Circular waveguide double-grooved linear to circular mode transducer
Image of the Circular waveguide double-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 double-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 several grooves in the wall of a section of circular waveguide.

In the double 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 circular waveguide used in the double grooved mode converter is designed in such a way that it allows the propagation of the first two fundamental circular waveguide modes (TE11 and TM01), but not the third mode (TE21). This limits the operational bandwidth to an absolute maximum of 24% (cutoff frequency ratio between the TM01 and the TE21 is approximately 1.27:1).

The graphs on the below are normalised to the cutoff frequency of the TM01 mode. The grey lines indicate the TM01 and the TE21 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 double-grooved converter.