Complex sentence: The range over which the dominant mode is guaranteed to be spurious-mode free is less than that in rectangular waveguide; the ratio of highest to lowest frequency is approximately 1.3 in circular waveguide, compared to 2.0 in rectangular guide.
Split into simpler sentences: The range over which the dominant mode is guaranteed to be spurious-mode free is less than that in rectangular waveguide. The ratio of highest to lowest frequency being approximately 1.3 in circular waveguide compared to 2.0 in rectangular guide.