Technology of theHigh power transmitter

     The rotatable curtain array is a versatile tool,which has come into prominence in the past decade,Traditionally, antennas for SW broadcasting havebeen of the fixed type; the one most commonly usedwas the fixed curtain array,an arrangement of dipolesstacked in a vertical and horizontal configuration,which, depending on the number of stacks, can realise a gain in the directivity from 17 to 23 dB.In the late l960s, the advent of the logarithmic-periodic antenna and its application to HF communications was later followed by its application to SWtransmitters with powers of up to 100kW or so. thehorizontally polarised log-periodic, wiht its single mast support, was found to be suitable for a rotatablearrangement either by motor-driven or y a maual arrangement of steeing it to the new angle of fire. Afew succeeded in companies building log - periodicantennas of the motor-driver roatable type for SWbroadcasting, but the design did not rea1ly take offbecause of problems of mechanical stress and an electrical naturd caused by very high voltages in the feeder and elements.By this time the curtain array had become universally Popular wiht SW broadcasting authorities, soit was logical that a rotatable version would follow.Practically all SW broadcasting relises on the use ofdirective antennas on a fixed azimuth bearing; to cover the world, therefore, a significant quantity is required. For example, a stat1on with15 SW transmitters might have as many as 36 directive antennas, andoften many more. This immediately raises the problern of land re(luired. Most of the BBC World Servicestations, fox example, have antenna sites that extendover several square kilometres; and in Westem Europe generally, land is an expersive resource.One antenna can only provide continuous transmission service from one transimitter, therefore forthe broadcasting authority with only one or twotransmitters the rotatable antenna is a cost-effectivesolution, able to cover different zones of the world atdifferent times. Equqlly, the rotatable has advantagesfor large broadcasters who might have 20 SW transmitters, in that one rotatable can act as a backup forany one of the curtains on fixed bearings.Disadvantages of the rotatble curtain array areits complixity of mechanics and the cost, which isvery high compared with any fixed antenna. With increasing power hand1ing these problem become rnagnified. As a result, there are few cornpanies in theworld with the necessary experience; those that havemet with the greategt success in this field are ABB,AEG and Thomson-CSF. AEG has been successfulin seIling its version to European broadcasting authorities; one in Netherlands, at Kvitsoy NTA, isprobably the most northerly such station in theworld. Others have been suppiled to Vatican City andORF Austia.ABB and Thomson-CSF hav been simi1arly successful in supplying thelr versions to various countries in the Middle East, including Iraq, Iran,Kuwait, Turkey and a number of Gulf states. Therotable curtain array is another product where thedirrerent companies have adopted quite different solutions to meet the same operational requirement.The AEG design is a structure of a circular railwaytrack.

     The ABB design is a Y -shaped stucture fromwhich hte curtain array is suspended. A single column, which rotates on a roller bearing, carries thecomplete Y structure. This design gives joltless startand stop, and continuous acceleration and deceleration, with a positioning accuracy within 1" o The Tomson-CSF design is similar to the AEG and ABB versions in certain respects, its unique feature is that oneside of the (otherwise perfectly balanced) structrue isweighted. Thus, in the event of a gale-force wind.the stucture rotates so that its main axis is parallelwith the wind direction. The cluth is designed to release when windspeeds exceed a certain figure, atwhich time the transmitter is shut down. The massivesize of these rotatable curtains-of the order of 100m'by 100 m highmeans a large surface area exposed towind forces.Possibly because of the high costs of rotable curtains there are at present no more than 20 or so countries equipped with them, but they certainly reprecenta current growth area in SW broadcasting. 大功率发射机技术 过去十年内突现的可转动幕式 天线是一种万能的工具。传统上，短 波广播使用固定式的天线；一种最 普遍使用的固定幕式天线是在垂直 和水平方向叠置了多个偶极子的结 构，取决于叠置的数目可以获得17 到23个dB的方向增益。

     在六十年代后期，对数周期天线出现并应用于短波通信，随后应用到大约100kW的短波发射机。由单桅杆支持的水平极化对数周期（天线）是一种适用于由马达驱动或手动转向调整装置以达到新的发射角度的可转动设备。少数几个公司成功地建造了用于短波广播的由马达驱动的可转动的对数周期天线，但是因为机械应力以及馈线和元件上的高电压的电气特性的问题，设计尚未真正完成。

     现时，幕式天线阵已普遍为短波广播主管部门所欢迎，因而跟着出现可转动的形式是合乎逻辑的。实际上所有短波广播都靠使用固定方位的定向天线覆盖全球，因此，需要量是很大的。例如，一个有15部短波发射机的电台可能有36付定向天线之多，并且通常还会更多。这就直接引发了土地需求的问题。例如，BBC的多数的环球广播电台的天线场地延伸到数平方公里以外：通常在西欧，土地是一项昂贵的资源。

     一付天线只能够为一部发射机提供连续的发射服务，因此只用一部或两部发射机的广播主管当局要在不同时间能够覆盖世界的不同地区时，转动天线是一种合算的解决方案。同样，对于可能有20部短波发射机的大广播电台，一付转动天线能够起到可以作为任何一付固定方向的幕式天线的备份作用。

     可转动幕式天线阵的缺点是其机械的复杂性和与任何固定天线相比的成本过高。由于功率的增大，处理这些问题变得更加突出了。结果，世界上只有少数几个公司具有必需的经验；那些已经在此领域取得巨大成功的是ABB，AEG和汤姆逊—CSF公司。AEG已经成功地向欧洲广播当局出售了其产品；一个在荷兰Kvitsoy的NTA，或许是世界上最北方的这种电台。其他的供应给了梵蒂冈城和奥地利的ORF。

     ABB和汤姆逊_—CSF在中东一些国家包括伊拉克、伊朗、科威特、土耳其和一些海湾国家同样成功地供应了它们的产品。不同公司的可转动幕式天线阵产品采用了完全不同的解决办法去适应同样的运用要求。AEG的设计是圆形的铁路轨道结构。

     ABB的设计是一个悬挂幕式天线阵的Y形结构。一个在滚柱轴承上旋转的单根圆柱，带动整个Y形结构。在定位精确到1°之内的情况下，这种结构提供了无摇动的启动停止和连续的加速减速，汤姆逊—CSF的设计在某些考虑上与AEG和ABB的产品相类似。它的唯一特征是在其结构的一侧增大了重量（其它方面完全是平衡的）。因而，当突然遇到强风时该结构要转向使其主轴与风向平行。将离合器设计为当风速超过某一确定值时关掉发射机。这些可转动的天线幕的100米乘100米的大尺寸意味着其硕大的表面面积被暴露在强风中。

     或许是因为转动天线的高价格，至今最多有大约20个国家装备了它。但它们确实代表着当前短波广播的发展领域。

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     作者：不详

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