500 kW Shortwave Broadcast Transmitter

photo Marconi B 6127 FEATURES


The Marconi B 6l27 transmitter is the most technologically advanced and the least costly to operate in its class.

The new PULSAM low loss modulation system ofamplitude modulation combines the virtues of pulse width and Class B modulation resulting in a transmitter which is psychologically acceptable to maintenance personnel in addition to its many other special features.

R.F circuits

The radio frequency drive for the transmitter is normally a synthesiser which can be mounted either at the front of the equipment or externally if preferred.

The synthesiser output is amplified in a wideband solid-state amplifier to a level of 100 W. This in turn is coupled to the penultimate r.f amplifier by a circuit having fixed input and output capacitors and a servo-controlled variable inductor.

The modulated amplifier is a single tetrode operating with grounded grid, modulation being applied to the anode and screen. The amplifier is coupled into an unbalanced load.

Output power and reverse power are monitored by wideband directional couplers and power is automatically removed if the reverse power level exceeds a safe value. An auto load matching facility is provided to minimise the possibility ofthe transmitter tripping off due to antenna or feeder mismatching as could be caused by icing conditions.

A television filter to attenuate unwanted emissions above 40 MHz to -80 dB or below is fitted as standard and is mounted on top of the r.f cabinet between the transmitter output and the balun.

The balun is normally included with the transmitter, unless a 50 ohm output is required, and is tuned automatically to minimise its v.s.w.r over the whole frequency range.

Power supplies

A medium voltage supply of 380V or415 V three phase, four wire is required for auxiliaries such as filaments, bias, screen h.t, fan and pump motors while a high voltage supply, typically 11 kV, is required for the main high voltage rectifiers. Silicon diodes are used as rectifiers for the high voltage supplies and applied peak voltages are limited by surge diverters, capacitor/resistor damping or other means of absorbing excessive voltages. All rectifiers have adequate thermal capacity and protection to survive direct short circuits across their output terminals.

As is usual for equipment of this power rating, 12 pulse rectification is used to limit harmonic currents from being introduced into the mains supply network.


The air cooling system which is designed as an integrated dual air handling unit includes:
a low pressure fan to remove warm air from the transmitter cabinets;
a high pressure fan to cool the vacuum tube seals.

The water cooling system comprises:
a condensed vapour cooling circuit which cools the modulator and modulated amplifier tube anodes;
an auxiliary water cooling circuit which cools the tuning inductors, vacuum tuning capacitors and the anode of the penultimate r.f amplifier.

An external heat exchanger system is used to cool the two pumped water circuits and this can be designed to suit the users particular climatic conditions. Typically, however, the system would consist of one twin fan water-to-air heat exchanger for the auxiliary circuit and three similar units for the main circuit. An automatic control system starts up the fans of the three heat exchangers in succession as the load increases which is more economical than running all the fans continuously regardless of the heat load. The usual alarms and safety measures necessary for a cooling system are incorporated into the system.


This unit uses the Marconi patented system of pulse width modulation in which the duty of the modulator is limited to the processing of the sideband information only.

In this system the switching tube peak anode current is half that required for series pulse width systems and the mean anode currents are much lower. It is possible, therefore, to use two relatively small tetrodes instead of one large tube thus reducing stray capacitance and minimising power losses at low modulation levels. In this transmitter the two modulator switching tubes are each about one quarter the electrical size of the r.f modulated amplifier and are substantially smaller than those required for an equivalent Class B modulator.

Although each tube requires a string of diodes, the rating being approximately a tenth of that required for a 'series' system, these are not highly stressed and need only be air cooled. The single wound storage inductor is relatively small as it only carries audio frequency current and, by ingenious design, further reduces the physical size by the use of ferrite cores.

The sub-modulator is completely solid state and does not require to be located on a high voltage platform. It processes the audio and switching frequencies to produce pulses whose width is proportional to the sinusoidal modulating signals.

