Admittance of the fat cylindrical antenna. Experimental study

The fat cylindrical antenna was used as a unipole over a vertical ground plane, and was driven by a slotted coaxial line carrying a current probe. The antenna was a continuation of the inner conductor of the coaxial line, and its radius was a. Admittances were measured and calculated for a/¿ in the range 0.0128¿0.0179, where ¿ is the wavelength. The experimental results are presented in rectangular plots of conductance G or susceptance B against ßh, where h is the length of the unipole and ß=2¿/¿. There is no comparison with theoretical results, since no theory for the fat antenna is available at the present time. However, the admittances have a behaviour similar to that of the admittances of thin antennas. Curves of ßh and G at resonance and antiresonance have been plotted against a/¿. The resonant and antiresonant lengths approach n¿/2 (n odd or even, respectively) as a/¿ gets smaller, i.e. as the antenna gets thinner. This agrees with the predictions of the King-Middleton theory. It is expected that, as the antenna gets very fat, the difference between the resonant and antiresonant lengths on the one hand and n¿/2 on the other will be very large, showing a radical departure from the behaviour of the thin cylindrical antenna. The conductances at resonance and antiresonance remain virtually independent of a/¿ for fat antennas.