A view of the main antenna switching station with the transmission lines leading to the antennas.

OWI 200-KW TRANSMITTERSAT BETHANY, OHIO

by R. J. ROCKWELL Director of Broadcast EngineeringThe Crosley Corporation

ON September 23, 1944, the Office ofWar Information, the Office of the Coordinator ofInter-American Affairs and the CrosleyCorporation officially dedicated three new200kw transmitters at Bethany, Ohio. This was the culmination of almost two years ofplanning, designing and building, of inventionand adventure in the field of radio engineering.This was the end of a trail that started with anurgent war necessity, and the beginning!: ofanother trail that leads forward to a new kind ofair supremacy for the United States of America.This was the loudest voice in the world trying itsyoung lungs.

On that fateful December Sunday in1941, when bombs rained down on PearlHarbor, Uncle Sam was internationallyinarticulate, as compared with the enemy. Sixinternational licensees were operating only 13short-wave stations, several of which wereincapable even of 5O-kW output. In contrast tothis, Germany had at least 68 short-wavetransmitters under her control, and it wasreported that from 12 to 20 additional units of

200-kW output might be in operation byDecember, 1942. Japan controlled 42 knowntransmitters, ranging up to 50 kW. Thus, the Axiswas able to carry the same program on as manyas 20 different frequencies. In fact, Germany'sregular service to the Americas was then using16 frequencies.

By the most conservative estimates, weneeded at least 36 international transmitters,operating on a total of 70 frequencies between 6and 20 mc, with powers ranging up to 200 kw, ifpossible. But that was a big IF! For, at that time.no vacuum tubes, output circuits or antennaswhich were capable of such powers were knownto exist.

In a drastic effort to remedy the situation,the Board of War Communications called acouncil of war in Washington. All interested.parties were invited - international licensees,equipment manufacturers, representatives of theFederal Communications Commission, Office ofWar Information, Office of Coordinator of Inter-American Affairs, the Department of State, and

others---to determine how soon existing facilitiescould be augmented and new facilities added.and what powers could be attained. As a resultof this and subsequent meetings, it was decidedthat, within one year, a total of 32 transmitterscould be in operation and at the disposal of OWland CIAA. It was felt that installations could begreatly expedited by duplicating existingmaximum power output, which was 100 kw, andrequired operation of tubes in parallel to achieveeven this power.

Figure 1. A general layout of the property where the 200 kWtransmitters are located, with building, substation, switchingstation and antenna system.

But, to be on a parity with the Axis, andto be able to override interference and cope withinferior receiving conditions in foreign countries,we had to do more than just duplicate existingpowers; we had to exceed them, we had todouble our greatest previous powers!

As a part of this mammoth program, the CrosleyCorporation undertook to redesign its equipment,for an output of 200 kW. Tubes were designedand developed, circuits were calculated, andantennae were devised to do the job. This paperwill describe the installation in some detail, andtell of its operation.

Figure 2. Floor plan of the transmitter building, showing thefireproof vaults where the high voltage transformers, filterreactors, filter condensers and high voltage controlcomponents are placed.

Figure 3. Block diagram of the complete transmissionsystem.

High Power Pioneering

The story of Bethany really goes backfar beyond Pearl Harbor. In the very early daysof high-power, high-frequency broadcasting,when 10-kW was the power maximum, Crosley'sW8XAL was a pioneering leader. Alwaysexperimenting with higher powers, its staff hadalready built a 50-kW transmitter when the FCCmade that output a license requirement. Throughcontinuation of these experiments, tubes weredeveloped. capable of 75 kW, and in 1940,another momentous dedication was held, of themost powerful shortwave station in this country

at that time, WLWO.

Even in the standard band, Crosley hadpioneered in high power. Its world famousW8XO, on the WLW frequency of 700 kc, hadoperated successfully for several years prior tothe war with 500 kW power. And even today, theunmodulated signal of W8XAL serves the U. S.Bureau of Standards, providing a variety ofInformation on radio skywave transmission,notably on absorption and its variation. Frommeasurements of this type, the Bureau suppliesOWl with advance information which enablesthem to schedule optimum usable frequenciesfor all American international stations for varioustimes of day, season and solar cycle.

Because of these qualifications, as wellas the fact that Crosley's offer to construct thethree 200-kW transmitters did not conflict withurgent military orders, this responsibility wasplaced in the hands of the Crosley Corporationby OWI. Moreover, since at the time the decisionwas made. it was still considered a possibilitythat bombings of the East coast might beattempted by Germany, a middle-west locationwas desirable for security reasons.

Figure 4. View of a section of the transmitter. Unit third fromleft is the RF driver which operates at carrier frequencythroughout: range is 6-22 mc.

Once the basic plan was evolved, the first bigproblem was the selection of a site on which tobuild the project. The specifications wereextremely exacting. About a section of land wasrequired for the building, switching station and 24rhombic antennae. And it was necessary toleave some space for future expansion withoutovercrowding. The ground had to be as high, orhigher. than surrounding territory, and relativelylevel throughout and for some distance beyond.A uniform slope under and ahead of theantennas in the direction of transmission was

needed, in order to lower the angle of maximumradiation of. the high frequency, directionalradiators. The land had to be virtually free ofwooded or populated areas for a considerabledistance. A source of 3000kva primary powerwas required, as well as provision of broadcasttype telephone circuits, leading from the mastercontrol room of WLW in Cincinnati.

Figure 5. Rear view of one of one of the RF generators. Shield has been romeved from second buffer stage coil toshow construction.

The Bethany site was chosen only after thoroughstudies had been made of U. S. topographical.maps covering a radius of about 50 miles aroundCincinnati, and after every other possible sitehad been investigated. It is slightly less than.afull section of high, relatively flat land. gentlysloping generally in all directions from center,and the surrounding land is also flat and slopesslightly in favorable directions. The area isremarkably free of housing; and the few slightlywooded areas could easily be avoided in layout.

The same dual-power circuit feeding the WLW.WLWO and WLWK transmitters near Mason,Ohio, passes along the southern boundary of theproperty, and the telephone cables for thosestations run along the eastern boundary. Thusexpense, and use of critical materials, could beheld at a minimum, in obtaining these vitalservices. Two rail lines are within a few miles ofthe place, and a good. hard surface road runspast the south entrance to the property, whilesatisfactory all-weather roads skirt the east andnorth boundaries. This was of considerableimportance, due to the physical size of some ofthe components of the project. such as antennapoles. transformers, etc., which had to betransported to the site.

Figure 1 shows a general layout of the property,with building, substation. switching station andantenna system indicated and identified.

Tailoring the transmitter building

The building is composed of two separate units.an administrative section and a transmittersection. Although, due to the character of thedesign, there is an expansion joint betweenthese sections, the two are actually one building.

