The planar spiral antenna being connected with the waveguide and operating in the long‐wave region of the millimeter range is proposed. The antenna has the bandwidth of 6 GHz and the axial ration which does not exceed −3dB in the frequency band 32–36.5 GHz. The radiation pattern shape is slightly changed within the operational frequency band. The results of simulations and experimental data are in good agreement. The nature of radiation pattern and axial ration changing within the operational frequency band is explained from the analysis of near‐field distributions measured at two orthogonal polarizations. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:293–297, 2014