This worksheet calculates the frequency of rectangular waveguide below which attenuation increase precipitously for the TE10 mode, or the waveguide "cutoff" frequency (Fco). It also calculates the attenuation in the TE10 mode for said waveguide at a frequency entered by the user. Formulae and sanity checks were provided by Tom WA1MBA.

Initial values for WR90 waveguide at 10368 MHz are inserted. To run WR90 as shown, click on "Compute". To run other examples, simply change the dimensions, frequency and units as desired and click on "Compute". Clicking the Reset button will reset the input to the intitial conditions, and clicking "Compute" will force a computation with the reinstated values.

NOTE1:.  The calculations are imperfect from just below Fco to about 0.5% below Fco. To help you avoid this anomaly, the frequency "0.5% below Fco" is shown.

NOTE2:  The "Useful" Frequency range calculated here is wider than that published for standard waveguide types. The calculated high end is the point that the next waveguide mode would exist. The published top end is specified some percentage below the next waveguide mode. When you hit the next mode, the same problem occurs as in coax; two or more modes exist and, as they propagate down the waveguide they can cancel or reinforce and the exact amount varies with the exact dimensions, frequency, and probably temperature. The published bottom end is specified some percentage above the cutoff.

NOTE3:  At higher frequencies, such as at 20 GHz and above, the surface roughness becomes an appreciable fraction (in many cases greater than) a few skin depths. The exact process of surface treatment (plating, polishing) is a specialty of waveguide manufacturers who provide the highest quality waveguide for frequencies above 20 GHz. The roughness causes loss to rise. Some very high quality waveguide can be measured with loss as much as twice the calculated value.