Since this paper deals primarily with thetransmitters, concentration will be on the detailsof that section, and the administrative portionwill be dismissed with the information that it istwo stories high, with a four-story central towerthat serves as a guard station and revolvingsearchlight beacon house. Offices and livingquarters for resident engineers, a machine shopand a garage occupy the two stories of thebuilding itself. while the first three levels of theguard tower contain radio-testing equipment, astorage tank for distilled water used in tubecooling, and two 1500 gallon water storagetanks, for general building purposes.

Figure 6. Partial view of the equipment bays in which the RFunits are mounted. Note coaxial patching panels. RFgenerator front panel has been removed to reveal internalconstruction.

The transmitter section is entirely functional. Itwas designed to accommodate six transmitterswith their associated equipment, and is actuallysix similar units, three on each side of a centerconcourse. This section is a monitor typebuilding 175' wide by 75' long, with a high baymeasuring 24' from floor to roof slab. Ceilingheight in the side bays is 17'. This sectioncontains some interesting features. A tunnelextends under the center of the concourse,

connecting with the boiler room under theadministrative section, and provides connectingfacilities for the transmitters on either side, Topermit the extraordinary flexibility for piping andwiring required. in this installation, a cellular steelfloor was set between two mats of concrete, theupper layer being the floor of the concourse.This. steel Q-floor extends from the sides of thetunnel walls to the undersides of the transmittercatwalks.

Figure 7. Cathode ray oscilloscopes used for monitoringcarrier of each transmitter. These are mounted in the controldesk.

Another unusual design feature its cantileverconstruction. The high bay has a span of 64'between column supports. The monitor width ofthe high bay is 99'. To reduce the depth of thegirders supporting bay beams, 17' 6?"cantilevers were designed on the outside of thecolumns, to support the extension of the monitorbeyond the column centers. This cantileveraction produced negative moments in the centerof the span, permitting a reduction of the beamdepth of the girders bridging the high bay. Theroof slab of the side bays is suspended from thebottom level of the cantilevers, producing adesign in which only three spans are necessaryto bridge a total distance of 175', and which, atthe same time, accomplishes the difference inceiling heights incident to monitor-type roofconstruction.

The transmitter section interior consists of thecenter concourse flanked. by catwalks, on whichare mounted six transmitter units. Directly behindthese units is a continuous wall, in which ismounted, behind each individual unit, a group ofcabinets which include 240-V power distributionpanels, magnetic switch panels, automatic aircontrol equipment, etc. Beyond the wall to therear are individual fan rooms, in which arecooling fans, water pumps, and plenum

chambers for distributing inlet and heated air asdescribed later. Directly behind these fan roomsare the transformer vaults. Under the catwalksare the filament transformers, and plumbing fordistributing the water to water-cooled tubes. Onthe slanted front edge are water-flow meters,temperature gauges, and water controls, Figure4.

The entire building is thoroughly grounded, a matof wires radiating on all sides for a distance of50', and extending to the reinforcing- bars whichare welded together within the concrete. Thiswas done to eliminate the possibility of cross-modulation, and to provide a good groundmatting for the transmitters.

Figure 8. Front view of the RF driver shich is composed ofthree stages. Each stage is separately tuned in both gridand plate circuits and is link coupled to the succeedingstages.

The Transmission System

A general overall picture of the completetransmission system appears in the blockdiagram of Figure 3. The diagram shows onecomplete transmission system with its threeessentials: primary power, rf power, af power,separately obtained and combined into highpower modulated rf energy, which is radiated bythe antenna system.

Radio frequency at carrier frequency isgenerated continuously on all frequenciesassigned to this plant. Separate 15-Watt exciters

for each frequency are mounted, six to a bay, inthe control room. Exciter power is fed to thetransmitters through a plug and jack panel, usingmultiple, double conductor coaxial lines andjacks.

The rf driver unit, third from the left in Figure 4,operates at carrier frequency throughout. Itsrange is 6 to 22 mc. Input power from theexciters is from 2 to 5 watts. A maximum outputpower of 10 kW can be obtained from this unit,which is coupled, through an adjustable link, tothe power amplifier in the adjacent cabinet to theright.

Figure 9. Output coupling adjustment, which is completedby swinging a small link coil into the field of the center of thedriver plate inductance.

The modulated class. C amplifier employs. twotubes, operating push-pull, capable of 200 kWcarrier output. Conventional lumped constantcircuits are ulsd in the grid while the plate circuitconsists of a lumped variable capacity, and ashorted transmission line section for theinductance. Variable inductive coupling isemployed to transfer power to the antennatransmission lines.

The a-f circuits originate in the control. room,where the modulating signal is taken fromincoming telephone lines or Iocal electricaltranscriptions. A transmitter circuit includes alimiting line amplifier, to amplify the audio voltageand compress high level peak; peak clipper tochop both positive and negative peaks at a pre-set level, usually equivalent to 100% transmittermodulation; and the usual level controls andvolume indicators, From the control room, a-fvoltage is fed at zero level to the modulator unitof the transmitter. The modulator contains threepush-pull voltage-amplifier stages; and amultiple-tube class B power stage capable ofdelivering 180 kW of audio power, Themodulation transformer, modulation reactor, antido blocking condenser are located in thetransformer vault, and are connected to themodulator through high~voltage cables.

Primary power for these transmitters is derivedfrom a substation at the side of the mainbuilding, and a stepdown transfomler in thetransformer vault. All rectifier tubes and theirfilament transformers are located in the rectifierunit of the transmitter, The rectifier unit also serves as a control andsupervisory unit for the entire transmitter.Momentary contact buttoos, pilot relays, andsupervisory relays and lights are mounted in therectifier cabinet, All 240-V power contactors forfilaments, low voltage supplies, etc., and a 240-Vbreaker and distribution panel are located in thewall cabinet behind the transmitter, as previouslydescribed, All equipment dangerous to personnelis complteIy interlocked, both electrically andmechankally,

Antenna System

The antenna system consists. of a transmissionline from each of the six transmitter bays to acommon transmission line switching systein. Inthis gear any transmitter line can be connectedto any outgoing antemta line. All antennas are ofthe reentrant rhombic type.

Each transmitter is equipped with a completeand separate cooling system, which supplieswater for water-cooled tubes, ane! ventilating airfor general cahinet cooling. This system is soarranged that all waste power is converted intoheated air, which is automatically used forbuilding heat in winter. In summer, heated air isforced through roof ventilators. causing cool airto be drawn into the building. A 50-gallon-per-

hour still in the boiler room and a 400-gal!onstorage tank in the guard tower with a commonfeeder to all systems, supply the distilled waterfor cooling.

A complete system. of modulation monitoring, bymeans of oscilloscopes, of audio monitoring bymeans of diodes on the transmission lines, andof frequency monitoring, by means of asecondary standard and WWV comparisonequipment is centrally located in the controlroom.Figure 10. Interior of the power amplifier cabinet. Smallcabinet between the two tubes provides shielding for gridcircuits of amplifier.

