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The effects of a Klopfenstein tapered transmission line used to feed a typical planar monopole UWB antenna have been investigated in terms of impedance bandwidth, phase linearity and radiation properties. The analysis shows that the taper allows manipulation of the resonances occurring within the operating bandwidth. A Klopfenstein tapered feedline that matches the 50 Omega input impedance to 30 Omega along 50 mm is able to reduce the maximum group delay variation by 85% with an acceptable variation of the impedance bandwidth and minor changes in the radiation pattern.
Ruvio, G., Ammann, M.: Effects of Klopfenstein tapered feedlines on the frequency- and - time domain performance of planar monopole UWB antennas. IEEE Antennas and Propagation Society International Symposium,pp.1-4. San Diego, California, 5-11 July, 2008.