Continental Electronics Type 420 B : 500,000 watt Short Wave Broadcast Transmitter

photo Continental Electronics 420 B Transmitter Overview

* User friendly

* Simplified control, operation

* High-efficiency digital modulation

* High-efficiency RF amplifier

* Rugged output cavity

* Low current contacts

* Efficient multi-phase cooling on power amplifier tube

* Full remote control capability

* On-board computer control

* Options: SSB operation, Dynamic Carrier Control, Tuned Balun, Dummy Load


Continental's Type 420 B is a versatile, state-of-the-art, high-performance, high-efficiency transmitter that can be operated in either of two modes: amplitude modulation or single sideband.

Designed for continuous operation, the transmitter can be tuned to any frequency within its range of 3.9 to 26.1 MHz in less than 30 seconds. Changes to adjacent frequencies can be accomplished in less than one second.

From its centralized master control panel, or from a remote computerized control and monitoring station, the 420 B can be set up and either pretuned or automatically tuned to deliver 500,000 watts of carrier power on any frequency between 3.95 and 26.1 MHz. The transmitter can also be manually tuned using controls located on the centralized control panel.

The transmitter uses advanced, but readily available, state-of-the-art components. Multi-phase cooled 4CM400,000 tetrode tubes are used in the power amplifier and modulator sections of the transmitter.

All critical components, including vacuum tubes and capacitors, tuning inductors and the solid-state broadband RF driver amplifier, are water or water-vapor cooled. The backswing diode is cooled and protected with insulation fluid. Other components are cooled by forced air.

The transmitter employs a very simple and reliable three-stage RF amplifier chain which uses only 9 tuning controls, including the balun, to develop 500,000 watts of carrier power.

The three-stage RF amplifier consists of a broadband solid-state amplifier, grounded-grid 3CW20,000A7 triode, and a 4CM400,000 tetrode final power amplifier.

The modulator is a series hard tube floating deck pulse-width modulator consisting of a single 4CM400,000 tetrode driven by a solid-state MOSFET driver.

Excellent noise immunity is obtained by feeding all low level signals to the floating deck and back to ground via fiberoptic cables.

Modern switching techniques, combined with a highly efficient tetrode tube, give the modulator an efficiency of better than 90%.

Overall transmitter efficiency can range from 65% to 72% depending upon the ambient temperature and frequency; typical efficiency is 70%.

The transmitter is relatively small and easy to operate and maintain. Operators can become proficient on the Type 420 B with a minimum of on-site training. Personnel who have operated the Type 420 B consider it to be very "user-friendly".

Optional peripherals allow complete tuning and operation of the transmitter outside of an existing network or system.

The Type 420 B joins a long list of Continental high power shortwave and medium wave transmitters installed around the world, with output powers from 10,000 to 2,000,000 watts.

Below, the transmitter is described from four aspects:
Simplified Operation
Efficient, Reliable Circuits
Superior Performance
General Specifications.

Simplified Operation

Regardless of its circuitry and internal components, the test of a modern shortwave broadcast transmitter is the ease with which it can perform its mission.

The Type 420 B is designed for easy, confident operation either from in front of the transmitter or from a remote location. Its straightforward control system and status indicators eliminate chances for confusion.

The left side shows the status of all transmitter functions; the right side is used for tuning the transmitter from the front panel.

Control functions use a combination of solid-state and relay logic. Tri-state, Stop/start control commands are provided for all basic transmitter functions.

Color-keyed indicators show command status.

A colored indicator column shows individual and summary status of transmitter cooling, interlock and fault systems.

A similar display system is used for all basic control system groups, with the groups arranged in a logical, sequential pattern.

All major operating parameters are displayed on easy-to-read meters which have easily set overload and circuit-prove functions.

Because of its easy-to-follow and understandable control read-out system, transmitter power up, tuning and operation can be learned in minimal time with a minimum of on-site training.

An on-board computer, the Intel 80/10B, provides remote and local status monitoring/metering, storage and retrieval of servo positions for 255 channels, and storage and retrieval of transmitter logs. The computer can be connected to an optional external printer and video terminal.

A numerical entry pad is used for tuning the transmitter from the central control panel.

Controlling and monitoring via the RS-232, RS-422A, or other system, allows remote or extended control of the transmitter from a video terminal.

