AbstractIn this paper, we study multifrequency antennas with planar omnidirectionality for application to mobile base stations. A method for frequency sharing is implemented by connecting the antenna elements resonating at their respective frequencies by microstrip lines so that they become an open terminal at the resonance frequencies of the other elements. We propose a loop slot antenna with parasitic elements having a configuration where a loop‐shaped slot having a circumference of about one wavelength of the resonance frequency is provided in the ground plane of the dielectric substrate; the feed is through microstrip wire lines; and the dielectric substrate is surrounded vertically by planar parasitic elements as the antenna elements. This loop slot antenna with parasitic elements is connected by microstrip lines to become an open terminal and share the frequencies. Experimental and analytic results demonstrated dual‐frequency sharing of 1.5 and 2 GHz and triple‐frequency sharing of 800 MHz, 1.5 GHz, and 2 GHz. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 90(2): 23–32, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20302