IIRC, 70 miles is limit because of curvature of the earth. Indoor antenna like Silver Sensor really is supposed to be used up to 20 - 25 miles away.
If you don't have mountains, something like a Terrestrial Digital DB-4 might work (you can get generic J-mount kit like tv installers use elsewhere). Old school large rooftop antenna might also work. They also make a DB-8, but you may not need that much gain (don't know if their DB-2 is enough given 40 mile from transmitter distance).
From my own personal experience, having a sensitive tuner chip is also important, in addition to antenna itself.