An RS-232 or RS-422A port set for 9600 baud, no parity, one stop bit, and eight character bits, allows communication with the Intel 80/10B computer which is built into the transmitter. The available commands consist of four characters each as shown below.

BGIN * HPWR NEXT nnnnn STOR nnnnn
BOOT HVOF POSN nnnnn * TIME nnnnn
DATE nnnn LOGS nnnnnn * RSET

Commands marked with an asterisk (*) are available at all times. Unmarked commands are not available when the transmitter is in the local control mode. Some commands require numbers, indicated by "n", to follow prior to a carriage return. Commands are not executed until a carriage return is entered at the terminal.

A brief explanation of the key board commands follows below.

AUTO toggles automatic log printing feature on and off.
BAUD toggles the computer and terminal EIA and AUX ports between 2400 and 9600 baud.
BELL alternately enables and disables terminal bell to signal a transmitter fault or out-of-bounds meter reading.
BGIN restarts program at the very beginning.
BOOT allows operator to re-write memory map or "boot loop"; of magnetic bubble memory.
BUBL tests magnetic bubble memory performance.
CURS toggles terminal cursor on and off.
DATE changes real-time date to new value of nnnn (day/month).
EXIT tells program to reinstate security check.
GRAF displays curve of servo position versus frequency.
HIST allows operator to examine past status and analog data which existed at time of a fault and just prior to the fault.
HPWR sends a momentary high to high power control for high power operation.
HVOF sends momentary high to high-voltage-off line.
HVON sends momentary high to high-voltage-on line.
LIMT places an upper and lower bound on each of 15 meter readings.
LOGS allows operator to review up to two years of stored logs.
LPWR sends momentary high to low power control for low power operation.
MAPS displays grid of Magnetic Bubble Memory channel and frequency-related servo position slots.
MNTR special command for use in software development.
NEXT specifies channel or frequency to which transmitter will be tuned.
POSN displays servo positions and date stored in bubble memory for particular channel (1 to 255) or frequency (3900 to 26100).
PRNT dumps current status screen to parallel printer port.
RAMT performs 16k RAM test.
RSET resets fault indicators.
STBY places transmitter in the standby mode.
STLG immediately stores current transmitter operating parameters in bubble memory.
STOP sends cooling stop command; filament off.
STOR stores current servo positions in any channel or frequency slot in bubble memory.
TIME changes real-time clock to new value of nnnn (hr/mm).
TUNE switches frequency synthesizer to NEXT frequency, inhibits modulator, checks NEXT servo positions for validity and interpolates if necessary, sends them out to the transmitter, then sends tune command to the transmitter. During tuning cycle, running servos are reported on the terminal. One minute after a successful tune command, a log will be stored in bubble memory if transmitter is on and no faults are reported.
YEAR sets real-time clock year.

All fault messages are tagged to appear in reverse video on an ADDS Viewpoint terminal. This warns the operator of a fault when viewing distance is too far from the terminal to read individual characters or when the warning bell is disabled.

Logging on command is accomplished by sending a HIGH to the parallel port enable line in the transmitter.

Spaces in keyboard entries are ignored.

Commands from the terminal will be ignored while the keypad is busy.

Several calculations are performed by the computer, based on the A/D inputs: VSWR, PA input power, net RF power output, and PA efficiency.

The Type 420 B can also be controlled from the keypad located at the front of the transmitter. Ten commands can be entered at this local keypad. Three of the commands are identical to their namesake in the remote terminal command list: NEXT, STOR, TUNE.

Keypad entries are shown on a two-line, 40 character fluorescent display mounted above the keypad on the control panel.


1 2 3 POS
4 5 6 PRT
7 8 9 STO

A brief explanation of the keypad commands follows below.

ENT terminator (similar to carriage return) for some commands.
BAK backspace (allows correction of previous entry).
BRI steps fluorescent display brightness through four levels.
CLR clears keypad buffer and fluorescent display.
POS allows operator to review or change servo positions. If servo positions are interpolated, the closest bracketing frequencies with valid position data will be displayed.
PRT dumps latest status and analog information to the local printer.
SEC allows operator to enter a 10- digit security code.

