Per ANSI C63.5 all calibrations are done with horizontal polarization, 2 meter transmit antenna height, and 10 meter distance. Antenna factors measured in this fashion should be utilized for measurements at 3, 10, or 30 meters either horizontal or vertical.
While on the surface it appears that different antenna factors for vertical and horizontal site measurement make sense some of the differences are already accounted for in the calculation of the theoretical site attenuation value. If these are also built into the antenna factors by using a vertical antenna calibration it would skew the resultant site attenuation results and can work either for or against you. A note of caution, just because the results are what you want to see (within the + 4 dB window) don't automatically assume they are correct. If a site does not meet the + 4 dB requirement I would first recommend careful scrutiny of the measurement setup. The most significant thing I found in my setup was the dress on the transmit antenna cable. I was able to establish a direct correlation between my problem frequencies and cable lengths (i.e. wavelength (or multiple of) = length of vertical cable). I switched to an EMCO 4630 Reference Comb Generator as my signal source to eliminate all the cabling issues. I mount the 4630 on the tripod directly behind the transmit antenna. I use this setup for site attenuation measurements as well as antenna calibrations. Tom Donnelly EMC Engineer Lucent Technologies
