Andy Fletcher

I came across an interesting design problem recently in an indoor PMR distribution system. The system comprised both radiating feeder and antennas all driven from two RF over fibre amplifiers in different locations.  The two amplifiers are used for resilience and drive the feeders from each end.

Because the antennas are receiving signals on the same frequency from two sources there exists a possibility of signal cancellation at certain frequencies. This would result in antennas failing to transmit (and receive) in certain areas as the signals cancelled at specific frequencies. The cancellation would depend on the relative distances from the amplifiers and the velocity factors of the intervening feeders.  The phasing of the signals would change with time and temperature making it near to impossible to predict the behaviour.

The solution was simple. Ensure that the signals at the antennas coming from the two sources are at different levels and therefore cannot completely cancel. This limits the variation in antenna gain across the band to a few dB. In this case a minimum distribution network design difference in power level of 6dB was adopted which results in a gain variation of 2.5dB across the band.

If the RF over fibre amplifiers are matched to within 2dB then this ripple could be about 4dB if the distribution network is set to 6dB difference. Obviously the coverage contour from the antenna would have to be at least at least 4dB above that level required for satisfactory performance to ensure that there are no dead spots.

The table below shows the worst case ripple resulting from the antenna being driven from two sources.