The four American channels listed are LOS Line Of Sight , moderetly strong and easy to receive. The other American Channels on down the line are Very weak. 0 NM(dB) is the 'reference number' for reliable reception with a simple 2 element dipole 30 feet in the air. Plus 10 NM(dB) gives some elbow room and more then that is better and better. Minus 10 NM(dB) is the point at which the signal is said to be not receivable with out extreme measures. You can go to the >>Start MAPS<< part of tvfool and see how high the antenna will have to be to Pull Up the other American stations NM (dB) numbers. And will also need a extreme fringe antenna such as Winegard HD 7084P. And Yes it is fun to get the very weak Tv stations coming in on the skip or tropo. Digitalhome may have DX'ers , D = Distance , X = Unknown , you can be involved with. And here is a web site
http://www.wtfda.org