R-F Generator

The r-f signal is initiated in the r-f generator units.A battery of these run continuously. All units areidentical, consisting of a 6V6 crystal-oscillatorstage, two 6V6 multiplier-buffer stages and afinal 807 amplifier.

The crystal-oscillator stage is of conventionaldesign, using broadcast-type temperaturecontrolled ovens. The plate circuit is tuned to

crystal frequency, The first buffer stage operateseither at crystal frequency of second, third orfourth harmonic, and the second buffer stageoperates in some cases as a multiplier and inother cases as a straight amplifier, depending onthe multiplication of crystal frequency required.

In all cases the 807 stage operates as a straightamplifier. The output is coupled to three parallelsets of coaxial jacks. Operating conditions aresuch that from one to three transmitters may beconnected without affecting the drive.

Each unit is assembled on a rackmounting 12 x20" chassis. all of which are identical. Thus anyunit can operate on any frequency, when propercoils and crystals are plugged in. Figure 11. Neutralizing condenser, which consists of twosemi-cylindrical sections, rotated in the cylindrical spaceformed by the tube jacket housings.

Tuning the crystal oscillator and. buffer amplifierstages is accomplished by midget variablecondensers mounted inside the coil forms. Thetuning condenser for the plate circuit of the 807stage is mounted in the chassis, and isadjustable from the front panel. Thus, to set upany unit on a given frequency, it is necessaryonly to plug in the proper coils and crystal, andpeak the tuning of the 807 stage. Figure 5 showsa rear view of one of the r-f generator units. Theshield has been removed from the second bufferstage coil to show the construction.

A partial view of the equipment bays in whichthese generators are mounted appears in Figure6, showing the coaxial patching panels by meansof which the outputs of the generators may bepatched to the inputs of the transmitter drivers.Immediately above and below the patchingpanels are the r-f generator unit front panels(one of which has been removed to showinternal construction) with the final tuning knob tothe right, tube metering; push buttons grouped atthe center, and crystal heat indicator at the left. Figure 12. Variable air condenser and transmission lineinductance, with movable shorting bar in plate tank circuit ofthe power amplifier.

Audio monitoring of the transmitters is

accomplished by speakers concealed aboveeach transmitter. Each speaker is driven from itsown amplifier. The input to this amplifier isnormally fed from a diode monitor coupled to thetransmitter output. By means of a switch locatedon the hand rail in front the transmitter, speakersmay be switched to the audio line so thatcomparison may be made between the signal asit enters the modulator input and as it leaves thetransmitter. Duplicate speakers are similarlylocated in the controI room above the controldesk, where monitoring is accomplished in thesame manner. In addition, there are monitorlevel controls on the desk.

Cathode-ray oscilloscopes, which may be seenin Figure 7, are provided for monitoring thecarrier of each transmitter, and are located in thecontrol desk. These have a 60-cycle sweep

voltage on the horizontal plates; the verticalplates are fed from pre-tuned circuits, coupled bycoaxial lines to the transmitter output. Thesetuned circuits are selected by band switch fromthe front of the oscilloscope The coil assemblyon each has 10 resonant circuits, each with amidget variable condenser for resonating thecircuit to any of 10 frequencies on which theassociated transmitter may be operating.

Immediately below each oscilloscope panel is asmall panel bearing the audio level control, thecarrier-on. light, the emergency-off button foreach of the transmitters, and a key which, whenthrown, drops the studio line, and picks up theoutput of a pre-set electrical transcriptionmachine, and simultaneously, through relays,starts the transcription machine motor. To theright of the public address speaker on the centerpanel is a master key, which operates all the pre-set electrical transcription machinessimultaneously.

At the center of the desk are the generalcontrols common to all transmitters. Immediatelyabove is the public address speaker, which alsoserves as a microphone, and which may beswitched to a two-way f-m transmitter,communicating with the antenna service car,without being disconnected from the regularbuilding intercommunication system.Immediately below the speaker is a volume levelindicator, with its range switch and channelselector switch. Below these are the level controland selector switches for the monitoringspeakers.

The frequency monitoring system is alsoincluded in the control room equipment, andconsists of a secondary frequency standard. afixed-tuned 5-mc WWV receiver a frequencycounter, and an r-f signal mixing panel.

All components of the 100 kc oscillator circuit areenclosed in a temperature controlled oven,including the tuned circuit, the crystal, theoscillator tube, and a voltage regulator. Theoscillator is electron coupled to an isolatingamplifier, which feeds the output to themultivibrator and the fixed tuned WWV receiver.

The WWV receiver is a three-stage t-r-f type, fedfrom a tuned loop. A detector stage mixes the100-kc signal with the WWV signal. An audioamplifier is included to drive the monitoringspeaker.

The multivibrator is of conventional design, withoperating frequencies at 10, 20 and 50 kc. It ispreceded and followed by isolating amplifiers.The output amplifier is of cathode-follower type,feeding one stator of two signal-balancingvariocouplers, in tandem, driven from a commonshaft, and completely shielded from each other.A faraday shield eliminates capacity couplingbetween rotor and stator of each variocoupler.The rotors are so oriented on the common shaftthat when coupling is at minimum in one unit, it isat maximum in the other. Multivibrator output isfed to one stator the unknown signal is fed to theother: and the two rotors in series are fed to theinput of the communications receiver. Thisprovides a means of balancing the amplitudes ofthe unknown and the standard signals. so that agood beat note is obtained.

The unknown signal input to the signal balancingvariocouplers may, by means of a jack. panel, beobtained from one of the r-f generators, or froman antenna. The multiple-band communicationreceiver and frequency counter complete theequipment of the frequency-monitoring unit.

R-F Driver

A front view of the r-f driver, which is composedof three stages, is shown in Fig. 8. Each stage isseparately tuned in both grid and plate circuits,and is link coupled to the succeeding stage. Boththe grid and plate circuits of the first stage, asingle 807, have eight pre-tuned, plug-in coils.Each grid coil is provided with an input windingwhich is selectively connected to the doublecoaxial transmission line terminating on the r-fexciter patching panel. The push-pull 813second stage has two sets of tapped coils inboth grid and plate circuits.. The Fl29B driverthird stage is push-pull, and has two sets oftapped coils in its grid. Both these stages haveadjustable tuning capacities, controlled from thefront of the cabinet. The F129B plate circuitconsists of a copper tubing' inductance withadjustable taps and shorting bars, and a variableair condenser, visible from' center front of Figure8, controlled from front of cabinet. A gangedband switch is mounted vertically and operatedfrom the rear of. the cabinet by means of ahorizontal handwheel, accessible from the topshelf. This mechanism switches all circuitsexcept the F129B plate circuit, which must bechanged manually , Other front controls includethe third stage neutralizing, and. the outputcoupling adjustment which is accomplished by

swinging a small link coil into the field of thecenter of the driver plate inductance, Figure 9.

