究本课题的研究意义42 雷达频率调节机构的工作原理和设计方案82.1频率调节机构工作原理82.1.1 蜗轮部分82.1.2 蜗杆部分92.1.3 连轴部分92.1.4轴结构设计102.1.5 轴扭转刚度102.1.6 磨损分析113 总体设计计算123.1蜗轮蜗杆的传动设计123.2.蜗杆、蜗轮的基本尺寸设计183.2.1蜗杆基本尺寸设计183.2.3 蜗轮轴的尺寸设计与校核193.3 轴的计算设计203.3.1 轴的基本计算203.3.2.轴的校核计算如表3-3213.3.3 轴承寿命的计算233.4 螺栓联结的强度计算263.4.1 螺纹连接的强度计算263.4.2螺栓联接的强度计算263.4.3螺栓强度计算263.4.4 螺栓组联接的设计273.4.5 提高螺纹联接强度的措施27结束语29致 谢30参考文献31黄河科技学院毕业设计(文献翻译) 第12页单位代码 0 2 学 号 080105037 分 类 号 TH6 密 级 毕业设计文献翻译 院(系)名称 工学院机械系 专业名称机械设计制造及其自动化 学生姓名 指导教师2012年 03 月 18 日1.6 EFFECT OF OPERATING FREQUENCY ON RADARRadars have been operated at frequencies as low as 2 MHz (just above the AM broadcast band) and as high as several hundred GHz (millimeter wave region). More usually, radar frequencies might be from about 5 MHz to over 95 GHz. This is a very large extent of frequencies, so it should be expected that radar technology, capabilities, and applications will vary considerably depending on the frequency range at which a radar operates. Radars at a particular frequency band usually have different capabilities and characteristics than radars in other frequency bands. Generally, long range is easier to achieve at the lower frequencies because it is easier to obtain high-power transmitters and physically large antennas at the lower frequencies. On the other hand, at the higher radar frequencies, it is easier to achieve accurate measurements of range and location because the higher frequencies provide wider bandwidth (which determines range accuracy and range resolution) as well as narrower beam antennas for a given physical size antenna (which determines angle accuracy and angle resolution). In the following, the applications usually found in the various radar bands are briefly indicated. The differences between adjacent bands, however, are seldom sharp in practice, and overlap in characteristics between adjacent bands is likely.HF (3 to 30 MHz). The major use of the HF band for radar (Chapter 20) is to detect targets at long ranges (nominally out to 2000 nmi) by taking advantage of the refraction of HF energy by the ionosphere that lies high above the surface of the earth. Radio amateurs refer to this as short-wave propagation and use it to communicate over long distances. The targets for such HF radars might be aircraft, ships, and ballistic missiles, as well as the echo from the sea surface itself that provides information about the direction and speed of the winds that drive the sea.VHF (30 to 300 MHz). At the beginning of radar development in the 1930s, radars were in this frequency band because these frequencies represented the frontierof radio technology at that time. It is a good frequency for long range air surveillanceor detection of ballistic missiles. At these frequencies, the reflection coefficient on scattering from the earths surface can be very large, especially over water, so the constructive interference between the direct signal and the surface-reflected signal can increase significantly the range of a VHF radar. Sometimes this effect can almost double the radars range. However, when there is constructive interference that increases the range, there can be destructive interference that decreases the range due to the deepnulls in the antenna pattern in the elevation plane. Likewise, the destructive interference can result in poor low-altitude coverage. Detection of moving targets in clutter is often better at the lower frequencies when the radar takes advantage of the Doppler frequency shift because doppler ambiguities (that cause blind speeds) are far fewer at low frequencies. VHF radars are not