Standard air – filled rectangular waveguides of dimensions a = 2.29 cm and b= 1.02 cm are designed for radar applications. It is desired that these waveguides operate only in the dominant TE₁₀ mode with the operating frequency atleast 25% above the cutoff frequency of TE₁₀ mode but not higher than 95% of the next higher cutoff frequency. The range of the allowable operating frequency f is.