These pulses are applied to the two switching tetrodes which operate in a 'push-pull' mode. The storage inductor and lowpass filter remove the switching frequency to a level well below that required to keep within the limits specified for spurious radiation. The resultant audio waveform is capacity coupled to the anode of the modulated r.f amplifier. With this system, which is analogous to the classical Class B system, there is no restriction to full modulation on negative peaks and spurious radiation levels do not present a problem.


All control and monitoring functions required for normal operation are grouped together on the front panel of the transmitter. Start-up and close-down can be controlled by a single control button and pre-selection of frequencies is also accomplished in this area. A comprehensive set of status and fault diagnosis indicators is provided and a built-in oscilloscope enables r.f and audio waveforms to be monitored in addition to modulation levels.

Controls, flow meters and pressure gauges for the water cooling system are located in the central passageway between the two rows of cabinets and these are accessible during normal operation. Manual tuning and auto selection of pre-selected frequencies is effected on the memory unit. Up to 128 pre-determined frequencies can be stored in the memory bank and each frequency is allocated a channel number which can be recalled by a 'Channel Selector' control. Change to channel then takes place when the 'Change Channel' button is depressed. the time required to achieve full power on a new frequency depends on the frequency separation. In the worst case, i.e from 3.95 to 26.1 MHz, the maximum time is 30 seconds, which is dictated by the time required to drive a vacuum tuning capacitor from one end of its travel to the other without undue wear. In practice most frequency changes are usually between adjacent bands in which case the time involved could be only 10 to 15 seconds.

Coarse range switching is achieved by varying the inductance of the tuning inductors in steps. In order to avoid the use of sliding contacts, which can be unreliable unless carefully maintained, the tuning inductors are arranged in the shape of a 'trombone' which can be shortened progressively by shorting-bar switches as the frequency required increases. These switches have replaceable, water cooled contacts and are pneumatically operated to provide a controlled rate of closure with a self-cleaning action.

Fine tuning is then accomplished by servo-controlled vacuum capacitors each of which is driven by a robust gearbox with a printed circuit zero inertia motor. The servo drive units are rack mounted at the front of the transmitter and are retractable on telescopic runners for servicing. If the tuning controls fail, the servo controls can be adjusted from the front of the servo drive units. Alternatively individual capacitors can be adjusted manually at the associated gearbox. R.F range switches can be manually set at the front panel or should this unit be faulty, by the use of switches on the pneumatic control unit. Thus there are two levels of emergency control in the event of a fault in the control system.


The transmitter embodies comprehensive protection circuits, utilising optical fibres where applicable. The operation of these circuits is initiated by sensors which monitor overcurrent, overvoltage, high reverse power, or malfunctioning ofthe cooling system and a three-shot recycling overload system is also incorporated. Whilst great improvements have been made in reducing the hazard of internal flash-arcs in high power vacuum tubes, tube manufacturers nevertheless specify a protection requirement to cover this eventuality. The specified requirements are more than adequately met in this transmitter. In the final r.f amplifier an incipient flash-arc triggers the 'crowbar' and also opens the vacuum switches between the high-voltage transformers and the rectifier bridges.


To ensure the safety of all personnel, a system of mechanical and electrical interlocks is provided. When the main supply isolator is open, access to all parts of the transmitter is possible with complete safety. When closed, filaments, fans and control circuits can be powered with certain doors open. However, where voltages over 50 V a.c are applied, covers and warning notices are fitted. Before high voltages can be applied, all doors have to be locked and the keys returned to an interlock panel. When all keys are restored to this panel, the earthing switch and power switch can be operated allowing power to be connected. While power is on it is impossible for any key to be released.

Earthing is provided for the r.f feeder and for high-voltage supplies. The equipment conforms generally with IEC.215.