Efficient, Reliable Circuits

The transmitter employs anode modulation of the final RF power amplifier stage to achieve amplified modulation (A3E) transmissions for International broadcast service. Anode modulation comes from a single tube floating deck pulse-width switching modulator. The modulator uses a patented circuit and operates at a maximum frequency of 100 kHz.

The pulse width modulator (PWM) signal is sent to the floating modulator deck via fiber optic cables. Switching signals and sideband modulation products are filtered in a low pass filter. The PWM uses patented circuits to control negative modulation troughs and to achieve smooth modulation in negative troughs to 100% modulation without distortion. The tube is driven by solid-state MOSFETS on two plug-in circuit boards.

The entire modulation system is straightforward, easy to comprehend and maintain, and uses basic, readily available, economical components.

Tuning is accomplished with a large, highly-efficient transmission line coupled cavity. Surface contact is made through 32 pneumatically driven silver points which are spring loaded to ensure contact.

The 32 points distribute the current flow to less than 8 amps per point, resulting in a trouble-free system.

Coarse tuning is accomplished under a "no power" mode; capacitors are used for fine tuning.

Contacts are released automatically during tuning, and the cavity is moved during the "no power" mode.

Fast-acting crowbar protects modulator tube and high voltage dc components.

All door interlocks and grounding switches are safeguarded by an interlocking door lock system located on back of the modulator-control panel cabinet.

RF shielding panels are used behind cabinet doors to provide extra safety to personnel and to keep unwanted RF signals from radiating into or around the transmitter area. Individual panels are installed or removed by means of quick-action slide locks.

Optional Equipment

An optional tuned balun can be mounted on top of the power amplifier cabinet, or in any other convenient location.

The balun delivers RF power to a 300 ohm balanced output transmission line, matching this impedance to the unbalanced output impedance of the final power amplifier. The highly efficient variable inductor employed is resonated by two parallel, water cooled, variable vacuum capacitors.

The inductor's variable shorting plate is driven by a dc motor to a coarse tuned position (an operator-selected frequency).

After coarse tuning, capacitors accurately resonate the circuit by tuning for a 90-degree phase difference between a voltage and current sample of the radio frequency at the output terminal of the power amplifier.

The plate does not move during fine tuning. Current flows through spring-loaded, pneumatically controlled silver-graphite points; each point carries less than 8 amps. The pneumatic system releases spring pressure on the contact points around the center conductor, and on its four edges, so that no contact is made while the plate is moving.

An optional, compact 825 kW dummy load consists of very stable deposited film resistor elements. Its load impedances of 50 or 75 ohms unbalanced and 300 ohms balanced, are unaffected by power changes.

Dynamic Carrier Control

Traditional practice in Amplitude Modulation (AM) systems is to provide a fixed carrier level and apply sidebands up to but not to exceed 100% modulation.

In Dynamic Carrier Control, sideband power is produced and carrier level is varied to provide only the necessary carrier power for the percentage of modulation required. The lower limit of carrier power is selectable from zero to -6 dB in one decibel steps.

In AM transmission systems, the signal-to-noise ratio is largely determined by sideband power, not carrier power.

Thus, if an AM transmitter is modulated at 50%, a listener will have to turn the volume up 6 dB to get the same loudness as a transmitter modulated at 100%. Background noise will also increase by 6 dB. If the carrier level of the same transmitter is lowered to obtain 100% modulation, no change in sideband power will occur. The AGC circuitry of the receiver will bring the volume up much as a volume control does, and background noise will also increase by the same amount.

In operation, the transmitter is set for full carrier power and modulated at or near 100% modulation. If full modulation is always available, the carrier will stay at full power. If there is a reduction in average modulation drive, carrier level will drop to bring the modulation level back to nearly 100%. If modulation is completely removed, carrier will drop to the operator-selected minimum level. As modulation is reapplied, the carrier level will increase to ensure that 100% modulation is not exceeded.

This technique greatly extends the life of transmitter components because during typical operation, the transmitter operates well below its design capability.

Superior Performance

High Fidelity

The Type 420 B is virtually transparent, and handles audio processing without degrading it. An oscilloscope picture shows the transmitter's extremely high fidelity. The oscilloscope was comparing the input audio waveform with the output envelope signal; the picture was taken during operations at a customer's site on January 19, 1987 while the transmitter was modulated at 100% with 1kHz.


