The weaker stations have hills or mountains between the transmitting antenna and receiving antenna in this situation 2 edge. This reduces signal strength and causes the signals to bend a little. The bending effect creates uneven signal strengths at the receiving antenna heights. In your situation have the antenna lower then 30 feet the signal is stronger. 15 to 20 feet will be a good height for the antenna. You can go to the >>Start MAPS<< part of tvfool , put in your exact address , select pending stations included , and enter different antenna heights , I suspect that improvements in signal strength of 10 NM (dB) or more will be at 60 feet antenna height and higher. Here are places to buy antennas and etc..
http://www.3starinc.com ,
http://www.solidsignal.com ,
http://www.starkelectronic.com ,
http://www.amazon.com . . If you will like to put a antenna 60 feet or higher thats Ok.