HD Antenna Specs / Distance
I wondered the same thing when I shopped for an antenna. Unfortunately, I couldn't find a set of specs or a way of comparing antennas from different manufacturers. All I found were reviews but they were all subjective and didn't help me rank one antenna over another, especially when the antennas were from different manufacturers.

That's when I bought various antennas and tested them myself using a Winegard Signal Strength Meter and my Tivo. Using the Winegard (which I have no affiliation with) was a very objective way of comparing them because you select a frequency and it gives you a signal strength. 

I test the antennas in two different ways. For the first test I rotate the antenna for each frequency and record the maximum signal strength. For the second test I point the antenna in the same direction and leave it stationary (which is how most people use an antenna). I then run through the frequencies and record the results.

Even though I can only test the frequencies in my area, I figured these results might be useful to others so I put them on my website at: http://byebyecabletvbill.com/tv_antenna_side_by_side_comparisons.php. As time progresses, I'll be testing more antennas and adding the results there.