Mi-Waves’ 340 series a critical consideration when using TE01 mode circular waveguide is the preservation of mode purity. Due to the similarities between the TE01 and TM11 modes, even the slightest irregularities in the circular waveguide will cause mode conversion from TE01 to TM11. And the large waveguide diameters will readily propagate TEM1 modes which degrade the purity of the TE01 signal. Extraneous TEM1 and TEMN modes cannot be reconverted to the TE10 rectangular mode – they show up as large spurious losses.
Each 340 series mode filter consists of a section of lossy wall waveguide. Because the higher order modes (TMM1, TEMN) have wall currents, they are sharply attenuated and do not propagate. Although the energy transferred to these modes is minimal, mode filters must be placed periodically along the transmission line. The TE01 mode, which does not have wall currents, passes through this section unaffected. The 340 series mode filters are available in circular waveguide sizes from 12.4 to 140 GHz. They are fitted with one male and one female type of Mi-Waves’ standard circular flanges.
The 340 series mode filters are used to prevent TE01 conversion to higher order modes. By attenuating unwanted TEM1 modes, the 340 series filters allow for the low loss transmission of TE01,02 modes in circular waveguide and eliminate unwanted resonances, it is recommended that the 340 series filters be placed at least every 10 feet in long.