bothered by echoes from rain, but they can be affected by multiple-time-around echoes from meteor ionization and aurora. The radar cross section of aircraft at VHF is generally larger than the radar cross section at higher frequencies. VHF radars frequently cost less compared to radars with the same range performance that operate at higher frequencies.Although there are many attractive advantages of VHF radars for long-range surveillance, they also have some serious limitations. Deep nulls in elevation and poor low-altitude coverage have been mentioned. The available spectral widths assigned to radar at VHF are small so range resolution is often poor. The antenna beamwidths are usually wider than at microwave frequencies, so there is poor resolution and accuracy in angle. The VHF band is crowded with important civilian services such as TV and FM broadcast, further reducing the availability of spectrum space for radar. External noise levels that can enter the radar via the antenna are higher at VHF than at microwave frequencies. Perhaps the chief limitation of operating radars at VHF is the difficulty of obtaining suitable spectrum space at these crowded frequencies.In spite of its limitations, the VHF air surveillance radar was widely used by the Soviet Union because it was a large country, and the lower cost of VHF radars made them attractive for providing air surveillance over the large expanse of that country.8 They have said they produced a large number of VHF air-surveillance radarssome were of very large size and long range, and most were readily transportable. It is interesting to note that VHF airborne intercept radars were widely used by the Germans in World War II. For example, the Lichtenstein SN-2 airborne radar operated from about 60 to over 100 MHz in various models. Radars at such frequencies were not affected by the countermeasure called chaff (also known as window).UHF (300 to 1000 MHz). Many of the characteristics of radar operating in the VHF region also apply to some extent at UHF. UHF is a good frequency for Airborne Moving Target Indication (AMTI) radar in an Airborne Early Warning Radar (AEW), as discussed in Chapter 3. It is also a good frequency for the operation of long-range radars for the detection and tracking of satellites and ballistic missiles. At the upper portion of this band there can be found long-range shipboard air-surveillance radars and radars (called wind profilers) that measure the speed and direction of the wind.Ground Penetrating Radar (GPR), discussed in Chapter 21, is an example of what is called an ultrawideband (UWB) radar. Its wide signal bandwidth sometimes covers both the VHF and UHF bands. Such a radars signal bandwidth might extend, for instance, from 50 to 500 MHz. A wide bandwidth is needed in order to obtain good range resolution. The lower frequencies are needed to allow the propagation of radar energy into the ground. (Even so, the loss in propagating through typical soil is so high that the ranges of a simple mobile GPR might be only a few meters.) Such ranges are suitable for locating buried power lines and pipe lines, as well as buried objects. If a radar is to see targets located on the surface but within foliage, similar frequencies are needed as for the GPR.L band (1.0 to 2.0 GHz). This is the preferred frequency band for the operation of long-range (out to 200 nmi) air-surveillance radars. The Air Route Surveillance Radar (ARSR) used for long range air-traffic control is a good example. As one goes up in frequency, the effect of rain on performance begins to become