R&D Report 1996-06 : The Crystal Palace FM filler experiment R.H. Evans, A.J. Bower In some areas of London, listeners to BBC Radio National FM services had suffered from poor reception for some time. The problem was particularly severe immediately south of the River Thames where around one million people suffered with unsatisfactory reception on portable and car receivers. This was due to path blocking of the direct signal from the BBCâs Wrotham transmitter, located several miles south east of London, and also high levels of multipath propagation. The lack of spare frequencies in Band II meant that the problem could not be solved by using a conventional filler transmitter in the locality. A novel solution was for a synchronised, low power filler transmitter to be set up at the BBCâs Crystal Palace site, using a frequency offset of only 304 kHz away from the main Wrotham transmission. For this technique to be successful, the two transmitters need to transmit the same programme feed, the frequency offset needs to be accurate to within a few Hertz, and the timing difference of the two signals at the receiver needs to be minimised to within a few microseconds. It was not known if the introduction of the filler would actually produce a worse service for a few listeners who were, at that time, adequately served by Wrotham, so test transmissions were established for one service only - BBC Radio 4, to allow trials of the technique. Due to the success of the experiment, the Crystal Palace filler has since been extended to all four national FM services.