50 kW Short Wave Broadcast Transmitter HHB 144

photo Bharat Electronics HHB 144 * Transistorised RF & AF preliminary stages

* Solid state control system

* Rapid and easy installation

* Small floor area

* Low running cost

The 50 kW Shortwave Broadcast Transmitter HHB 144 operates in the frequency range of 3.2 MHz to 26.1 MHz. It employs four tetrode tubes, 2 in RF and 2 in AF. Solid state devices are used for the lower power AF & RF stages, control system and the rectifier. The transmitter employs the well proven standard class B modulation system using a modulation transformer. The condensed vapour cooling system is employed for the power tubes which enhances the reliability.

The transmitting frequency is derived from a synthesiser unit external to the transmitter. The synthesiser output is fed to a broadband amplifier which delivers 10W to the grid of the aircooled penultimate RF amplifier tube CQL-2. The output power of 50 kW is obtained from a CQK-25 valve stage. The tube operates with grounded cathode and feeds into a π circuit. The harmonic filter provided, ensures the transmitter conforms to CCIR Radio Regulations on harmonic radiation. The output of π circuit is transformed to 300 ohms bal. by a balun fitted inside the transmitter cabinet.

In the AF chain, a series of solid state amplifier is followed by two tetrodes, CQK-25, in a zero grid current push pull class B system. Overall negative feedback ensures that harmonic distortion and hum remain within acceptable limits. The modulation is applied to screen of PA also.

The transmitter operates on standard 415 V, 50 Hz, 3 phase power supply. Twin HT transformers feed two series connected silicon diode rectifiers. The phases of the 2 transformers are shifted by 30°. This arrangement reduces interference feed back into the mains and also requires a minimum of filtering capacity which reduces the protection requirements for the diodes. The reliability is further enhanced by the thyristor control unit (Thycon) to feed the primary windings of the HT transformers. The thycon unit allows a smooth variation of HT rectified output from 0 V to the full value.

The final RF tube and the 2 modulator tubes are condensed vapour cooled. Distilled or demineralised water is used as coolant. A water to air heat exchanger is provided for secondary cooling. A single blower is used for general cooling and ventillation of Tx cabinets.

The personnel safety system fulfils the requirement of IEC publication 215. It comprises electric door contacts and door key interlock panel. In addition, a blinking light system gives warning, whenever, for tests or for any other purpose, the door interlock contact is bridged.

The transmitter is fitted with comprehensive protection circuits, initiated by sensors, which monitors over voltage, over current, excessive VSWR or cooling system failure. Also, the HT is blocked by means of the thycon unit thereby disconnecting HT from the tubes much faster. A three shot recycling overload system forms part of the protection measures.

The basic transmitter comprises two MNS cabinets. One cabinet contains the RF driver and final stages with the output circuit and harmonic filter. The second cabinet carries the control and supervisory system, the RF drive preamplifier, and the AF preliminary stage. At the rear of this cabinet, the modulation stage, 380 V auxiliary distribution board and the auxiliary rectifiers are located.

Frequency range 3.2 to 26.1 MHz
Frequency control Synthesiser
Frequency stability As per CCIR requirements
Frequency change time 2 mts. (ave.); 3 mts. (max.)
Type of transmission A3 broadcasting
Type of modulation High level plate modulation
Maximum VSWR 1:1.7
Cooling system Condensed vapour cooling
Carrier power output 50 kW into 300 Ohm (bal.)
RF harmonics and spurious radiations According to ITU Radio Regulations, Geneva 1968
Carrier voltage shift (Mains voltage drop excluded) <5% from 0 to 100%
sinewave modulation at 1 kHz
Modulation capability Sinewave modulation
(a) Programme material 50Hz to 10kHz with peaks to 100%: Continuous
(b) Tone tests (at any frequency between 100 Hz & 5 kHz): 10 mts./hr. at 100% modulation; at 70% modulation 50 mts. continuous
(c) Short term test: 100% modulation from 50 Hz to 10 kHz
Modulation linearity ±1 dB 100 Hz - 7.5 kHz
±2 dB 50 Hz - 10 kHz
AF input 0 dBm into 600 Ohm (bal.) for 100% modulation at 1 kHz
AF response Within -1 dB & +0.5 dB from 60 Hz to 7.5 kHz at 70% modulation
AF distortion <4% from 20 to 95% modulation from 100 Hz to 5 kHz
<4% upto 80% modulation between 100 Hz & 50 Hz and between 5 & 7.5 kHz
Hum & Noise level Better than -55 dB for 100% modulation at 1 kHz
Supply voltage 415 V/380 V, 50 Hz, 3 phase and neutral
Consumption 150 kVA max.
Power factor Better than 0.9 at 100% modulation
Overall efficiency
at 0% modulation
at 30% modulation fm = 1 kHz
at 100% modulation fm = 1 kHz
Mean value during one day
Below 15 MHz
Above 15 MHz
Temperature range +1°C to +45°C
Relative humidity 95% max.
Transmitter cabinet (H) 2100 x (W) 3600 x (D) 1600 mm
Total transmitter installation (W) 7200 x (D) 6200 mm
Specifications may change without notice

RF stages AF stages and modulator
Number Type Number Type
1 CQK 25-2 2 CQK 25-2
1 CQL 2-1


ITU Country
ITU Country