significant, so the radar designer might have to worry about reducing the effect of rain at L-band and higher frequencies. This frequency band has also been attractive for the long-range detection of satellites and defense against intercontinental ballistic missiles.S band (2.0 to 4.0 GHz). The Airport Surveillance Radar (ASR) that monitors air traffic within the region of an airport is at S band. Its range is typically 50 to 60nmi. If a 3D radar is wanted (one that determines range, azimuth angle, and elevation angle), it can be achieved at S band.It was said previously that long-range surveillance is better performed at low frequencies and the accurate measurement of target location is better performed at high frequencies. If only a single radar operating within a single frequency band can be used, then S band is a good compromise. It is also sometimes acceptable to use C band as the choice for a radar that performs both functions. The AWACS airborne air-surveillance radar also operates at S band. Usually, most radar applications are best operated in a particular frequency band at which the radars performance is optimum. However, in the example of airborne air-surveillance radars, AWACS is found at S band and the U.S. Navys E2 AEW radar at UHF. In spite of such a difference in frequency, it has been said that both radars have comparable performance. 9 (This is an exception to the observation about there being an optimum frequency band for each application.)The Nexrad weather radar operates at S band. It is a good frequency for the observation of weather because a lower frequency would produce a much weaker radar echo signal from rain (since the radar echo from rain varies as the fourth power of the frequency), and a higher frequency would produce attenuation of the signal as it propagates through the rain and would not allow an accurate measurement of rainfall rate. There are weather radars at higher frequencies, but these are usually of shorter range than Nexrad and might be used for a more specific weather radar application than the accurate meteorological measurements provided by Nexrad.C band (4.0 to 8.0 GHz). This band lies between S and X bands and has properties in between the two. Often, either S or X band might be preferred to the use of C band, although there have been important applications in the past for C band.X band (8 to 12.0 GHz). This is a relatively popular radar band for military applications. It is widely used in military airborne radars for performing the roles of interceptor, fighter, and attack (of ground targets), as discussed in Chapter 5. It is also popular for imaging radars based on SAR and ISAR. X band is a suitable frequency for civil marine radars, airborne weather avoidance radar, airborne doppler navigation radars, and the police speed meter. Missile guidance systems are sometimes at X band. Radars at X band are generally of a convenient size and are, therefore, of interest for applications where mobility and light weight are important and very long range is not a major requirement. The relatively wide range of frequencies available at X band and the ability to obtain narrow beamwidths with relatively small antennas in this band are important considerations for high-resolution applications. Because of the high fre-quency of X band, rain can sometimes be a serious factor in reducing the performance of X-band systems.Ku, K, and Ka Bands (12.0 to 40 GHz). As one goes to higher radar frequency, the physical size of antennas decrease, and in general, it is more difficult to generate large