I am trying to use an Elgato Eyetv Diversity DVB-t usb tuner with my macbook but am having some problems with signal quality.

I am located in Hamilton City and am using the small indoor aerials that the tuner came with (I know, get a larger aerial...) and I can get 80% - 90% signal strength however sometimes I can get ~ 90% signal quality and other times it is only around 20 - 40% and switches between quite frequently. 

And ideas what causes the signal quality to change so much and so often?