HF - Short wave 300 kW broadcasting transmitter

photo Thomson-CSF TRE 2335 Feature

Layout and access

The transmitter is composed of 2 or 3 main sub assemblies (depending on installation) which can be rapidly interconnected on site: wiring and piping between those assemblies are limited to a minimum.
Description of the transmitter

There are basically two separate amplification lines, RF and AF.

RF circuit

Last refinement of the proven 300 kW RF THOMSON CSF circuits, its structure is unbalanced. Equipped with only 2 tetrodes, the RF circuit amplifies the RF signal from the output of the synthesizer (1 V/50 ohm) up to the rated level (300 kW/50 ohm) in 3 steps:
RF 3 power tube operates in class C. It is modulated through anode control, with automodulated screen. It is followed by three distinct networks in series which ensure the following functions:
AF circuit

The AF high voltage necessary to modulate the anode of the final RF tube is produced by integration of width modulated pulses at 50 kHz. AF1 generates pulses through a complete digital process which introduces precise precorrection to compensate for non linearity of AF stages. Any carrier variation (DACM or reduced power) may be programmed in the microprocessor system. Transmission of the signal to AF2 and AF3, located on an insulated chassis, is performed through optic fibers. AF1 and AF2 are fully transistorized, while AF3 uses a TH 558 or 573 tetrode.
Double coil, free wheel diode, and low pass filter complete the AF circuit.

Transmitter and frequency control

Transmitter and frequency control units gather the means for controlling, monitoring and insuring internal security of the transmitter, trying to minimize the need for operator intervention. They are based on a microprocessor central unit surrounded with several interface boards to shape data exchanged with the transmitter, the operator, or a remote control computer through an RS 232 or IEEE 488 interface.

Monitoring and trouble shooting:

Through a TV screen located on the front panel of the transmitter. A printer may additionally be connected to get a complete logbook. Most informations displayed locally can be remote transmitted on the bus. An automatic self diagnosis for quicker trouble shooting is performed in case of a fault. Result is automatically displayed on the monitor.

R.F. characteristics
Power output (carrier) 300 kW
Reduced power operation
DACM operation
Frequency range 5.9 to 26.1 MHz. Extension to 3.9 or 3.2 (optional)
Tuning automatic on any frequency within broadcast bands (and optional on most frequencies outside broadcast bands)
Transmission type A3, A3 with DACM, SSB (optional), AM stereo (optional)
Modulation type digital PDM for anode amplitude modulation
Frequency stability ±2 10-7 per month (or better optional)
RF output 50 ohm unbalanced SWR less than 2 (300 ohm balanced with balun)
RF harmonics better than CCIR specifications (50 mW or less on any frequency)
AF characteristics
AF input impedance more than 5000 ohm balanced between 50 Hz and 7500 Hz, compatible with a 600 ohm balanced line
AF nominal level to achieve 100% modulation at 1000 Hz =0 to +10 dBm adjustable of ± dB by 0.5 dB steps
AF band with 50 Hz to 7500 Hz (or 4500 Hz with a filter) in A3 mode
Frequency response Reference curve stands within the limits below, measured with a 50% modulation rate. For any modulation between 30 and 80%, the curve stands within ±0,5 dB of the reference curve
Harmonic distortion 50 to 100 Hz: 3% at 30 to 80%
100 to 5000 Hz: 2,5% at 30 to 80%, 3% at 90%
5000 to 7500 Hz: 3% at 30 to 80%
Noise Unweighted noise referenced to a 100% modulated carrier with a 100 Hz tone less than 58 dB
Modulation capabilities
Maximum modulation factor 100% from 100 to 5000 Hz
80% anywhere in band
Maximum average modulation factor in program operation 70% (can be sustained continuously)
Carrier shift Less than 5% at 100% modulation with 1000 Hz sine wave
Power supply
Three phases 5,5 to 24 kV (or more optionnally) 1200 kVA
Single phase 220 V 3 kVA permanent
Three-phases 220/380 V (can be internally generated) 50 kVA
Overall power efficiency
70% typically (64% minimum guaranteed at any modulation on any broadcast band)
Temperature +1°C to +50°C (extended optionally)
Humidity 95%
Altitude 1500 m (or more optional)
Specifications may change without notice

RF stages AF stages and modulator
Number Type Number Type
1 TH558 1 TH558 or TH573
1 TH562


ITU Country
ITU Country