twisted waveguide minimum length?

hi all.
i want to design a twisted waveguide (WRD650).
i saw in the manufactured twisted waveguides that the manufacturers set the length of the waveguide about twice times of maximum wavelength. i have to design it with shorter length. is there any rule of thumb for designing twist waveguides?

thank you in advance

You already mentioned the rule. A 90° twist should be done over a distance greater than two wavelengths of the frequency in use.
If a complete inversion is required, (e.g. inversion of 180°) the twist should be done over a four wavelength distance.

thank you vfone.

could you explain more about the reason of this.

i have 45 degree for twisted double ridge waveguide in 6-18GHz, so according to your description it should be grater than one lambda. correct?

For a linearly polarized TE10 field the inversion rule is linear with wavelength, so for a 45° inversion the distance should be greater than one lambda.