"Z"11 and "Z"22 are simply the ordinary driving point impedances of a dipole, thus 73 + j43 ohms for a half-wave element (or purely resistive for one slightly shorter, as is usually desired for the driven element). Due to the differences in the elements' lengths "Z"11 and "Z"22 have a substantially different reactive component. Due to reciprocity we know that "Z"21 = "Z"12. Now the difficult computation is in determining that mutual impedance "Z"21 which requires a numerical solution. This has been computed for two exact half-wave dipole elements at various spacings in the accompanying graph.