Radio broadcasters compete for airwave space in the saturated FM range, in detriment to AM wavebands with a poorer sound quality. Nevertheless, frequencies at wavebands less than 30 MHz (AM ones) could well have a new opportunity, with terrestrial digital radio broadcasting. The Digital Radio Mondiale (DRM) international consortium is working on the development of a universal system that will provide clarity which is close to that of FM, besides other advantages such as the multilingual option in programmes or a reduction of 40-50 % in electrical consumption.
Engineer Iván Peña, who works with DRM through the Signal Processing and Radiocommunication Groupat the University of the Basque Country (UPV/EHU), focused concretely on the study of the 26 MHz waveband. His thesis is entitled, Planning factors for digital local broadcasting in the 26 MHz band.
Mr Peña's research involved studying broadcasting models of propagation and planning factors in order to implement DRM services in the 26 MHz waveband. To this end, he carried out trials at a local level in countries such as Mexico, Brazil and Germany. Concretely, those of Mexico and Brazil were the first undertaken with this system. Besides, MHz is a waveband that has rarely been possible to use until now, and so its optimisation will have great repercussion. This is why Mr Peña's (jointly with the team he belongs to) research has been received at international congresses, as well as by the IEEE Transactions on Broadcasting journal, occupying, as it does, seventh position in the JCR classification for telecommunications.
To date only by ionospheric wave
As explained in the thesis, the only mode used to date for long-range radio broadcasting on the 26 MHz waveband was propagation through ionospheric wave. Here, the wave is refracted progressively, until regressing to the terrestrial surface from the ionosphere. Nevertheless, due to the fact that this method is affected by solar activity and frequency, it is not always possible to use it.
With the goal of using 26 MHz frequencies in a more efficient way, other modes of broadcasting that could facilitate the implementation of DRM services on this waveband have been experimented with in a local manner, this specific use having been started to be tested over the past decade. In fact, two techniques are being applied: one involves propagation by direct vision (the wave is made up of a direct ray, a reflected ray and rays refracted by the irregularities of Earth's surface), and the other, diffusion by surface wave (the wave is propagated by the earth-air discontinuity, adapting itself to the curvature of Earth). As explained by Mr Peña, despite the trials, there is a lack of knowledge about the system when applied to this waveband. His thesis contributes to characterising this type of radio broadcasting at a theoretical level, through the interpretation of the data obtained experimentally.
To begin with, Mr Peña undertook an analysis of propagation by terrestrial wave (in direct vision and surface wave modes) in this waveband, and drew up a prediction model that could identify local coverage of DRM networks found at each portion of the spectrum, in each situation. Moreover, he studied the conditions of ionospheric propagation which, at these frequencies, could cause interference amongst local digital radio broadcasting services. Finally, another contribution by this engineer was to determine the current levels of noise and to characterise other electromagnetic perturbations that might influence the quality and reliability of reception with these kinds of signals.
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