Figure13. Verticaladjustable portionof the

coupling loop, which also has a horizontal fixed portionrunning parallel to the horizontal portion of the transmissionline tank circuit.

Final Amplifier Unit

The final amplifier of each transmitter isdesigned to deliver 250-kw 100% platemodulated, on frequencies between 6 mc and21.65 mc, with plate voltage up to 15 kV. Figure10 shows a view of the Interior of the power

amplifier cabinet. Mounted between the twotubes is a small cabinet which provides shieldingfor the grid circuits of the amplifier. The grids ofthese tubes are driven through a pi network, fedthrough coaxial transmission lines from the drivercabinet, where it is inductively coupled to thetank circuit of the driver. The doors of the gridcabinet are shown open, so that grid coils maybe observed. These coils are so proportionedthat the circuit is tuned at 6 mc, when the entirecoil is in the circuit. By placing shorting strapsacross turns of the coils, higher frequencies areobtained, until at 21.65 mc the entire coil is shortcircuited. The pi network uses the tubecapacities and inductances in its circuit.

The grid circuit of the final amplifier has a fixedbias which protects the tubes to a safe value ofplate dissipation, in case of failure of drive. Thisis accomplished by obtaining bias voltagethrough a high resistance from one of the lowvoltage rectifiers. The voltage is appliedcontinuously, and in case of failure of drive, thetubes are automatically protected from overloadswithout. the operation of any relays. When tubesare being driven, the bias becomes negligible.

On each side of the cabinet interior, cooling air isprovided through a vertical duct, connecting withthe header duct above the entire transmitter unit,and providing a blast of air on the glass of thepower amplifier tubes, of sufficient velocity foreffective cooling over the entire periphery of theglass.

In Figure 10, the arrangement of the neutralizingcondenser and the plate tank condenser mayalso be seen. In the center of the picture, threeconcentric shafts run toward the rear of thecabinet, through a corona shield, housing bevelgears. This mechanism, operating. by chaindrive from controls on front of the cabinet,adjusts the plate tank condenser, the antennacoupling loop, and the neutralizing condenser.

The neutralizing condenser consists of two semi-cylindrical sections (Figure 11) rotated in thecylindrical space formed by the tube jackethousings. This design provides a balanced-bridge circuit, with essentially no detuning of gridor plate circuits during neutralizationadjustments.

The plate tank circuit of the power amplifier(Figure 12) consists of a variable air condenserand a transmission line inductance with movable

shorting bar. The constants of this circuit aresuch that a frequency range of 6 to 21.65 mc iscovered by adjustment of the capacity andinductance. The moving portion of the plate tankcondenser is suspended from a carriage whichrides on three brass pipes. In addition, thiscondenser is bypassed to the shield at practicallythe same point to which the filaments of thetubes are bypassed. The grid cabinet is alsoconnected to this same point. This serves to holdthe circulating currents in the shielding to aminimum, and provides stable operation atpowers in excess of 200 kW.

The stationary plates of the plate condenser arecarried directly on the housings which containthe tube jackets. Figure 13 shows an assemblyof tube jackets in housings, incorporatingthermometers and water pressure gauges. Also.shown are the antenna coupling loop, verticalsection of the transmission line, and the bottomheader castings to which are to be connectedthe horizontal four-pipe transmission line, onwhich the shorting bar slides. These horizontalpipes also carry cooling water to the tubes.

The transmission line tank is housed in a metalshielding duct, which is a continuation of a shieldwithin the cabinet, but which is insulated from themetal of the cabinet itself. The vertical section ofthe transmission line consists of a pair of largecopper pipes on each side, each pair encased ina. copper sleeve. This offers an exceptionallylow impedance circuit, and makes it possible toobtain a high distributed capacity, even thoughthe conductors are separated from each other farenough to permit mounting of a coupling loopbetween them.

The coupling loop is in two portions: a verticaladjustable portion shown in Figure 13, and ahorizontal fixed portion which runs parallel to thehorizontal portion of the transmission line tankcircuit, and which is automatically coupled ordecoupled, depending on the position of thetransmission line shorting bar. Tuning of thiscoupled circuit is accomplished by vacuumcondensers, either at the bottom of the verticalcoupling loop, or at the rear of its horizontalportion, depending on frequency. The verticalportion of the coupling loop is so arranged that itcan be rotated into a bucking position withrespect to the horizontal section, duringoperation at lower frequencies .

All of the foregoing adjustments for frequency

change can be made from the front of thecabinet in a few seconds.

Low Level Audio and Control RoomEquipment

All program material is supplied to thetransmitters either by telephone circuits, or byelectrical transcriptions. No provisions are madefor live pick-ups at the transmitter building. Atpresent the OWl and CIAA programs originate instudios in the East, and are fed through theWLW master control room at Crosley Square indowntown Cincinnati, to the transmitter audioequipment. Figure 14 shows a panoramic view ofthe equipment bays in the control room. Aseparate amplifier channel is provided for eachmodulator input, plus one spare. Each channel isinstalled in a separate equipment bay.

Figure 14. View of the equipment bays in the control room.

The incoming lines are terminated in matchingnetworks which provide a means of feeding oneto three amplifier channels from any one line.The output of the matching network may bepatched directly to the desired amplifier input, orit may be patched through a selector switch,affording a quick means of transfer from oneprogram channel to another. This switch affordsa selection of either of two circuits, and is set upby patch cord, so that the selection may be

between any two telephone lines, or betweentelephone line and electrical transcription output.

The signal then is routed to one of fourequipment bays which contain the amplifierchannels associated with the particulartransmitter inputs, and passes through a variableattenuator and all electrical transcriptionswitching relay to the line amplifier. The volumeindicator, mounted . in the bay, monitors thelevel at the output of the line amplifier. From thispoint, the signal passes through a peak clipper,and out of the amplifier bay to the transmitterinput attenuator, located on the control desk.

Figure 16. The 42 Ohm surge-limiting resistors which are inseries with each modulator plate circuit, at bottom of unit inview below.

The electrical transcription switching relaymentioned, is controlled by keys on the controldesk, as previously described. Monitoringspeaker amplifiers and switching equipment arealso located in the control room equipment bays.Each program channel has a monitoring take-offat the output of the line amplifier, and this signalis fed through speaker selector switches soarranged that the operator is able to pre-set

levels before the signal is fed to the modulatorinput. The e-t machine output is fed through apre-amplifier and variable attenuator to theswitching relay, and to the selector switches ofthe monitoring speaker, so that here again thelevel may be pre-adjusted before the signal isfed to the modulator.

Throughout all of the speech equipment in thecontrol room, complete flexibility is maintained byuse of jacks and patch cords, by which anyelements, from amplifiers to fixed pads,. may beisolated. In most cases, back contacts on thejacks route the signal through its normal channel,but the presence of the jacks in the circuit makesit possible to lift a defective element quickly, andpatch in a spare.

