A contour map of the supernova remnant HB 21 has been constructed from observations at a frequency of 2695 MHz. The integrated flux density at 2695 MHz is flux units. Variations of the spectral index across the remnant were found by combining the map at 2695 MHz with previously published maps made at frequencies of 178, 610.5, and 1430 MHz. The flux-density spectrum appears to be relatively straight in the central and southern parts of the supernova remnant, and has a mean slope of about -0.40. In the northern part of the supernova remnant, the spectrum is curved, with a turnover at about 250 MHz. Several possible interpretations of this spectral feature are discussed.