transmitter power. Thus, the range performance of radars at frequencies above X band is generally less than that of X band. Military airborne radars are found at Ku band as well as at X band. These frequency bands are attractive when a radar of smaller size has to be used for an application not requiring long range. The Airport Surface Detection Equipment (ASDE) generally found on top of the control tower at major airports has been at Ku band, primarily because of its better resolution than X band. In the original K band, there is a water-vapor absorption line at 22.2 GHz, which causes attenuation that can be a serious problem in some applications. This was discovered after the development of K-band radars began during World War II, which is why both Ku and Ka bands were later introduced. The radar echo from rain can limit the capability of radars at these frequencies.Millimeter Wave Radar. Although this frequency region is of large extent, most of the interest in millimeter wave radar has been in the vicinity of 94 GHz where there is a minimum (called a window) in the atmospheric attenuation. (A window is a region of low attenuation relative to adjacent frequencies. The window at 94 GHz is about as wide as the entire microwave spectrum.) As mentioned previously, for radar purposes, the millimeter wave region, in practice, generally starts at 40 GHz or even at higher frequencies. The technology of millimeter wave radars and the propagation effects of the environment are not only different from microwave radars, but they are usually much more restricting. Unlike what is experienced at microwaves, the millimeter radar signal can be highly attenuated even when propagating in the clear atmosphere. Attenuation varies over the millimeter wave region. The attenuation in the 94 GHz window is actually higher than the attenuation of the atmospheric water-vapor absorption line at 22.2 GHz. The one-way attenuation in the oxygen absorption line at 60 GHz is about 12 dB per km, which essentially precludes its application. Attenuation in rain can also be a limitation in the millimeter wave region.Interest in millimeter radar has been mainly because of its challenges as a frontierto be explored and put to productive use. Its good features are that it is a great place foremploying wide bandwidth signals (there is plenty of spectrum space); radars can havehigh range-resolution and narrow beamwidths with small antennas; hostile electroniccountermeasures to military radars are difficult to employ; and it is easier to have a military radar with low probability of intercept at these frequencies than at lower frequencies. In the past, millimeter wave transmitters were not capable of an average power more than a few hundred wattsand were usually much less. Advances in gyrotrons (Chapter 10) can produce average power many orders of magnitude greater than more conventional millimeter-wave power sources. Thus, availability of high power is not a limitation as it once was.Laser Radar. Lasers can produce usable power at optical frequencies and in the infrared region of the spectrum. They can utilize wide bandwidth (very short pulses) and can have very narrow beamwidths. Antenna apertures, however, are much smaller than at microwaves. Attenuation in the atmosphere and rain is very high, and performance in bad weather is quite limited. Receiver noise is determined by quantum effects rather than thermal noise. For several reasons, laser radar has had only limited application.1.6 工作频率对雷达的影响雷达已在低至 3hz (刚好高于 AM 广播频段)的频率上工作过,也在高至数百 GHz(毫米波段)频率上工作过。