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The present paper considers the connection between complex input impedance and the physical dimensions for planar ultra wideband (UWB) antennas. The first time the effect of both the actual radiator width and length for impedance behaviour is comprehensively presented. Also the effect of feed point dimensions on complex impedance is studied. The investigations involve both UWB single-resonant dipoles to cover bandwidth≥ 500 MHz and a multi-resonant dipole for the entire Federal Communications Commission's (FCC) frequency band of 3.1-10.6 GHz. Lumped-element equivalent circuits are used identically with 3D antenna simulations in order to observe the corresponding impedance behaviour with the studied antennas. The used equivalent circuits consisting of series-and parallel-resonant stages are widely accepted in the open literature. The series-resonant circuit of equivalent is observed to have the analogue to the antenna feeding area. The physical dipole dimensions in terms of a length and width are connected to parallel-resonant part, which mainly determines the antenna input impedance. The resistance of a parallel-resonant stage behaves as the maximum value of real part of dipole impedance with an influence on bandwidth together with the ratio of parallel capacitance C and inductance L. The increase of the antenna physical width has an effect on bandwidth, because of the wider the antenna, the higher the capacitance in the antenna feed. Since the traditional dipoles are used for these studies, the results can be extended in several ways for other antenna types or, for instance, to verify the effect of body tissue, close to …