It is known that the configuration of the input driving system may be of importance to the impedance of antennas at high frequencies. In this paper, a region in the vicinity of the input is considered as a transducer between the TEM mode on the driving transmission line and the spherical nearly‐TEM mode on the antenna system, and the constants of this transducer are found both by calculation and measurement for a particular case. The calculation is based upon quasi‐static concepts, utilizing an electrostatic field map obtained from an electrolytic tank to give the approximate form of the electric field lines. The measurement is made by finding three input reactances corresponding to the input region closed by three spun aluminum hemispheres of different radii. The three sets of data, taken at each frequency, are sufficient to determine the constants of the transducer. They are expressed here as a pi‐network for comparison with calculated values.The main portion of the antenna is analyzed substantially as in Schelkunoff's approach, with the TM modes in space and in the antenna region expressed as a shunt admittance at the end of the antenna in the TEM mode equivalent circuit. This admittance is then transformed to the desired reference on the antenna by a perturbation calculation. From there it is transformed to a desired reference on the feed line by means of the input network constants. Values calculated in this way are compared with measured values over the range 200 to 1000 mc/sec. for the particular monopoles studied. Agreement is not perfect, but appears considerably better than without consideration of the specific input configuration.