4-17. An air-filled circular waveguide has a diameter of 4 cm and is to carry energy at a fre- quency of 10 GHz. Determine all TEлp modes for which transmission is possible. 4-18. A circular waveguide has a cutoff frequency of 9 GHz in dominant mode. a. Find the inside diameter of the guide if it is air-filled. b. Determine the inside diameter of the guide if the guide is dielectric-filled. The rela- tive dielectric constant is e, = 4.