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We use observations of the continuum radio emission at 1420, 820, 408, 34.5and 22 MHz to estimate the mean brightness temperatures of the HB 21 supernovaremnant (SNR) at five frequencies. We also presented mean spectral index of HB21. The spectra of HB 21 are estimated for mean temperatures versus frequencyfor 1420, 820, 408, 34.5 and 22 MHz. We also presented $T-T$ plots of threefrequency pairs: between 1420--34.5, 1420--22, 34.5--22 MHz. We noticed flatterspectral indices at frequencies below 408 MHz. Probably this is due to theabsorption by thermal plasma at low frequencies.

Spectra of HB 21 supernova remnant: Evidence of spectra flattening at the low frequencies