Typical average tuning time for 10 auto-tune frequencies is 15 seconds. Transmitter can be tuned to any fre quency between 3.95 and 26.1 MHz in less than 30 seconds in or out of the International Shortwave Broadcast Band.

A combination of digital and analog techniques ensure accuracy and reliability of auto-tuning as well as simplicity and ease of manual tuning when desired.

High "Q"

Transmitter offers maximum performance and reliability with basic, easily understood circuitry. Highly efficient tuning and matching circuits in the RF amplifier and balun maintain a constant and optimal circuit "Q" over the entire tuning range.

Basic Components

Readily available, proven state-of-the-art, cost-effective components are used wherever possible to minimize down time and hold maintenance costs to a minimum.

Easy Operation

Patented circuits and techniques used in the modulator yield superior performance with minimal circuit adjustment.


Several doors on the modulator and power amplifier cabinets give easy, uncluttered access to interior components. The modulator deck and other sub-systems can be reached in a walk-around mode.

Interchangeable Parts

Servo amplifiers, circuit boards, computer systems and mechanical components are interchangeable among Type 420 B transmitters, and second sources are available for most major components.

Simple, Logical Control

"User-friendly" layout and design enables operators to become proficient in minimal time with minimal hands-on training. Control panel is easy to read and displays all critical tuning and operating parameters.

Single Knob Tuning

Unique manual tuning system enables operator to hand tune any of the various tuning components using a single knob on transmitter's front control panel.

Excellent Harmonic Attenuation

Optional, high efficiency balun provides superior tuning.

Easy Remote or Local Control

Single key command is available at remote locations via RS-232, RS-422A, or other system with a video terminal, or from entry keyboard on front control panel.

Continuous Operation

Transmitter is designed to provide continuous operation at 100% modulation for extended time periods

.9994 Availability

MTBF and MTTR experience indicates high availability.

High Efficiency With Easy Maintenance

Circuit design and component selection offer highest efficiency within the parameters of circuits and components which are easy to understand and to trouble shoot.

Logic Programming

All semiconductor logic devices used in the transmitter's computer control system are programmed at Continental's plant in Dallas. This in-house facility enhances field-support and custom programming capabilities.

Carrier Output 500,000 watts, min.
Modulation Pulse-width modulator
Emission A3E
Frequency Range 3.9 to 26.1 MHz
AF Input Impedance 600/150 ohms, balanced or unbalanced
AF Input Level for 100% Sinewave Modulation -5 dBm to +10 dBm
AF Response ±1 dB 50 Hz to 7.5 kHz
AF Distortion Less than 4% THD 50 Hz to 7.5 kHz at 95% modulation
Carrier Shift Less than 3% excluding line variations
Modulation Capability 100% positive and negative peaks 50 to 7.5 kHz sinewave for 10 minutes per hour; up to 80% modulation continuously
Residual Carrier Noise 60 dB unweighted below 100% modulation at 1 kHz or better
RF Harmonic Output and Spurious Response Below 75 dB
Output Impedance 50 or 75 ohms unbalanced, 300 ohms balanced
Maximum VSWR 2.0:1
Power Source 4160 v ac, 3 phase, 50/60 Hz (other on special order)
Power Factor 0.9 or better
Power Consumption Carrier, unmodulated: 800 KVA, max.
Carrier, modulated: 100% sinewave, 1200 KVA, max.
Efficiency 65% minimum, including all cooling, auxiliary power and balun, with worst case operating conditions, to 72%; 70% typical
Exciter Any suitable exciter with 0.5 watt output, 3.95 to 26.10 MHz
Operating Environment: Altitude Sea level up to 6,000 ft (1828.8 m)
Ambient Temperature +32° to +122°(0° to +50°C)
Relative Humidity Up to 95%
Size and Weight Transmitter's main cabinet group and power vault area require approximately 4,300 cubic feet (129 m3) including doorways and walk space, but not including outside heat exchanger. Shipping weight, packed for export, is approximately 47,300 lb (21,285 kg).
Specifications may change without notice

RF stages AF stages and modulator
Number Type Number Type
1 4CM400,000A/AG or 4CM500,000G or TH558 or TH576 or RS 2078 1 4CM400,000A/AG or 4CM500,000G or TH558 or TH576 or RS 2078
1 3CW20,000A7


ITU Country
ITU Country