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A simple formula based on transmission line modeling (TLM) is proposed to determine the VSWR < 2 impedance bandwidth upper frequency limit for a staircase-profile planar monopole antenna. After a brief description of TLM analogy for planar monopoles, the formula is derived from a theoretical exposition. Accordingly, four prototypes are designed, implemented and measured in such a way that they are required to cover direct sequence ultrawideband (DS-UWB) first band (up to 4.85 GHz), UWB (up to 10.6 GHz) and possible future broadband systems (e.g., up to 15 and 21.4 GHz). The formula predicts this limit with a maximum error of 7.4% in the prototypes studied. The height of the antennas is chosen so that they help to filter WLAN 5 GHz band on the H-plane. This condition determines the lower impedance bandwidth limit (1.3 GHz for all of them). The resulting improvement on antenna system transfer functions (ASTF) is also discussed. Index Terms--Broadband antennas, monopole antennas, transfer functions, transmission line modeling (TLM), ultrawideband (UWB) antennas.

From the current distribution on planar monopoles, transmission line modeling is applied to study this kind of antennas. Some reported techniques for broadband monopoles are approached by using this model from a qualitatively point of view. Conclusions are derived that help to match the monopoles over an ultrawide bandwidth regardless of whether they are folded or not. Folded configurations are obtained in order to provide solutions to specific designs and improve radiation pattern maintaining the planar monopole broadband behavior. Three compact folded prototypes with greater than 1:38 bandwidth are implemented and tested. Pulse distortion is also discussed for this type of applications. Index Terms--Folded-plate monopoles, impedance bandwidth, planar monopoles, transmission line modeling, ultrawideband (UWB) antennas.