MARCONI BD 272

GENERAL DESCRIPTION

HF Broadcasting Transmitter (250 kW) Type BD 272

photo Marconi BD 272

THIS transmitter, designed for long-range broadcasts, embodies techniques for overcoming interference and enhancing modulation depth whilst preserving the maximum possible fidelity of transmission.

A speech clipping device can be used in the audio input to produce a much higher average depth of modulation such that the intelligibility of the received signal from this transmitter has been made equal to that obtained from a conventional sine-wave-modulated transmitter of more than twice its power.

The transmitter is housed in three free standing cubicles, with an additional enclosure for high voltage oil-filled equipment.

FEATURES

Drive. A ten-way crystal drive is provided, with provision for external drive if required.

Frequency changing. The number of tuning controls has been reduced to a minimum by the use of wide band amplifier techniques in the early stages of the transmitter.

Harmonic filter. Harmonic filters reduce the harmonic content in the radiated signal to comply with international specifications and in particular suppress harmonics which could interfere with television reception.

Monitoring. A cathode ray tube is included for envelope monitoring, RF monitoring points are provided.

Cooling. The major valves use vapour cooling. The associated heat exchanger can be cooled by air or water as required. In the latter case the water can be used for heating the building. The cubicles are cooled by a low-pressure suction fan.

Protective circuits. Access to high-voltage compartments is controlled by mechanical and electrical interlocks which ensure the fullest protection for operating personnel.

CIRCUITS

The ten-way crystal drive, which is optional, uses crystals operating at radiated frequency with wide-band amplifiers to provide an output of 2 watts. At this point switching is provided to permit selection of an external drive.

This unit is mounted in the RF drive unit which has an output of approximately 5 kW. Wide-band techniques are used in all but the output stage of this unit.

The penultimate and final HF amplifiers are contained in a single cubicle. Both employ grounded grid push-pull amplifiers with magnetic coupling to the feeder.

In the modulator phase correction is applied to compensate for distortion due to clipped sinusoidal input signals. Four vapour-cooled cathode-follower valves in parallel push-pull are driven by a submodulator unit, and feed the two final modulator valves.

Overall negative feedback is employed in the modulator circuit.

TECHNICAL SPECIFICATIONS
Frequency range	3.95 to 26.1 Mc/s
Power rating	250 kW from 3.95 to 17.9 Mc/s, falling to 200 kW at 26.1 Mc/s, at nominal mains voltage
Frequency stability	Better than 5 parts in 10⁶ per month
Output impedance	Balanced 270 to 330 ohm
Drive input	5 W at radiated frequency
Frequency response	±1 dB between 50 and 7500 c/s, referenced 400 c/s, measure at 75% modulation
A.F harmonic distortion	Less than 2.5%; 50 to 7000 c/s, at 50% modulation. Less than 4%, 50 to 7000 c/s, at 95% modulation
Noise and residual modulation	At least 56 dB (unweighted) below level corresponding to 100% modulation by sine wave signal at 1000 c/s
Power supply	All auxiliary circuits 380 to 415 V, 3-phase, 4-wire, 50 c/s a.c.
Supply voltage regulation	+6% to -10% deviation from nominal mains voltage (a.v.r fitted to give ±1% for filaments etc.)
Frequency regulation	±4% deviation from nominal mains frequency
Overall power factor	Better than 0.9
Power consumption	575 kW at 40% sine mod. 860 kW at 95% trap. mod.
Floor space required	Transmitter 27 ft x 16 ft (8.2 x 4.9 m) HT enclosure 27 ft x 15 ft 9 in. (8.2 x 4.8 m)
Specifications may change without notice

TUBE COMPLEMENT
RF stages		AF stages and modulator
Number	Type	Number	Type
2	BY1144L	2	BY1144L
2	BY1161	4	BY1654F
1	4CX5000A	4	813
7	4CX250B	2	TT21

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	GHA	GHANA	RRW	RWANDA
	SNG	SINGAPORE	G	UNITED KINGDOM