What is the strength of a TV signal at the antenna?
The signal strength at the antenna is measured in dBm, decibel-milliwatts. A 0 dBm signal corresponds to 1 mW, -90dBm is 1picowatt (pW), one billionth the strength of 0 dBm. 

Given the power of the transmitter, transmitting antenna gain, distance, receiving antenna gain and a few other parameters, such as obstructions and frequency, it's possible to calculate the approximate power at your receiving antenna. A television may need -65 dBm or stronger signal for clear reception.

An example of signal-strength calculation is shown at Mathworks.

Some stations make public a chart showing the signal strength at various locations on a map. If you can't find one for a local broadcaster, contact the station for info.