What is effect of a small change in wire radius?

—Modeling Center-driven Dipole with Microstripes

Figures 1 and 2 show the input impedance for the same configuration with a 2.5 mm wire radius. The resonant frequency is 143.6 MHz, the impedance at 150 MHz is 84.2+j39.0 ohms. This is a relatively small change; however the amplitude of the peak values near the resonant frequency change significantly.

Fig. 1. Input Impedance (cylinder radius = 2)

Fig. 2. Input Impedance at the first resonant frequency (cylinder radius = 2)