Figure 15. Modulator unit which consists of a three-stagevoltage amplifier and a two to six-tube class B amplifier.

Modulator

The modulator (Figure 15) consists of a three-stage voltage amplifier, followed by a 2- to 6-tube class B power stage. The first two stagesare push-pull 807 and 813 tubes, respectively,and are resistance coupled. These stages aremounted horizontally on a vertical panel in theupper center of the unit, while the circuitcomponents are mounted on the rear. The 813sare resistance coupled to push-pull 891s, locatedslightly above the main group of tubes, andoperating class A as the third voltage amplifier.

The 891 stage is coupled with a 1:1 ratiotransformer to 2, 4, or 6 F125 tubes, operatingclass B with zero-grid current, as the powerstage. In the 4 or 6 tube arrangement, 2 or 3tubes operate in parallel on each side of thepush-pull circuit. Three 125s in the frontconstitute one side of the class B stage, whilethree more in the rear constitute the other side.

In order to accommodate tubes of normaldifferences in characteristics, each tube hasindividually combined bias and drive adjustment.The circuit is such that these two adjustmentsare accomplished simultaneously, with onepotentiometer in each grid circuit. .Eachmodulator tube plate circuit has a 42 Ohm surgelimiting resistor in series, which may be seen atbottom of unit in Figure 16.

The modulation transformer, weighing more than24,000 pounds, and its reactor and dc blocking-condenser, are located in the transformer vault.This combination was designed to cover afrequency range of from 50 to 10,000 cycles, at apower output of 180 kW maximum.

Power Supply Substation

Power is supplied to the plant from a 33-kV loopcircuit, with remote controlled disconnectswitches on each side of the take-off point.These switches, both normally dosed, may beopened by pushbutton controls in the transmittercontrol room, in case of a fault on either line.

At the substation a 3000-kVA transformer bankprovides 3phase, 2400 Volt service, divided intosix main circuits to the transmitters, eachequipped with fused disconnect switches; Threeof these are for the high-voltage plate-supplytransformers, and the other three are fed throughan induction regulator, and supply a 200-kVA,2400/240-V, 3-phase transformer in each vault.All 240-V power for operating filamenttransformers, pump and blower motors, lowvoltage supplies. etc., is obtained from thesetransformers. Each transmitter becomes anindependent unit from the substation to theantenna switching station.

Transformer Vault

All high voltage transformers, filter reactors, filtercondensers, and high-voltage controlcomponents are located in three fireproof vaultswithin the building, one for each of the three

transmitters, built into the bay directly behindeach transmitter unit. In the floor plan (Figure 2)the location of the vaults and the majorcomponents of one vault: may he seen.

The high-voltage plate transformer is a 750-kVA,three-phase unit. with a special high speedmotor-operated tap switch, connected in itssecondary winding, which operates under load.The transformer windings and taps are such asto provide variable dc voltages at the load from5500 to 15,000-V in 32 steps. Incorporated in thetransformer is a voltage regulating control, tooperate the tap switch motor, and thus maintainthe voltage at any selected value, regardless ofinput line variations or load on the transformer.Voltage adjustment is accomplished from thetransmitter control panel, by means of a rheostat,which is the series calibrating resistor for thecontact-making voltmeter in the control unit. Theshaft of the rheostat is connected to the panelcontrol through a torque switch, which causesthe tap changer to become instantly responsiveto adjustments of the control. At all other times, ahalf-minute time delay is automatically operative,as the tap changer corrects for line voltagefluctuations. The transformer also returnsautomatically to its lowest voltage tap each timeprimary voltage is removed for periods in excessof a few seconds in order that reapplication ofpower may occur at minimum voltage.

The unregulated three-phase power for the highvoltage system from the substation is fedthrough manual disconnect switches in the.vaults, and then through series line reactors forlimiting fauIt. or rectifier arc-back currents,through a magnetically operated contactor, andthe ignitron interrupter (see' high voltagecontrols) before reaching the tap changingtransformer. Normally, the magnetic contactorremains closed, and the power interruption isaccomplished by the ignitron unit. But if an a-cline overload should occur or a transmitter doorshould be opened, this contactor operates,following the ignitron interruption, to provideadditional protection.

The high-voltage supply filter consists of a .5/2henry reactor. and 20 mfd of 20,000-Vcondensers. Power from filter is distributed toother components in the vault on copper buses.All d-c: power circuits to the r-f section haveseries surge-limiting resistors, and magneticallyoperates isolation switches, which are mountedon the vault walls, The 24-Ohm surge-limiting

resistor for the final amplifier is a bank ofedgewise wound resistors, mounted in a rack.For the lower voltage circuits, standardopenwound resistors are used. The isolationswitches are standard 25,000-V 3-ampere singlepole switches with arcing horns added tominimize burning of the contacts. The bias-power supply consists of a 7.5-kVA transformer,with 2-10 henry 2.5-ampere chokes in the full-tapcircuit. and a .5-ampere 10-henry choke in thehalf-tap circuit. With only the low voltage andbias supplies operating, the a-c power for thelatter is fed through the 240-V breaker panel; butwhen the high voltage power supply isenergized, the power is fed through a 7.5-kVAstep-down transformer from the secondary of the750-kVA plate transformer, which causes thebias voltage to vary directly in proportion to thechanges in the high voltage supply. The lowvoltage supply uses a 5-kVA transformer, andthe same size filter chokes as the bias supply.The dc power from the low voltage and biassupplies also incorporate a set of surge resistorsand standard three-pole isolation switches.

All of the condensers used in the power suppliesare equipped with a special fuse mechanism,which automatically disconnects a faultycondenser from the bank, and places a shortacross it.

Figure 17. The rectifier cabinet.

Rectifier Cabinet

The rectifier cabinet (Figure 17) contains threerectifiers. two of which are low voltage bridgecircuits, one providing bias voltages of 2000 and4000 V, and the other providing plate voltages of1000 and 2000 V. The third rectifier is the highvoltage six-phase single Y circuit, employing six870A tubes, and provides plate voltage for themodulator and final amplifier. Two spare 870tubes are kept continuously heated, making atotal of eight tubes, all of which are mounted ona thermostatically-controlled air duct, withnozzles blowing on their individual mercuryradiators. This duct is supplied from a verticalduct in which are mounted strip heaters and theircontrol switches, and which connects with theoverhead cold air supply. Figure 18. Tube switching panel.

Below the tubes, at the rear of the cabinet,(Figure 18) is a tube switching panel, soarranged as to make switching of spare tubesinto the circuit an error proof procedure. Six ofthe eight tubes are always connected into thecircuit by means of six jumper straps, in pairs ofthree different lengths, no two of which are

identical due to the left or right twist of the topend of each strap. The upper switch jaws justbelow the tubes are mounted at correspondingangles, slanting alternately left and right, andconnecting alternately to cathodes and anodesof all eight tubes.

