On the Octave Bandwidth Properties of Octagonal-Shaped Waveguide Mode Transformers

An octave bandwidth concept for compact rectangular-to-circular and square-to-circular waveguide transitions based on the use of intermediate octagonal-shaped sections is presented. Apart from its inherent bandwidth capability and spurious mode free operation, this architecture offers some other useful advantages such as easy fabrication, low losses, and extremely short length (less than 0.75 λ₀ , with λ₀ being the wavelength at the center frequency), which allows their use in the millimeter-wave frequency range. Experimental verification is provided through the measurement of a rectangular-to-circular mode transformer covering more than the super-extended C -band (3.6-7.025 GHz) with 30-dB return loss and 0.1-dB insertion loss, which represents state-of-the-art performance in terms of bandwidth.