Keep in mind that, when behind hills or other obstacles that block the direct signal path(s), signals propagate by diffraction (
http://www.hdtvprimer.com/antennas/siting.html) which will tend to weaken the signals and to cause the signals to cause aggravated hot and cold spots. You may need to try multiple spots to find one that happens to work for all your channels, so be patient if the first spot you've picked out isn't successful.