When a fault occurs, the annunciator light on thefront meter panel indicates which tube isdefective, and it is necessary only to remove thecorresponding jumper strap and insert it into theonly vacant position into which it will fit. Thisarrangement provides one spare tube each forthe parallel cathode side and the parallel anodeside of the circuit. Below this switching panel aremounted the high-leakage reactance-typefilament transformers for the 870 tubes, whichautomatically limit starting filament current.

Tube CooIing

The heat dissipation system consists of aseparate blown-air and water cooling system foreach transmitter. The fan and pumpingequipment for each unit is housed in the fanroom directly behind the unit. Each system is.designed to dissipate 350 kW of electrical power,and provide for removal of waste heat in thetransmitter cabinets and high velocity air forcooling the mercury radiators on the high voltagerectifier tubes; as welt cooling water to theignitron. units. The system is also arranged tofurnish byproduct ventilation of the building insummer, and heating of the building in winter.Auxiliary steam coils are installed in connectionwith the air units, to operate automatically whenthere is insufficient waste electrical heat foradequate building heating.

Each fan room .is equipped witb two double-inlet, double-width centrifugal fans, individuallymotor driven. The fan suction plenum isconnected through dampers to a filtered outdoorair plenum. to the building concourse, and to arecirculated warm air bypass. The fansdischarge the air through four, finned copper,distilled water cooling coils, from where theheated air is wasted to atmosphere or deliveredto the building for recirculation. Forced airventilation for the transmitter cabinets andignitron cooling system is taken directly from thefan discharge plenum which is maintained atconstant pressure. An evaporative coolingsection is provided for the ignitron unit to supplywater at lower temperature for this unit. Theremainder of the system utilizes dry air cooling to

obviate maintenance of coil surfaces andattendant spray water nuisances.

Each transmitter is equipped with a separatedistilled-water circulating system. The pump is ofthe horizontally split case type, direct connectedto a 1750-rpm motor. The pump takes its suctionfrom an open surge tank and discharges thewater through the tinned cooling coils previouslymentioned. From here it goes to a distributingsupply header below the transmitter cabinets.where it is fed through individual circuits to thevarious tube jackets and coils. The heated wateris then collected in a similar supply header whichcarries it back to the surge tank, which serves asan air release and automatic make-up chamber.A similar system of much smaller capacity isused for the ignitron system.

Water is distilled in a 50-gallon-per-hour capacitystill in the boiler room and is pumped from thereto a gravity tank in the tower, then to the varioussurge tanks. The still is operated by steam fromthe heating boilers.

A complete pneumatic control system operatesthe various dampers in the blown-air system,from a master panel housed in the main controlcenter at the rear of each transmitter unit. Flowmeters, pressure gauges and thermometers areconveniently located on the slanting front edge ofthe transmitter catwalk, so the operator caninstantly determine the water flow and air andwater temperature at critical points in eachcircuit.

Since the complete transmitter unit is composedof two r-f sections, one power supply and onemodulator, the control circuit had to be arrangedto provide for individual or simultaneousoperation of the r-f sections. As it is set up, eitherr-f section can be operated as an individualtransmitter, or both can be operated from thecommon power supply and modulator. In case ofa fault or tube overload when both units are inoperation, automatic isolation of the units isprovided, to minimize loss of air time of the unitin operating condition.

The heart of the control system is located in therectifier cabinet, and each function in theoperation is initiated from the push button on thefront control panel. The relay panel contains theprimary-control relays plus the annunciatorrelays. On the meter panel at the top of therectifier cabinet. a set of 50 annunciator lights

indicate open transmitter or vault doors, and theoperation of overload relays, each of which hasan associated annunciator relay, which locks upafter a fault occurs, lighting the correspondingannunciator light until a reset button is pushed.Another button closes all annunciator relays totest for burned out lights.

For checking faults in the control system. aseries of 40 small 1/25-watt neon lights areprovided on the annunciator relay chassis, eachof which is connected across a particular set ofoverload relay contacts, door interlocks, orcontrol relay contacts, so that in normaloperation none of the lights would be lit; but, if afault occurs, the neon light number, inconjunction with a chart, tells which circuit isopen. In turn, each overload relay has a neonlight across its contacts, so the exact location ofthe trouble can be determined quickly.

This same equipment is duplicated in a dummyrectifier cabinet in the companion transmitter onthe opposite side of the concourse, bothoperating with a common power supply andmodulator.

All overload and arc-back relays, with theexception of the high-current delayed a-coverloads, are. dc telephone type relays, withspecial coil and contact insulation. The coil of therelay is in series with a current adjustingrheostat, and this combination is shunted byanother resistance of the proper value for theprotected circuit. This is a very flexible andsatisfactory means of overload protection.Because of high operating speed. this type ofrelay gives better protection than the standardoverload device. In eases where the coil must beat a high potential to ground, such as for arc-back protection of the rectifier tubes, thecontacts are removed from the frame andoperated by an insulating rod.

All 240-V power is fed through a breaker panel inthe wall cabinet behind the transmitters, andthen through contactors which are operated byrelays of the control circuit.

Filament Control

Conventional water flow and water temperatureprotection are provided in the filament circuits.Distilled water pumps, fans and filaments arecontrolled by the first pair of push buttons on thepanel. A time delay provision is incorporated, to

continue operation of pumps and fans until 15minutes after filament shutdown. In addition,filament switches are provided for individualwater-cooled tubes or banks of tubes (dependingon number of tubes in series on one watercircuit), which permit changing of tubes withoutturning off all tube filaments. These switchescontrol the primary power contactor, and shortout the water flow interlocks associated witheach particular tube or bank of tubes. The circuitis so arranged that turning on the filaments of theseparate r-f section also turns on the filaments ofthe main transmitter, since the modulator andrectifier are needed for its operation. Conversely.turtling off the filaments of the main transmitterturns off the filaments of both units. Figure 19. Elements composing the antenna switch. They

are the same diameter and spacing as pipe conductorsconnected to switch.

Low Voltage and Bias Control

The second pair of push buttons provide controlfor the low voltage and bias systems. Theinterlock system is divided into three sections,one of which contains the door interlock switchesof the rectifier, modulator, catwalk andtransformer vault, and the overload relays.contacts associated with these units, In serieswith this are the interlocks and overload relaycontacts for each of the r-f sections, Anemergency off push button is located in thecenter of the control panel of each cabinet.These are also in series with the interlocks.Magnetically controlled isolation switches areused to remove the d-c voltage from an r-fsection when not in use, or if a failure in oneshould occur while both are on the air. Theseswitches are designed to break circuits with thepower on. The a-c power for the bias supply isfed through the same contactor that supplies thelow voltage power, when the low and biasvoltage supplies are first turned on. However,when high voltage is turned on, the bias power isobtained as described in the discussion of thetransformer vault.

Bias switching is accomplished with a fast-operating reversing contactor, mechanicallyinterlocked to prevent an interconnection of thepower sources.