雷达更常用的频带可能为 5阳也95GHz 以上,这是一个巨大的频率范围,所以应该可以预期的是雷达技术、性能及应用会显著依赖于雷达工作的频段而变化。不同频段的雷达通常具有不同的性能和特性。般,在低频段易于获得远程性能,因为在低频易于获得大功率发射机和物理上巨大的天线。另一方面,在更高的雷达频率上,容易完成距离和位置的精确测量,因为更高的频率能提供更宽的带宽(它决定距离精度和分辨率),以及在给定夭线物理尺寸时更窄的波束(它决定角精度和角分辨率)。下面简要介绍不同波段的雷达应用。然而相邻波段的区别在实践中没有显著差别,在特性上可能会有重叠。高频(HF. 330MHz) HF频段的主要用途是被雷达用来探测远程目标(标称可达到 2000n mile) ,方法是利用高频电磁波能量被远离地表的电离层折射的特性。无线电爱好者称这为短波传播并用它来在远距离上通信。 HF 雷达的目标可能是飞机、舰船和弹道导弹,以及来自海面本身的回波(可提供驱动海面的风向及风速的信息)。甚高频(VHF30300MHZ)20 世纪 30 年代开发的大多数早期雷达都工作在该频段,因为在当时这些频率代表无线电技术的前沿。它对远程空中监视和探测弹道导弹是很好的频率。在这些频率上,地球表面特别是水面散射的反射系数会非常大,所以直达信号和面反射信号之间的相长干涉会显著增大 VHF 雷达的作用距离。然而,当有这种效应使作用距离翻倍时,会有伴随而来的相消干涉减少作用距离,这是由于在某些仰角上,天线方向图有深的零点。同样,相消干涉会导致低空上差的覆盖。雷达利用多普勒频移探测杂波中的动目标时在低频上经常会更好,因为多普勒模糊(导致盲速)在低频段要少得多。 VHF 雷达不受雨杂波困扰,但受来自流星的电离和极光的多次时间折叠回波的影响。在 VHF 频段,飞机的雷达截面积一般比在更高的频率上大。 VHF 雷达在获得同样的距离性能时比工作在更高频段上的雷达花费要少。尽管甚高频雷达对远程监视有许多诱人的优点,但也有很多严重的局限。俯仰上的深零点及差的低空覆盖之前已经提到了。分配给 VHF 雷达的可用频谱宽度很窄,因此距离分辨率经常很差。天线波束宽度通常比微波频段的宽,因此角精度和分辨率也差。 VHF 频段中拥挤着许多重要的民用服务,如电视和调频广播,这进一步减少了雷达可用的频谱空间。通过天线进入雷达的外部噪声电平在 VHF 频段比微波频段高。工作在 VHF 频段,雷达的主要局限可能是在这个拥挤的频段中获得合适频谱空间的困难。尽管有局限, VHF 对空监视雷达在苏联曾广泛使用,因为苏联国土广大,而 VHF 雷达的低廉,对提供疆域这么广阔国家的空中监视很有吸引力8 。据说苏联生产了大量的 VHF 对空监视雷达一一一些有着非常巨大的只寸和远的作用距离,但多数是可容易运输的。有意思的是 VHF 机载拦截雷达曾在第二次世界大战中被德国广泛使用。例如, Lichtenstein SN-2 机载雷达在不同型号中工作在 60100MHz 上。在这些频率上的雷达不受称为辅条(也称为窗口)的对抗措施的影响。超高频(UHF.300MHZ1GHZ)工作在甚高频雷达的许多特点在一定程度上也适合于超高频。 UHF 特别适合于机载预警雷达系统 (AEW) 中的机载 AMTI (动目标检测)雷达(参见第 3 章),也适于探测和跟踪卫星和弹道导弹的远程雷达。在这个波段的上段可找到远程舰载对空监视雷达和测量风速及风向的雷达(称为风靡线雷达)。地面穿透雷达 CGPR) ,是所谓超宽带 (UWB) 雷达的例子,参见第 21 章。它宽的信号带宽有时同时覆盖 VHF 和 UHF 波段。这种雷达的信号带宽可能从 50MHz延伸到 500MHz。宽的带宽对获得好的距离分辨率是需要的。低频率对允许雷达能量穿透地面传播是需要的(尽管如此,在典型土壤中传播衰减迅速,因而简单的机动 GPR 作用距离可能仅有几米)。这个距离适合定位掩埋在地F的电线、管线和其他物体。如果雷达要发现位于地表但被树木遮盖的目标,也需要同 GPR 所用类似的频段。Ku,K和Ka波段(14.040.0GHZ)在更高的雷达频率上,天线物理尺寸减小,一般更难产生大的发射机功率。因此, X 波段之上频段的雷达的距离性能一般不如 X 波段的雷达。军用机载雷达有 X 波段的,也有 Ku波段的。对必须要有小的尺寸而不需要远距离的雷达应用,这些频段具有吸引力。机场表面探测设备 CASDE) ,通常在大型机场控制塔的顶端可以找到,工作在 Ku 波段,主要因为它比 X 波段有更好的分辨率。在原先的 K 波段中,在 22.2GHz处有一条水蒸气吸收线,这导致的衰减在一些应用中是个严重的问题。这个问题当 K 波段雷达在第二次世界大战期间研制开始以后被发现,这就是后来引入 Ku 和 Ka 波段的原因。雨杂波会限制该波段雷达的性能。毫米波波段尽管这个频段很宽,多数毫米波雷达感兴趣的频率位于 94GHz附近,此处的大气衰减有一个极小值(称为窗口,是指相对于其附近的频率衰减小的区域, 94GHz附近的窗口和整个微波频段一样宽)。如上面所提到的,对雷达的目的,毫米波范围实际上一般从 40GHz甚至更高的频率开始。毫米波雷达的技术和环境的传播效应不仅不同于微波雷达,而且通常有更多的限制。不同于微波波段雷达所经历的衰减,毫米波雷达信号即使在洁净的空气中传播也会有很大的衰减。衰减在毫米波段是变化的。 94GHz 窗口中的衰减实际上比大气 22.2GHz处的水蒸气吸收线还要高。在 60GHz 氧气吸收线处的单程衰减约为 12扭曲,基本上排除了雷达在其邻近频率的应用。雨的衰减对毫米波波段也是一种限制。对毫米波雷达感兴趣的主要原因是因为它作为研究和有成果的应用的前沿带来的挑战。它的好的特点在于它是采用宽带宽信号的极好场所(有大量的频谱空间);雷达可使用小的天线得到高距离分辨率和窄波束:敌方难以对军用雷达使用电子对抗措施:它使位于这些频率的军用雷达比低频率的雷达有低的被截获概率。在过去,毫米波发射机平均功率无法超过数百瓦一一通常要低得多。回旋管上的进展(参见第 10 章)使得可以产生比传统的毫米波功率源大几个数量级的平均功率。因此,获得大功率不再成为限制。激光雷达激光器在频谱的光学和红外区可以产生可用的功率。它可使用宽带宽(极短脉冲)并具有非常窄的波束宽度,而天线孔径比微波段的小很多。大气和雨的衰减非常高,因此在恶劣天气中的性能十分有限。接收机噪声由量子效应而不是热噪声决定。由于几种原因,激光雷达的应用有限。1.8 雷达过去的一些进展(1)第二次世界大战之前和第二次世界大战期间,开发为防空部署在地面、舰船和军用飞机上的 VHF 雷达。(2) 第二次世界大战早期微波磁控管的发明和波导技术的应用,以获得能在微波频段工作的雷达,从而可使用更小和机动性更强的雷达。(3 )MIT 辐射实验室在第二次世界大战期间存在的五年中开发了超过 100 种不同的雷达型号,为微波雷达奠定了基础。(4) Marcum 的雷达检测理论。(5) 速调管和行波管放大器的发明和发展,提供了稳定性好的大功率源。(6) 使用多普勒频移来检测淹没于杂波中的移动目标。(7)适于空中交通管制的雷达的开发。(8) 脉冲压缩。(9) 单脉冲跟踪雷达有高的跟踪精度,以及比以前的跟踪雷达对电子对抗措施有更好的抵御能力。(10) 合成孔径雷达,对地面场景和地面上的物体成像。(11) 机载动目标显示 (AMTI) ,用于在有杂波情况下远程机载空中监视。(12) 稳定的元件、子系统和超低副瓣天线,使可大量抑制无用杂波的高 P盯脉冲多普勒雷达 (AWACS) 成为可能。(13) 高频超视距雷达,把飞机和舰船的探测距离扩大了一个数量级。(14) 数字处理,从 20 世纪 70 年代早期对雷达性能的改善有重大影响。(15) 监视雷达的自动检测和跟踪。(16) 电扫描相控阵雷达的批量生产。(17) 逆合成孔径雷达 CISAR) ,提供目标成像,如对舰船等非合作目标识别需要的图像。(18) 多普勒气象雷达。(19) 太空雷达,适于对如金星等行星进行观测。(20) 计算机对复杂目标雷达截面积的精确计算。(21)多功能机载军用雷达,体积和质量相对小,适于安装在战斗机前端,具有执行大量不同的雪一空和空地任务的功能。以上是对雷达过去一些主要发展的一点观点。其他人或许有不同的看法。并非每种重大的雷达成就都包括在内。如果包括本书其他章节的内容,这个列表可能会更长并包含更多的例子。但是这个列表己足以显示出对雷达性能改进很重要的进展类型。
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