IEEE TRANSACTIONS ON BROADCASTING, VOL. 34, NO. 2, JUNE 1988 147

The Evaluation of 500 kW Shortwave Transmitters at the Voice of America

Gerald A. Berman and Thomas R. Garlington Voice of America 601 D St., N.W.

Washington, Dc 20547

ABSTRACT

The Voice of America (VOA) has initiated a major program to modernize and expand its worldwide broadcast system. The program includes the procurement of 500 kW shortwave transmitters both for new stations and as replacements for aging and less efficient equipnent in existing facilities.

To develop and verify a specification for new transmitters, four currently available 500 kW transmitters were purchased from separate vendors for evaluation at the VOA facility in Greenville, North Carolina. The intent of this "off-the-shelf" approach was to ensure a proper level of technical rigor in the specification without unduly restricting competition among suppliers. The evaluation focused on a variety of factors including installation, performance, operability, and maintainability. This paper describes the procedures followed during the evaluation and presents some of the salient findings.

INaEoDUCTION

The Voice of America has enbarked on a multi-year billion dollar modernization program to upgrade existing transmitters and antennas , construct new sites, and examine state-of-the-art methods of radio transmission. policy guidelines to complete this enormous task espouse the application of demonstrated design principles, standardization of reliable and affordable equipment, and allowance for operational flexibility. To facilitate sound judgments in the selection of equipment and services, technical specifications must be formulated for various functional subsystems of a station. Thus, much effort has been devoted to providing a solid basis for technical specifications in terms of experience and current awareness.

Faced with the need to procure a large nuber of new 500 kW shortwave transmitters, VOA sought to develop a performance specification document that defined a precise standard of quality but was not so stringent as to exclude competition from qualified bidders or involve open-ended costs of custom-made equipment. To gain input for the specification, several briefings and meetings were held with transmitter manufacturers and others to seek technical information, examine products, and explore production capabilities. As a means for providing feedback a preliminary specification was issued for comnents. An evaluation project also was conceived as a means for obtaining firsthand experience and data for refining and validating the specification. The project was initiated in March 1984, with the issuance of a request for proposals for currently available 500 kW shortwave transmitters. In January 1985, four firms were awarded contracts to install one of their stock item transmitters. The firms were AEG, Brown Boveri & Company, Continental Electronics Company, and Marconi. To ensure an impartial setting, free of cmrcial pressures and extraneous influences, the evaluation was conducted in an operational environment at VOA's Edward R. Murrow Transmitting Station in Greenville, North Carolina. The station had space available to accomnodate the transmitters (two at each of the transmitting sites associated with the station) and the antennas and primary power to handle the load,

with minimal disruption to normal station activities. It should be emphasized that the evaluation was conducted to obtain information to formulate and validate a specification; it was not intended as a "run-off" contest to establish a winner. Cklce the evaluation was completed, these four transmitters entered into regular service relaying VOA broadcasts.

The evaluation was scheduled as a 12 month effort starting immediately after transmitter commissioning and acceptance. Numerous construction and comnissioning delays compressed the schedule into eight months. It should be noted that, while the transmitters were "off-the-shelf" models capable of producing 500 kW carrier output over the shortwave broadcast bands, their specific designs, features and architectural and component configurations differed markedly. All used some sort of pulse modulation which, for three, was accomplished using a switching tube. The fourth used a switching power supply. All reflected the latest state-of-the-art in control and monitoring systems. Appendix A smrizes specific details of each transmitter.

The scope of the evaluation included preparing an evaluation plan and data collection procedures, collecting data, reducing and analyzing data, and preparing reports and briefings for teams within VOA responsible for formulating the final specification for future transmitters.

EVALUATION CONCEPT

The approach taken in the evaluation was to use four mutually independent activities for gathering broad-based data that would be applicable to various portions of the specification. These activities were an analysis of the transmitter installations, an analysis of ancillary equipnent, an expert panel evaluation, and the conduct of formal evaluation procedures involving inspections, examinations and measurements.

The data collected from the various activities overlapped sonewhat and ranged from objective quantitative measurements to subjective responses of operating staff to questionnaires. The sections that follow describe the evaluation activities and the nature of the resulting data.