High Voltage Controls

All unusual feature of this system is the anti-pumping circuit. Should a fault occur while the onpush button is held down. the power supplyinterrupter has only one closure, and will notreclose until the button is released and pushedagain.

D-c voltage to both the r-f driver and finalamplifier is fed through individual magneticallyoperated high-voltage isolation switches. Thecontrol of these switches is arranged to operateonly when the d-c voltage is off. Thus. if one r-fsection is turned on while the other is operating,the power supply shuts down long enough forswitching to take place, then automaticallycomes on again. The same occurs if one of theunits is turned off, or if an overload occurs ineither section .To facilitate neutralizing of thefinal amplifier, a switch is provided on the relaypanel of this cabinet for individual control of theisolation switch, thus removing high voltage fromthe unit. and permitting it to be applied to allother units.

Figure 20. A view of one of the transformer vaults.

The Ignitron

Another novel feature of this installation is theuse of the ignitron as the 2300-V high-speedcircuit breaker. The overload-relays protectingequipment connected with the high voltagesupply have one set of contacts in aconventional interlock circuit, which serves toremove power supplying the thyratrons whichignite the ignitrons. A second set serves to biasthe thyratrons in advance of removing power, sothat if an overload occurs, there is all . almostinstantaneous interruption of power; many timesfaster than with conventional a-c relays. Thecontrol system is, of course, tripped by the firstset of contacts when a fault occurs, andinterrupts the control voltage for the ignitron.Thus, two means have been included to insureshutdown of the high voltage supply, making ahigh-speed circuit that operates in 3 to 10milliseconds after the overload relay operates,depending on the portion of the cycle theignitron is passing at the time. The average timeof the telephone-type overload relay used isabout 5 milliseconds. Thus the overall time forinterruption of power is only a fraction of thatrequired for the conventional overload relay andair or oil-circuit breaker.

The high-voltage tap changer remote rheostat,with its associated torque switch and front panelcontrol, as previously explained, is duplicated inthe “dummy" cabinet on the opposite side of theconcourse. Associated with this control on bothmain and dummy rectifier cabinets is a push-button to switch the tap changer control circuitsfrom one rheostat to the other.

Automatic Reset

The fourth pair of push buttons on the controlpane! operate the automatic recloser circuit.When on. this circuit automatically reenergizesthe power supply after an overload occurs,should a second overload occur, or the first faultstill exist, within fifteen seconds, that particular r-fsection in which the fault took place is isolatedand the remainder of the transmitter turned onagain. If no further trouble develops within thatfifteen-second period, the system resets itselfand is ready to operate again.

Carrier Alarm

A relav in the cathode circuit of one of the filialpower amplifier tubes operates the carrier-alarmsystem. This relay is set so that it will pull inunder normal operating conditions. but drop outif excitation or plate voltage should fail. And inconjunction with another relay, it operates thealarm, a warning light above each transmitterand a horn common to all six final amplifiers. If atransmitter goes off the air, the carrier alarm canbe switched off temporarily, but becomeseffective each time the transmitter is turned on.

Personnel Protection

For the safety of personnel, all equipmentoperating at high voltage is completely enclosedand interlocked. Each transmitter cabinet has asolenoid-operated shorting mechanism whichgrrounds all high voltage terminals in the cabinetwhen any cabinet door is opened. This is locatedin the upper part of the cabinet where it is easilyvisible from both front and rear. Catwalk grillsand doors are interlocked to operate the shortingmechanism in the rectifier cabinet.

The high voltage components in the transformervault are enclosed with wire fencing, Before thissection of the vault can be entered. a lever mustbe thrown which operates disconnect andgrounding switches, and a mechanical interlockon the cage and ignitron doors.

Figure 21. An interior view of the antenna service car. Equipment in this car includes two-way FM communicationsequipment , affording constant contact with the control room.

Transmission Lines

The transmission lines from the transmitters tothe main antenna switching station are all of 300-Ohm surge impedance. The section leading outof the building is of 2" copper pipe in order to beself-supporting across 18' driveway clearance,and to present good corona conditions throughthe glass entrance pane. These pipes arespaced 13". From this point to the mainswitching station. the line consists of a 4-wireconstruction, using two pairs of 1/0 copper weld(each pair as one conductor). spaced 2-1/8" oneabove the other, and 7" between pairs, which isthe same spacing as used in the 300-Ohmswitch structure, where two I" copper pipes areused for each line. To maintain spacing betweenwires on each side of the line, cast clamps areused at intervals of about 10'.

To meet the requirements of multiple switchingof antennas and transmitters at 200-kW powers,the following specifications had to be met in thedesign of the main antenna switching station: (1 )- Provision for. connecting any transmittertransmission line to any antenna transmissionline. (2)- Provision for unlimited expansion of numberof transmitter and antenna transmission lines. (3)- Maintenance of 300-Ohm impedancethroughout, regardless of switching operations. (4)- Elimination of all dead-end effects. (5)- Provision for breaking up all unused lines toprevent possibility of resonance. (6)- Simplicity of operation and identification forscheduled switching.

These requirements were met by designing astructure carrying incoming transmittertransmission lines on a lower level, and outgoingantenna transmission lines on a higher level, atright angles with the incoming lines. Betweenthese two levels are vertical spiral. riser lines,terminating top and bottom with double-pole,double-throw switches. On short posts near theground are operating cranks, which operate theswitches through mechanical linkage.

Electrically, all lines continue through thestructure as long as. all switches are inhorizontal position. Operation of a lower switchbreaks the line at that point, and connects thetransmitter with its vertical riser. Operation of theassociated upper switch breaks the antenna lineat this point and connects the antenna to theriser, thus completing the circuit from transmitterto antenna No unused lines remain connectedto the system to cause dead end effects. Theunused lines which have been disconnected maybe further broken up into shorter sections, ifdesired, by operating other switches beyond thepoint where the connection was made, in orderto prevent any possible resonant effect.

As seen in Figure 19, the elements composingthe switch are of the same diameter. and.spacing as the pipe conductors connected to theswitch, providing constant impedance throughoutthe structure.

No limitations are imposed on possible futureexpansion, since it is necessary only to addmore lines and switches as transmitters orantennas are addled.

Matching Section

Immediately upon leaving the main antenna-switching station, impedance matching sectionsarc inserted between the 300-Ohm gear and the500-Ohm antenna transmission lines. Thesesections comprise 3 lines, each one-quarterwave long in tandem, varied in spacing; andconductor size to accomplish the transition. Twotypes are used: type A sections have a designfrequency of 12.3 mc, and are connected to linesserving both large and small antennas requiringmaximum frequency range of 6 to 18 mc; type Bsections are designed at 13 mc. and are used inconnection with lines serving antennas operatedbetween 7.5 and 15.5 mc.

