QRM on 70cms

After suffering QRM on the Bracknell 70cms repeater, GB3BN, for several months now, it was interesting to note that within several days of us finding the cause, a similar case was reported elsewhere.

The interference is noticed as a regular “chuffing” sound whenever the repeater is open but isn’t being overriden by a stronger signal.  Thanks to the excellent efforts of the TVRG, the QRM was been traced to a building site in Harrow on the Hill, where the 10mW transmitters on top of the cranes there have a direct line of sight to the GB3BN antenna high above Coppid Beech roundabout.  The transmitters appear to be linked to a crane collision avoidance system, tranmitting in this case on the GB3BN input frequency of 434.6MHz.

It was interesting, therefore, to read this report from G4UVJ about a similar problem affecting 433.5MHz in the Thames estuary area.  I wonder if it’s the same system in both cases?


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