Transmitter Installations

The U.S. Army Corps of Engineers (CE) was retained by VOA to monitor the installation of the transmitters from initial site preparation through the arrival of shipping crates , transmitter assenbly, and commissioning. Inspectors from the CE mintained notes and daily logs detailing the progress of each installation, noting problems and successes alike. They monitored a variety of items including the techniques used by the manufacturers' field staff in layout and assenbly of the equipment, both indoors and out, the quality of material and workmanship, and the sequencing of the installation as cownents and subsystems were received from the factory. Based on its observations, the CE made recmndations for the specification and acquisition process.

0018-9316/88/0600-0147$01.~ 0 1988 IEEE

148

Ancillary Equipment

Holmes and Narver, Inc., an architect/engineering firm under contract to the VOA to produce a prototypical design of future stations, performed a site investigation that focused on ancillary support equipment, system, and components from the incoming high voltage power supply to the FtF output of the transmitter. With concern for issues related to design , safety , reliability , operability , and maintainability, they addressed construction techniques of the turnkey installation, circuit breakers, transformers, motors, controllers, cabling, piping, valves, pumps, ductwork and fans. They also made recommendations for the specification and acquisition process.

Expert Panel Evaluation

A panel of four experts from VOA, having over 80 years of collective experience in transmitter design, operation, and maintenance spent approximately one month examining and observing the operation and maintenance, of the four transmitters. Their evaluation was guided by a comprehensive checklist that addressed topics ranging from workmanship to electronic and mechanical design. They identified advantages and disadvantages of the different designs and equipment realizations, and assessed each transmitter's operation, maintenance and reliability. They translated their findings into recommendations applicable to the transmitter specification and acquisition process.

Evaluation Procedures

Develoyt. To obtain uniform objective data from all four transmitters a series of formal evaluation procedures was established in five mjor categories selected to encompass the majority of transmitter characteristics. The categories were performance, operability, maintainability, safety, and training program effectiveness. The procedures within each category consisted of detailed step-by-step actions that involved observing, inspecting, and measuring various transmitter parameters.

Developing the procedures represented a significant effort because compromises had to be made between activities that could be conducted within the constraints of time, instrumentation requirements and budget, and the relative importance, quality and precision of the data that could be derived from the effort. For example, destructive tests or intentional perturbations to the primary mains power supply were eschewed.

Table I shows the evaluation procedures that were addressed under the five categories. The performance procedures measured inherent characteristics of the transmitters such as efficiency, frequency response and linearity, and spurious output. The operability procedures evaluated the transmitters' controls and indicators, including the extent of remote control and automation capability, the quality of operating

1 While not a direct participant in the evaluation activities in Greenville, Thomson-CSF conducted several measurement procedures on its TRE 2355 500 kW HF transmitter at its factory in Gennevilliers, France. Because VOA did not witness this testing, the results were not included in the data base of the evaluation report. However, the Thomson data was considered in the validation of the VOA's transmitter specification.

instructions, and the ease and time required for executing in-band and between band frequency changes. The maintenance procedures dealt with the skills, tools, and test equipnent needed to diagnose and repair faults and sustain the transmitters in good operating condition. The safety procedures examined the features of each system that concern accident prevention and hazards to the health of operating and maintenance staff. Training support assessed the effectiveness of the manufacturers' operator training programs.

TABLE I

SYSTEM CATEGORIES AND

CORRESPONDING EVALUATION PROCEDURES

Performance

Overall Efficiency Af Response Af Distortion Signal-to-noise Ratio Af Modulation Linearity Carrier Amplitude Shift Rf Phase Distortion Harmonic And Spurious Radiation Rf Stability Second Source Power Tube Backup Battery Power Req'mts Variable Carrier Power Water Purity Autotune Characteristics Power Line Disturbances Total Harmonic Distoltion AC Mains

Remote Control And Monitoring Autotune Time Start/Restart Time

Maintainabilitv

Maintenance Operations Built-in Test Equipment Accessibility, Lighting 8 Outlets Scheduled Maint-enanie Req'mts Mean Time Before Failure Mean Time To Repair Spares Determination Special Tools

