Tom:
From your distance of 12 to 13 miles, any antenna will work with the amplifiers turned off. The incoming signal is overloaded (for your test), and actually drives down the "strength meter" reading.
My Sony Bravia from 2005 has Menu for TV repairmen. It gave all the necessary information they needed. It included dB readings, actual RF channel, and other useful technical information.
Cutting back any unneeded amplification could either keep their best reading, or actually raise it for really close stations, or stations transmitting under higher power.
Jeff's method is best. He could do readings using the Sensor Pro, but the test was for comparison of the antenna-heads.
Mark B.
