Sorry, but there aren't any omni antennas that will provide consistent, reliable reception at that distance. A station 75 miles away is "over the horizon:" The curvature of the earth itself is almost certain to block the direct path between your antenna and the transmitting antenna, even if both are far above ground. This lack of "line of sight" plus the distance mean weak signals. A high-gain antenna compensates for this weakness, but the higher an antenna's gain, the more directional it is -- the opposite of an omni!
For another thing, because they pick up signals equally well in all directions, omni antennas are also less than ideal for DTV. They pick up signal reflections as well as the main signal. If these reflections are strong enough, they can "confuse" the tuner to the point that it's no longer able to lock on to the stream of packets that make up a digital broadcast. That's when picture and sound are lost.
When trying to capture weak signals in several different directions, the only practical path to reliable reception is a high-gain antenna that can be pointed at the desired station using a rotor.