The r.f circuits, the modulator and all low-power supplies are contained in free-standing cabinets occupying a floor area 4.6 m wide by 4.8 m deep. The layout of the high voltage and cooling components can usually be tailored to suit particular building arrangements.

For ease of transportation and installation the two main cubicles can be split up into smaller units and these are provided with strong steel bases which give great rigidity to the units. Reconnection of inter-unit wiring is made simple by the use of special mating terminal blocks which obviate the possibility of wrong connections being made during installation.


Remote controls and indications are provided for in the transmitter and these are categorised as follows:
Controls (commands);
Status indications;
Proving indications (intermediate start-up conditions);
Memorised indications of trips and overloads.
Details of these facilities can be supplied on request.


The Pulsam modulation system improves upon the overall efficiency of series p.d.m systems, particularly at average modulation levels, and at the same time obviates some of their main disadvantages. Full modulation of both positive and negative peaks is achieved.

Spurious radiation due to the switching frequency components is more readily kept to acceptably low levels because the switching is carried out at the relatively low levels of sideband power. This is increasingly important as international limits for spurious emissions in the shortwave bands are bound to become tighter in the future.
Carrier amplitude shift is reduced by virtue of the use of a separate power supply for the modulator compared with systems using a single supply. The use of separate supplies also simplifies tuning and maintenance as the r.f amplifier can be powered independently of the modulator and as mentioned above the solid-state sub-modulator does not have to be located on a high-voltage platform.

R.F Data
Power output 500 kW +0.1, -5 dB.
Output impedance 300/328 ohm balanced, 75 ohm unbalanced or 50 ohm unbalanced.
V.S.W.R 1.8:1 max at 300/328 ohm balanced or 2.0:1 at 50/75 ohm unbalanced
Frequency range 3.95 to 26.1 MHz. (WARC 1979 broadcast bands).
Autotune facility Any of the 128 pre-set channels selected in less than 30s.
R.F harmonics Better than -70 dB ref. to carrier.
Spurious radiation -80 dB for harmonics above 40 MHz.
Harmonics related to switching frequency -70 dB ref. to carrier.
Modulation Data
A.F input impedance 600 ohm balanced.
A.F input level -5 to +10 dB for 40% modulation at 400 Hz.
A.F input attenuator Range of 19.5 dB in 0.5 dB steps.
A.F response 60-6000 Hz; +0.5, -1.0 dB. 6000-7500 Hz; +0.5, -2.0 dB relative to 400 Hz at 75% modulation.
A.F distortion Less than 3% t.h.d at 50% modulation (60-7500 Hz); less than 4% t.h.d at 75% modulation (6000-7500 Hz); less than 4% t.h.d at 90% modulation (60-6000 Hz).
Carrier noise At least 56 dB below 100% modulation at 400 Hz.
Modulation capability 100% modulation for 10 min per hour then 70% modulation for 50 min per hour (with sinewave 60-5000 Hz) or 75% modulation continuously (with sinewave 60-5000 Hz).
Electrical Data
Power supplies Auxiliary circuits 380/415 V 3 phase 4 wire 50/60 Hz main rectifiers nominally 11 kV 3 phase (other supply voltages to order).
Voltage fluctuation (permissible) Auxiliary circuits ±10% main rectifiers adjustable +5% to -10% in steps. Full performance except power output, with maximum voltage variations of ±2%.
Frequency tolerance ±2 Hz.
Line inbalance (permissible) Not greater than ±1% for full performance.
Power factor Better than 0.9.
Environmental Data
Ambient temperature 50°C maximum. External cooling equipment can be designed for particular conditions including temperatures below freezing point.
Daily average 45°C.
Yearly average 35°C.
Altitude 2300 m maximum.
Humidity 95% maximum.
Overall efficiency Better than 62% at any depth of modulation.
Specifications may change without notice

RF stages AF stages and modulator
Number Type Number Type
1 4CM500,000G or TH558 2 TH581
1 4CW25,000A


ITU Country
ITU Country