No abstract

Abstract-A semi-lumped balun transformer for UHF and ultrawideband (UWB) dipole antennas is presented in this paper. The proposed structure is based on two asymmetric filters that also transform the variable antenna impedance into the desired source impedance. These asymmetric filters make use of a binomial impedance transformer in each filter section. The asymmetric filters (one low pass filter, LPF, and other high pass filter, HPF) allow the balun bandwidth to be increased while the binomial transformer matches the variable balanced dipole impedance. In this way the ripple in the balun response due to the variation of the UWB dipole impedance is reduced. A balun for a UHF and UWB dipole antenna working from 220 to 820 MHz (bandwidth of 4:1) has been achieved with losses lower than 1 dB. These types of baluns are particularly useful in the low microwave frequency band.

The paper shows that a mode similar to the TMo, in circular geonietries can be also excited in non circtilar patches, short circiiited,in the center: the rectangular geometiy has been chosen /or it. Most oJthe interesting characteristics and excellent behaviour of the TM,, short circuited ring, mainly irsefiil j a r niobile user teriiiiiia1.s. have been also found in the proposedgeomerry.

A complete study of the bandwidth extension of asymmetrical stacked patches as been carried out. A method that combines the effect of upper patch size and its displacement along the E-plane axis has been analyzed. Results show new possibilities in the design of such antennas for the two studied cases: square and circular patches. ᮊ 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 31: 53᎐56, 2001.