Transmission lines of 500 Ohms from these

impedance matching sections to the antennasare, in some cases, as long as 3750'. To keeploss at a minimum consistent with cost and useof critical materials, 4-wire construction wasused. consisting of two pairs of number 2 copperweld wire, each pair spaced 3/4" as oneconductor, with 20" spacing between pairs. andwith all four wires in the same horizontal plane.As in the case of the lines leading from thetransmitter building, clamps maintain the 3/4"spacing.

Lines run at about 15' above ground in mostplaces. Turns are accomplished by verticalspiraled jumpers to maintain equal conductorlength and spacing. Where more than onetransmission line is carried by the same poleline, a spacing of better than four times the 20"conductor spacing is maintained, to keepcoupling at a reasonable minimum.

Figure 22. Water controls on front ledge of transmittercatwalk.

Line Switches

The 24 antennas are arranged in 9 groups, 6 ofthese containing 3 antennas each, the other 3containing only 2 each. Each group is fed by apair of transmission lines. In the case of the two-antenna groups, lines run directly from theswitching station to the antennas; but in each ofthe 6 triple antenna groups, one transmissionline runs from the switching station to the middleantenna, while the other is arranged to beswitched between the small and the large antennas, since these two are never usedsimultaneously. A special double-pole, double-throw switch is located along the transmissionline near the antennas, so constructed as tohave 500-Ohm surge impedance. Thus nodisturbance is introduced into the transmissionsystem. The switch itself is located attransmission line level, and is operated through a

mechanical linkage system by a lever at waistheight.

Reentrant Rhombic Antennas

Because of relative ease and low cost ofconstruction, amount of critical material required,and satisfactory performance over a rather widefrequency range. rhombic type antennas areused for the 24 radiators. Three sizes cover thefrequency range of 6 to 18 mc. with adjacent sizepairs being used simultaneously.

Figure 23. Rhombic antenna, showing reentrant feature andstub-tuning arrangement.

These three' sizes are designed at mid-frequencies of 16.33 mc. 11.67 mc, and 9.33 mc,with a low vertical angle of approximately 10and a beam width of 20 at these frequencies.Antennas are four wavelengths long on eachside, with a tilt angle of 70, and an averageelevation of 1.4 wavelengths above ground.They are constructed of three number 6 copperweld wires on each side, spaced at the sidepoles, and converging to a point at each end.Due to low inherent coupling between adjacentrhombic antennas, common support poles wereused between antennas. Each pole is actuallytwo poles, spliced butt-to-butt with a steel sleeve.Poles rest in a 6" recess in a concrete base, andare guyed at three points.

In order to avoid wasting up to 50% of the powerdelivered to the antenna, by the conventionaldissipative rhombic termination method.reentrant transmission lines have beenincorporated, whereby the normally dissipatedpower is returned to the input line, properlyphased and adjusted as to voltage magnitude,

through the use of stub lines of the proper valuesand spacings along the return line. Impedance ofthe input line is corrected in a like manner, and insome cases combined with one of the reentrantstub lines. All stubs of the shorted variety aregrounded at the midpoint of the short to providestatic drain, and lightning protection.

Operation

Transmitters are scheduled in pairs, so that, asone frequency is becoming less effective in achosen coverage area, the other is already inoperation on the frequency next coming intomaximum effectiveness.

To change frequency, two men are required tomake the necessary technical adjustments in theshort period allowed (usually 15 minutes). Oneman re-tunes the transmitter, and the othermakes the necessary antenna changes andadjustments. The control operator shuts downthe transmitter after the scheduled sign off, anddisconnects the frequency generator which hadbeen driving the transmitter, connecting it to theproper generator to supply the new frequency.This change is made on the r-f generatorpatching panel. Then he moves the dials on thetransmitter unit to pre-determined settings. orchanges pre-tuned coil condenser circuits asrequired for each stage of the transmitter,beginning with the lower power stages, andending with the high power final stage and theadjustable antenna coupling loop.

While these adjustments are being made at thetransmitter, the second man proceeds, in anantenna-service car, to the main switchingstation, where he connects the proper transmittertransmission line with the proper antennatransmission line. Then he proceeds to theantenna group. and connects the transmissionline to the desired antennas through the lineswitches previously described. Finally, theproper stubs are inserted by use of switches, toadjust the antenna for its assigned operatingfrequency.

Upon completion of the necessary adjustments,he advises the transmitter operator, who is thenready to put the transmitter on the air on its newfrequency.

Antenna Service Car

The car which is used for these operations is

especially designed and equipped for antennawork. Its equipment includes a complete two-wayf-m communications set, with which thetechnician is in constant communication with thecontrol room.

The f-m receiver is just one part of the completeintercom system, which also includes telephonesand public address systems. The combination ofthe three affords a complete and flexible meansof communication between personnel anywhereon the property.

The public address system employs permanent-magnet speakers which serve both as speakersand microphones. These are located throughoutthe building; and at the main antenna switchingstation at the rear of the building. On theconcourse, speakers are concealed above thetransmitter units, with a switch key on the handrail. At each of these positions, a key is alsoprovided to mute the monitor to preventinterference with the public address speakers.When one of the concourse speakers is used asa microphone, the other five are silencedautomatically by a relay, to avoid feed-back. Thesame system is used at the switching station,where there are six speakers.

The f-m system is tied into the public addresssystem in such a way that any calls picked up onthe receiver are fed directly into the publiaddress line. Through relay control, the f-mtransmitter may be operated from the publicaddress station at the control desk. The f-msystem is used to maintain contact with theoperator of the antenna service car, and withmaintenance crews in the field.

Acknowledgments

The inception and realization of this project wasmade possible largely through the confidence,cooperation and active assistance of J. D.Shouse, vice president in charge ofbroadcasting, The Crosley Corporation; J.O.Weldon, chief of the Communication FacilitiesBureau, OWI; and .E. J. Boos. businessmanager, broadcast division, The CrosleyCorporation.

It would also not have been possible to completean undertaking of this size without the untiringefforts and close cooperation of the entirebroadcast engineering staff, as well as outsideengineering organizations, who contributed

materially to the project, in spite of facilitiesalready overloaded with war contracts.

Acknowledgment is hereby gratefully extendedto the engineering staff, particularly: William S. Alberts, chief propagation engineerJames A. Baysore, supervisor, antennaconstruction L. G. Bertenshaw, purchaser and expediter Calvin C. Bopp, design and development engineer D. H. Ehlman, supervisor, machine shop J. L. Hollis. chief design engineer E. F. Jenkins, equipment test engineer P. J. Konkle, chief management engineer Floyd N. Lanser, chief transmitter engineer H. Lepple, design and development engineer J. M. McDonald. assistant engineering director W. A. Moore, chief design draftsman W. B. Nester, design draftsman C. A. Robson, design draftsman R. L.. Schenck, design and developmentengineer C. B. Sloan, design and development engineer R. H. Uphouse, design and developmentengineer P. A. Young, propagation engineer

Reprinted from November and December 1944Communications Magazine