Transmission lines/Antennas power delivered to a load, why halved?
The 1/2 coefficient is from the use of peak values. To convert an undistorted sine or cosine wave from peak to RMS you divide by sqrt(2).

Since P = V*I=V^2/R = I^2*R the 1/sqrt(2) is squared no matter how you slice it and this formula assumes you are plugging peak values. It's important to be mindful of whether you are using peak or RMS values each time you have a sinusoid!