Safetv

Interlock System Acoustic Noise Levels System Grounding Warning Labels Ionizing And Non-ionizing Rad. Toxic Substances

Visual Aids Lectures Documentation Schedule Requirements

All of the evaluation procedures were written in a standard prescriptive format to ensure that uniform data would be collected regardless of the transmitter under evaluation or the individual performing the tests. The format included a statement of the test purpose, the definition of the parameters to be measured, and a list of conditions to be controlled such as carrier frequency, modulation depths, power level, etc. The test equipment was described both in the text and by means of an interconnect diagram. Mathematical formulas were given whenever it was required to calculate a parameter from raw data, and finally a data sheet was provided for recording the raw measurements.

Although most of the procedures were designed to be consistent with standards of the Electronics Industries Association (EIA) and the European Conference of Postal and Telecommunications Administrations (CEPT), as well as recommendations of the International Radio Consultative Committee (CCIR), some special tests also were performed. One example was an evaluation of "variable carrier power" (VCP), a type of amplitude modulation in which, for a given sideband power, the average power of the carrier is less than with standard AM. Therefore, transmitter operation using a VCP mode offers a potential for

149

substantial savings in electric power consumption. TWO of the transmitters were equipped with VCP options. The electric energy consumption during operation of these transmitters over a fixed interval was measured in both the VCP and standard AM modes. At the same time, the test program signals originating from the transmitters were monitored at a remote site to compare the quality of received VCP signals to received AM.

Another special test involved replacing the standard qipp"d final RF power tube of one of the transmitters with an equivalent from another vendor. A goal of the VOA's transmitter procurement is to avoid relying on components that are manufactured by a sole source. merefore, the operation and performance characteristics of that transmitter with the substitute tube were evaluated to determine whether another manufacturer could supply a satisfactory replacement.

Data Collection and Analysis. Figure 1 illustrates the signal interfaces between a transmitter system, the test equipnent, and a microcomputer-based controller. The computer contained spreadsheet software that allowed for analyzing and displaying data in a report format.

FIGURE 1 DATA COLLECTION AND ANALYSIS

Transmitter System

--

instruments

Dummy 471 Inlet Tern p

Outlet Temp

Coolant Flow Rate

External RF Exciter

IEEE 488 Control Bus instrument Control and Data Acquisition

I -- e -- ~

Data Anaiysls and Display

The transmitters included the following components and interfaces:

A balun to convert from an unbalanced to a balanced output impedance (An exception was the AEG transmitter which used an unbalanced dummy load that was switched into the transmitter output before the balun. ) ;

An instrumented dunany load (Each manufacturer supplied a calibrated d m y load with its transmitter to facilitate commissioning and testing. 1;

A 4160 V three-phase AC primary power supply;

Audio input terminals for either test signals or program material;

An RF input connection that permitted an external source of RF excitation to drive the transmitter:

A test connection to sample the transmitter's RF output signal (Each manufacturer calibrated its test connection); and

A standard data interface to permit control and monitoring of transmitter operation from a remote terminal.

In addition, each transmitter had its standard complement of front panel operating controls, meters, status indicators, alarms, and interlocks.

The test instrumentation used during the evaluation was supplied by VOA and consisted of modern laboratory precision instruments that were purchased new to support the effort. When available, an instrument included the option for microprocessor control of data collection. An IEEE-488 control bus supported the automatic instruments. Before the start of a performance evaluation, procedure self-tests were run on several instruments with automatic calibration capabilities.

During the data collection phase of a given procedure, a "quick-look" analysis was performed to ensure the "completeness" and "reasonableness" of the data, particularly to determine if there were anomalies that might dictate further investigation (e.g., a rerun of some portion of that procedure). Most of the testing was accomplished by relay station staff during normal daytime working hours. However, because many of the procedures took long periods of time to complete, particularly those conducted at a variety of carrier frequencies and modulatinq frequencies and depths, flexibility in the testing schedule and times was required. Complicating matters was the fact that some procedures had to be conducted when the ambient RF level at the station was low, usually at odd hours. Testing staff and RF specalists from VOA engineering headquarters in Washington provided oversight and assistance in the testing. When necessary further testing assistance was provided by contractor staff from the MITRE Corporation which played a major role in reducing and analyzing the data.

