Upon examining the formulas for self‐ and mutual impedances of antennas, it is found that while the mutual impedance formula for separately driven collinear standing wave antennas may be used directly in determining the radiation impedance when such antennas are connected in cascade, certain modifications must be made in the case of traveling wave antennas under similar circumstances. Accordingly, formulas are derived for rhombic antennas spaced in tandem and are modified to permit the determining of the radiation impedance of two identical rhombic antennas connected in cascade.