BBC R&D

Abstract

BBC Strategy and Distribution have asked Kingswood Warren staff to look into a question that has existed since the advent of digital broadcast services in the 1990s: Given the nature of COFDM signals, what voltage rating is required for transmitter antenna feeders - or, for that matter, the associated antennas and combiner/splitter units? The conventional wisdom is that, statistically, a COFDM signal is similar to Gaussian noise; hence there is a possibility of very large voltage peaks that could initiate flashovers. Until now, the problem has been contained by the relatively low power levels of the COFDM signals. However, the situation will change as the television 'digital switchover' takes place over the coming few years.

Not surprisingly, the BBC needs some reassurance that terrestrial television will continue to work after switchover. There is also the related question of how to carry out realistic acceptance testing. A transmitter might work perfectly overnight, for instance, but does that mean a flashover is unlikely over the subsequent year?

This Technical Note looks at the questions of feeder ratings and acceptance testing. The conclusion is that the current philosophy regarding feeder ratings is adequate, and that an overnight acceptance test at slightly enhanced power should be sufficient to highlight any potential flashover problems.