The principal data analysis tool was an IBM PC computer equipped with word processing, spreadsheet data manipulation, and graphics software. The IBM PC was used to organize and manipulate much of the data and was essential to their rmid and efficient processing.

As opposed to the procedures that required direct measurements from dedicated instruments, the demonstration and inspection procedures involved the observation of personnel and transmitter responses to various controlled inputs. The data from these procedures were in the form of completed checklists, written comments, and subjective observations. In these procedures, the analysis was usually a comparison of such results with national and international standards and codes.

DESCRIPTIONS OF SELECTED PROCEDURES AND RELATED DATA

Each of the four transmitters was subjected to the battery of evaluation procedures identified in Table I. Space does not permit a discussion of each; however, those described in this section are included because they involve fundamentally important characteristics of high-power transmitters.

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Overall Transmitter Efficiency

The purpose of this procedure was to characterize transmitter efficiency at half and full power operation. "Transmitter Efficiency" was defined as the ratio of RF power output consumed by a dunnny load, to the mains power input to the transmitter and its auxiliary subsystems of coolant pumps , heat exchangers, and air handling equipment. Efficiency was determined using a 1000 Hz modulating tone at modulation depths of O%, 25%, 50%, 85% and 95% at each of the representative carrier frequencies from the 11 shortwave broadcast bands in Table 11.

TABLE II

TEST CARRIER FREQUENCIES

SW Broadcast Band (Meters)

Carrier Frequency (MHz)

75 60 49 41 31 25 21 19 16 13 11

3.975 4.905 6.075 7.200 9.700 11.850 13.700 15,350 17.712 21.650 25.885

RF output power was determined calorimetrically. The flow rate of coolant through the load as well as the inlet and outlet temperatures of the coolant were measured with instruments supplied with the dumy load. Output power, Po, was calculated using the formula, Po= kQ AT, where k is a calorimetric coefficient specific to each dummy load, Q is the coolant flow rate in litres per minute, and AT is the difference in degrees Celsius.

Transmitter efficiency, 9 , was calculated using the formula, v = PdPi x loo%, where Pi was the total AC power into the transmitter. A power analyzer was used to determine this power.

Approximately 450 data points were collected for this procedure to characterize the "efficiency" of the transmitters. Figure 2 shows the efficiency range of data from all four transmitters. Based on these results a minimum efficiency of 65% at 500 kW was specified for future VOA transmitters.

FIGURE 2 EFFICIENCY RANGE 500 KW

ALL TRANSMITTERS ALL MODULATION LEVELS 90

- 80 3 - 6 6

70 U U. Lu

60

50

MAXIMUM

3

2 6 10 14 18 22 26 FREQUENCY (MHz)

Audio Frequency Response

The purpose of this procedure was to characterize audio frequency response and bandwidth at half and full power operation. "Audio Frequency Response" was defined as the ratio of the demodulated transmitter output to the reference output level at 1000 Hz, expressed in decibels. The frequency response was determined for 50% and 95% modulation respectively at each of the representative carrier frequencies from the 11 shortwave broadcast bands in-Table 11.

The oscillator of an audio analyzer provided audio input to the transmitter. The transmitter's output was sensed and routed to the RF input of a measuring receiver, which measured the modulation depth of the negative peak and detected the test tone. Tne output of the measuring receiver was connected to the audio analyzer which measured signal level.

The level of a 1000 Hz audio input signal was adjusted to attain a modulation depth of 50% and 95% respectively as measured on the receiver. This was the reference level to which each audio input test frequency was set by control software that automatically conducted the test over the frequency range of 40 to 10,000 Hz.

Approximtely 2200 data points were collected under this procedure to characterize the "bandwidth" of the transmitters. Figure 3 shows the average response for all four transmitters at 50% modulation. Based on these results the audio frequency response was specified as _t 1dB from 50 to 5000 Hz and +1, -2dB from 5000 to 7500 Hz.

