cutoff freq formula for conical horn antenna's waveguide?

hi,

kindly if any knows the formula used for conical waveguide used in conical horn antenna.

please inform me if rectangular waveguide's formula is applicable to it?

also please mention the reference from where you are coding the formula.

thanks.

Try to read a textbook on microwave basics.
All hollow-pipe waveguides, rectangular or circular, exhibit a cutoff frequency , or rather a wavelength which simply would not fit into a narrow pipe. The smallest cross-section determines it.
Similar rules hold for sound waves- given a pipe diameter, a wavelength long enough could not pass through the narrow cross section.