I assume you are referring to the feedpoint resistance and reactance. Stearns, k6oik, antenna impedance models, presented at arrl pacificon 2004 is an excellent treatment of the subject. For example, fair-rite's 73 material:

What do these numbers represent, and how can i use them in amateur radio? Do they allow me to predict inductance or losses of … The wikipedia s meter article cites: Iaru region 1 technical recommendation r. 1 defines s9 for the hf bands to be a receiver input power of -73 dbm. This is a level of 50 microvolts at the receiver's … Per the above; Although correct based on a free space dipole impedance of 73 ohms, i would suggest that very few practical dipoles are close to this based on the typical proximity to ground and the …

This is a level of 50 microvolts at the receiver's … Per the above; Although correct based on a free space dipole impedance of 73 ohms, i would suggest that very few practical dipoles are close to this based on the typical proximity to ground and the …