FIGURE 3

AUDIO FREQUENCY RESPONSE 500KW 5

AVERAGE OF ALL DATA 50% MODULATION

- 4 - 3 1

I

40 60 80 100 200 400 600 800 1K 2K 4K 6K8KIOK

AUDIO FREQUENCY (Hz)

Audio Frequency Linearity

The purpose of this procedure was to characterize the degree of modulation linearity at half and full F e r operation. In other words, whether equal increments in audio input level result in equal increments in modulation depth. "Audio Frequency Linearity" was determined from the variation of the transmitter's output level and modulation depth for

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9 -

ap 8 -

t m 7 - U1 2 6 -

$ 5 -

3 3 -

5 4 - z

2 -

1 -

audio frequencies of 100 Hz, 1 kHz, and 4 kHz at each of the representative carrier frequencies from the shortwave broadcast bands in Table 11.

0 P o_ - 2 -

g - 3 + -6:

2 - 1 0 -

5 - '-14-

-18-

- 2 2 -

The test setup for this procedure was the same as in the procedure for audio frequency response. The oscillator of an audio analyzer provided audio input to the transmitter. The transmitter's output was sensed and routed to the RF input of a measuring receiver, which measured the modulation percentages of the positive and negative peaks and detected the test tone. The output of the measuring receiver was connected to the audio analyzer which measured signal level.

The level of the audio input was adjusted to attain a modulation depth of 70% as measured on the receiver. This was the reference level from which the input signal level was varied.

As in the bandwidth procedure approximately, 2200 data points were collected to characterize the "linearity" of the transmitters. Figure 4 shows the average data from all four transmitters. Based on these results the specification limit for the deviation from linearity was set at 1dB with a maximum of 5% difference between positive and negative modulation percentages.

- .

FIGURE 4

MODULATION LINEARITY 500 KW AVERAGE OF ALL DATA

6

......

- 2 6 1 , I , I , I I I 1 I

- 2 6 - 2 2 -18 - 1 4 -10 - 6 - 2 0 2 6

INPUT VOLTAGE (dB)

Carrier Amplitude Shift

The purpose of this procedure was to characterize the deviation of carrier amplitude from its umdulated value when single-tone modulation was applied. The procedure was conducted at half and full parer operation. "Carrier shift", 6 , is defined as,

6 = (1 - E d E c u ) X 100%

where Ea is the unmodulated carrier amplitude in volts and E, is the carrier amplitude in volts when single-tone modulation is applied. The carrier shift was determined for 95% modulation at each of the representative carrier frequencies from the 11 shortwave broadcast bands in Table 11.

The oscillator of an audio analyzer provided an audio input to the transmitter. The transmitter's output was sensed and fed to a power splitter from which the signal branched to the RF inputs of a measuring receiver and a spectrum analyzer. The receiver measured the modulation depth of the negative peak and the spectrum analyzer measured carrier level.

The level of a 1000 Hz audio input signal was adjusted to attain a modulation depth of 95% as measured by the receiver, and the carrier level was noted from the display of the spectrum analyzer. Then the input signal was switched off and the change in carrier level recorded. The sunnnary data presented in Figure 5 are averages over all four of the transmitters evaluated. Based on these results the specification limit for carrier shift was set at 5%.

FIGURE 5

CARRIER AMPLITUDE SHIFT AVERAGE OF ALL TRANSMlTERS

10

I 0 1 ' 1 ' 1 ' 1 ' 1 ' 1 "

2 6 10 14 18 22 26

FREQUENCY (MHz) Q500KW a250KW

Harmonic and Spurious Cutput

The purpose of this procedure was to measure the levels of unwanted emanations, specifically those harmonically related to the carrier or any others from various sources including power supply and modulator switching. This procedure was conducted at full pier operation for all four transmitters.

The transmitter's output was sensed and routed through a tunable band-reject filter to the RF input of the spectrum analyzer. The band-reject filter was employed to notch out each carrier frequency, thereby preventing the analyzer from saturating and displaying erroneous values of harmonic and spurious outputs.

