TIPAZA, Algeria — Télédiffusion d’Algérie, the national broadcaster in Algeria, is operating a 252 kHz longwave transmission system with a nominal operating RF power of 1.5 MW. The broadcasting station is located close to the city of Tipaza. TDA contracted with Transradio SenderSysteme Berlin AG to upgrade and refurbish the entire system. The project required cooperation between Transradio (for transmitters and indoor equipment) and Ampegon (for the antenna structures).
Ampegon engineers were challenged by the need to make the existing 356-meter antenna into a fully Digital Radio Mondiale-compatible system that would operate at top efficiency. In addition, Ampegon carried out standard refurbishment measures, such as changing-out guy wires and painting the steel structure.
Ampegon designers modified and optimized the layout and the shape of the cage, based on the existing folded monopole antenna with a bicone cage. Structural engineers and RF engineers worked hand-in-hand running sophisticated simulations of the antenna system on computers. These simulations finally led to the required result of a radiating system with outstanding electrical and mechanical performance.
The old cage was replaced by the new one and RF measurement validated the predicted bandwidth performance. Despite optimizing the radiating system, more work was necessary to meet the DRM requirements of VSWRmax < 1.1 for f0 ±10 kHz. Therefore, a completely new antenna tuning network had to be designed and implemented. Due to the high RF power level of 1.5 MW, the components of the antenna tuning unit are loaded with enormous RF current and voltage levels, consequently, high-performance-matching components, i.e. extra-large inductivities and vacuum capacitor arrays, had to be installed to achieve the maximum efficiency of the ATU.
Also, to realize top-of-the-line performance for the new transmission system, the old 60 W open-wire coaxial RF feed line was replaced with 50 W open-wire technology. This line connects the 50 W output of the Transradio long-wave transmitter with the input of the antenna tuning unit.
Another important factor for the efficiency of a long-wave antenna is the ground screen. A large radial screen made from more than 15 tons of copper was put into the ground. This screen ensures the highest efficiency while the loss of RF power in the surrounding ground is minimized.
The end result was that objectives in optimal system performance and energy efficiency were met and TDA’s long-wave transmission system in Tipaza is ready for DRM transmission.
Norbert Stangl is head of antenna R&D for Ampegon.