Part of Radio Tele Luxembourg’s Directional Antenna System based at Beidweiler, Luxembourg
BEIDWEILER, Luxembourg — Transmitter operator Broadcasting Center Europe (BCE) manages short- and longwave broadcasts for Radio Télévision Luxembourg (RTL) from its long-standing site in Beidweiler, Luxembourg, including the French-language programming “RTL France,” which is transmitted toward Paris on 234 kHz (main lobe southwest 227 degrees) via a directional antenna system comprising three 290-meter masts.
NEW TRANSMITTERS Transradio was contracted to replace the existing tube transmitters at Beidweiler with solid-state transmitters that possessed high operational reliability and high energy efficiency.
For this challenging project, Transradio developed an air-cooled DRM-capable solid-state AM transmitter, the TRAM/P 1500 LS.
Aside from designing the medium-voltage system and the air-cooling system, the project also involved designing a wideband high frequency transformation network for a top RF power of 1.5 MW. This transformation network matches the 50-ohm output impedance of the transmitter system to the antenna impedance of 120 ohms.
The transmitter itself was derived from the Transradio LW/MW family of TRAM transmitters.
The TRAM/P 1500 LS consists of two single TRAM 750 LS transmitters (each delivering an output power of 750 kW) and a paralleling unit (PU) for the loss-free coupling of both transmitters. In case of maintenance work or in the unlikely event of a fault, the PU automatically switches the unaffected transmitter to the antenna so that the system remains on air with 3 dB reduced power.
Each TRAM 750 LS transmitter comprises six identical 125 kW amplifier blocks connected via a star combiner. Each 125 kW block consists of two 62.5 kW amplifier cabinets (19-inch cabinets), each of which incorporates 48 identical power modules delivering 1.3 kW. Hence a TRAM 750 LS transmitter has 576 identical final stages, which provides unsurpassed redundancy.
The high signal quality is mirrored by the excellent output spectrum in DRM operation. The spectrum in Figure 1 was measured at the dummy load of a TRAM 750 LS transmitter delivering a DRM output power of around 400 kW.
Figure 1: Measured Output Spectrum of a Transradio TRAM 750 LS Transmitter
At a modulation error ratio (MER) of 36 dB, a mask reserve of nearly 10 dB was achieved, in accordance with ITU-R SM.1541/ETSI EN 302. Similar good values can also be realized using a suitable operational antenna.
Using the Transradio DMOD 3 as an exciter, the TRAM 1500 LS can deliver a DRM output power close to 1 MW with the help of the Transradio Digital Power Enhancement (DPE). An MER of 35 dB and conformity to the ITU/ETSI-Mask for DRM transmissions are also ensured.
Apart from the high efficiency of the transmitter (approximately 90 percent), additional savings in energy costs in AM operation can be achieved through the use of dynamic carrier control.
Beidweiler is the second BCE site to be equipped with Transradio TRAM transmitters. The RTL station in Marnach, Luxembourg, operates with a TRAM/P 600 at 600 kW carrier output power, which has been providing reliable service on 1440 kHz for several years.
With the development of the TRAM/P 1500 LS transmitter system, Transradio has demonstrated that the combination of reliability, redundancy, high efficiency and signal quality, expressed especially in outstanding DRM capability, is also possible in the longwave band at a power capability in excess of 1 MW.
Harald Zank is senior engineer for AM transmitters for Transradio SenderSysteme Berlin AG.
For information, contact Transradio SenderSysteme in Germany at telephone: +49-30-339-78-106, or visit www.transradio.de.