Spectral scans were made with the transmitters operating at each of the representative carrier frequencies (f,) from the 11 shortwave bands in Table 11. For spurious radiation, scans were taken across five ranges of frequencies. The first spanned the relatively narrow band of 30 kHz centered on fc. The second and third ranges spanned successively wider bands of 150 kHz and 4 MHz also centered on fc. The last two scans were broad-band checks, the fourth covered the entire VHF band from 30 MHz to 300 MHz, while the fifth covered part of the UHF band from 300 MHz up to 1.5 GHz.

For the close-in range of 30 kHz about the carrier, three scans were taken for each transmitter. The first was a check of the spectral background with the

152

transmitter turned off to identify spectral components caused by external sources. The second and third scans were checks with full power carrier only and then carrier modulated at 95%. "do additional scans were taken to check for out-of-band radiation in the intermediate range of 150 kHz about fc. As above, the first scan was a background check with no carrier present. The second scan was conducted with unmodulated full power carrier.

The 4 MHz range and the two broadband ranges were also checked with two scans in the same fashion.

Two more scans were made at each carrier frequency to assess its first 10 harmonics. First the background was scanned with the transmitter off starting at slightly less than fc and stopping just beyond 10fc. Then the same frequency span was swept again with the unmodulated full power carrier turned on.

Expressing the results of this procedure compactly to aid in formulating and verifying the specification posed a significant problem. This was addressed by examining the spectral plots and identifying significant peaks of unwanted radiation. These peaks were tabulated and averaged at each carrier frequency for all four transmitters. Figure 6 shows a graphical example of the results for spurious output. Many peaks were down more than 80dB. Some were only down in the 50 to 60dB range. The average was down approximately 72dB.

FIGURE 6 SPURIOUS RADIATION LEVEL 500KW

ALL TRANSMITTERS 0

-10

m ̂s -20 a: II! a: LT -30 4 0

- 4 0 -I w rn

-50 > !3

-60

-70

- 80

1

acquisitions. A detailed review of how the data from all of the measurement procedures, from the expert panel, and from the installation and site inspection teams were used in formulating and verifying the specification is beyond the scope of this paper. However, analyses of data from each of these sources were encouraging in that, compared to VOA's existing transmitter inventory, most of which is over 20 years old, significant improvements in key performance characteristics could be expected in future transmitters.

This was especially true in the areas of efficiency and operating cost. The data confirm that overall transmitter efficiency was 30% to 40% better on the average than performance of the old transmitters, without any measurable degradation in audio response or fidelity. Furthermore, the efficiency remained reasonably good even when these new transmitters were operated at reduced power levels. Having the ability to reduce transmitter power from 500 kW to 250 and even 125 without a severe loss of efficiency will allow VOA to save on energy costs when propagation conditions are favorable. This will also help achieve satisfactory coverage without overwhelming the already conjested broadcast bands. The VCP tests showed that additional energy savings of 20% or more could be realized while maintaining comparable signal quality.

Overall, the evaluation demonstrated that high-power transmitter design and manufacturing have continued to improve. The application of microprocessor-based control and monitoring has made transmitters more flexible and easier to operate while use of new materials in critical components makes today's transmitters more reliable and easier to maintain.

I I I I I l l [

APPENDIX A

SUMMARY OF HIGH POWER TRANSMITTER CHARACTERISTICS

AEG Model S4005

Broadcast Characteristics

Frequency Range: 3.95-26.1 MHz Carrier Power: 500 kW Tuning: Fully Automatic Control: Local or Remote

2 6 10 14 18 22 26

OGREATEST LEVEL 0 AVERAGE LEVEL CARRIER FREQUENCY (MHz)

The specification limit for unwanted output was expressed in terms of spurious emissions without modulation and in-band and out-of-band emissions with modulation. Based upon a review of the raw spectral plots and the reduced data, spurious emissions in the VOA specification were limited to a minimum of 70dB down from the carrier. This included any contribution from harmonics, parasitics, switching or step modulator, or frequenLy conversion products.

