The HD switching is looking for about 55dBf to switch from analog to HD. Also, the signal must be in the neighboorhood of +50 S/N to allow switching. Some antennas do contribute noise, especially the amplified ones, which is a no-no. My Terk FM -Pro does not work well on HD. An antenna that contributes little noise is a coxial sleeve. In addition, perfect impedance match is also important because it causes self-inflicted multipath. Don Scott