Circular waveguide single-grooved linear to circular mode transducer
Image of the Circular waveguide single-grooved linear to circular mode transducer.

Circular polarised antenna systems often require a mode converter to convert between a single mode (generated by a transmitter or required to drive a receiver) and two orthogonal modes with a 90 degree phase difference (at the antenna feed point). The requirements for such converters are high performance (low loss) and compact size.

The single-grooved linear to circular mode converter operates on the TE11 mode and generates orthogonal modes with the required phase shift through the simple addition of a single groove in the wall of a section of circular waveguide.

In the single grooved mode converter, grooves are aligned in a straight line, positioned on opposite sides of the waveguide and orientated +/-45 degrees with respect to the circular waveguide cross section. Each row of grooves consists of two centre and two end grooves.

The single grooved converter employs a circular waveguide that supports the propagation of only the first fundamental circular waveguide mode (TE11) but not the TM01 mode. This limits the operational bandwidth to an absolute maximum of 26% (cutoff frequency ratio between the TE11 and the TM01 is approximately 1.3:1).

The graphs on the right below are normalised to the cutoff frequency of the TE11 mode. The grey lines indicate the TE11 and the TM01 cutoff frequencies, respectively. In order to show the circular output mode, transmission coefficients in a plane parallel to - and perpendicular to the incident E-field of the TE11 mode are plotted. These two orthogonal TE11 output modes and the 90 degree phase difference between them show how the mode converter splits the power equally and introduces a 90 degree phase difference between the two orthogonal modes when the operating frequency is between the grey lines.

Typical reflection and transmission coefficient magnitude (top) and phase (bottom) versus frequency for the single-grooved converter.