APPLICATION OF EYALUATION RESULTS

As noted earlier, this project developed information and experience for the construction of rigorous yet realistic specifications for future VOA transmitter

Major Components

Tubes : 3 (PA-TH558; IPA-TH561, Modulator TH558; All Thomson-CSF )

Servos: 12 Variable Capacitors: 9 Variable Inductors: 5 Band Switches: 1

Electronic Design

The RF Section consists of a 400 watt solid state broadband amplifier, a tetrode driver, and a high power tetrode final. The RF matching network consists or two cascaded low-pass Pi Sections, followed by an L Section and a final VHF Filter. Principal feature is only three closed-loop fine tuned elements, which are inductors, tuned under full power. Vacuum capacitors are coarse tuned for selected frequency.

The modulator, a patented PDM design, uses a special tightly-coupled pulse transformer, supplied only by AEG .

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The transmitter is capable of controlled carrier T network, loading capacitor and a tunable balun. modulation characterized by a residual carrier of 60% Exclusive of balun tuning, there are five servo-driven for low levels of nodulation rising to 100% at full tuned elements in the output network, three of which modulation. are closed-loop fine tuned.

Brown Boveri and Company, Model SK-55C3-2P

Broadcast Characteristics

Frequency Range: 5.9-26.1 MHz (Optional 3.9-26.1 MHz)

Carrier Output Power: 500 kW Tuning: Fully Automatic Control: Local or Remote

Major Components

Tubes:

Servos : Variable Capacitors: Variable Inductors: Band Switches:

2 (PA-CQK 650-1, IPA-CTK 12-1; both BBC)

12 12 5 8

Electronic Design

The RF Section consists of a low-power broadband amplifier, a grounded-grid Class C driver, and a high power tetrode final. The plate output circuit consists of a triple Pi network with three coarse-tuned variable inductors and five vacuum variable capacitors, three of which and one inductor are closed-loop tuned. The 50 ohm output leads to a TVI filter, a tunable balun and a 200 to 300 ohm exponential transmission line.

The modulator uses a BBC-patented Pulse Step Modulator design involving 32 power supplies. The input audio is sampled at 33 kHz, and the PA plate modulating voltage is synthesized accordingly. The microprocessor control system allows the modulator to function normally, except for a small limitation on the maximum percent modulation, with as many as seven power supplies inoperative.

The modulator is a series pulse width design employing a variable frequency switching feature to improve high frequency response.

Marconi, Model 86127

Broadcast Characteristics

Frequency Range: 3.95-26.1 MHZ Carrier Output Power: 500 kW Tuning: Preset Channels Control: Local or Remote

Major components

Tubes: 4 (PA-TH558, Modulator (2 ea) TH581 (All

25,000A (Eimac) Thomson-CSF), IPA-4CW

13 Servos: Variable Capacitors: 11 Variable Inductors: 6 Band Switches: 9

Electronic Design

The RF Section consists of a 100 watt broafband solid state amplifier, a Class B driver, and a class C high power tetrode final. The plate output circuit is somewhat complex starting with a tunable harmonic trap and shunt tank, followed by a Pi network, a length of 109 ohm transmission line and a double Pi network. There is also a tunable balun and a harmonic filter. A l l inductors are band-switched with shorting bars (except the driver grid inductor which is variable), and all capacitors, except two, are coarse tuned for each of the 128 preset (fix tuned) channels. TWO capacitors are closed-loop fine tuned to accommodate load changes.

The transmitter is capable of controlled carrier The modulator is a parallel pulse width design using modulation characterized by a residual carrier of 70% a storage coil, switching diodes, and two cascode at no modulation rising to 100% at full modulation. connected tubes.

Continental Electronics Manfacturing Co., Model 42013

Broadcast Characteristics

Frequency Range: 3.95-26.1 MHz Carrier Output Power: 500 kW Tuning: F u l l y Automatic Control: Local or Remte

Major Components

Tubes : 3 (PA-4CM 400,000Ar IPA-3CW 20,000 A7 Modulator-4CM 4000,00OA, All Eimac)

Servos: 9 Variable Capacitors: 5 Variable Inductors: 5 Band Switches: 3

Electronic Design

The RF Section consists of a broadband solid state amplifier, a Class B driver, and a Class C high-power tetrode final. The output circuit uses a double-tuned transformer in an adjustable cavity, followed by a