Fox Problem

My back garden is about 12m x 12m surrounded by a 2m high wooden fence.  I live in an urban area.  I have a problem with fox wandering the garden during the night.  i have 2 - 3 year old lhasa apso dogs who run around the back garden during the day and early evening.  If there was a confrontation betweeen the fox and the dogs I am concerned the dogs may get harmed or worse !   -   Any suggestions please ?