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,