The ultrawide‐band antenna with original transformer providing the required electromagnetic matching of the coaxial cable with the small‐size single‐arm equiangular spiral antenna is proposed. The measured input reflection coefficient of the small‐size spiral antenna is less than −10 dB within the frequency band from 7.5 to 40 GHz. The near‐fields, radiation patterns and axial ration within the impedance antenna bandwidth are presented and discussed. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2190–2194, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27018

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The stop-band filter with the new band-rejection element is proposed. The element is a coaxial waveguide with the slot in the centre conductor. In the frame of this research, the numerical and experimental investigations of the amplitude-frequency characteristics of the filter are carried out. It is noted that according to the slot parameters the two typical resonances (half-wave and quarter-wave) can be excited. The rejection band of the single element is defined by the width, depth, and dielectric filling of the slot. Fifth-order Chebyshev filter utilizing the aforementioned element is also synthesized, manufactured, and tested. The measured and simulated results are in good agreement. The experimental filter prototype exhibits the rejection band 0.86 GHz at the level −40 dB.