Horn antennas occur in a variety of shapes and sizes, ranging in application from electromagnetic sensing, RF heating, biomedicine, reflector feeds, and as reference source for antenna testing. In some situations, it is required that the horn antenna operate over a wide frequency band, such as in UWB applications. It may also be required that the horn be able to handle two orthogonal polarizations and minimize interference between them.
Corrugated horns are able to operate up to bandwidths of about 2.4:1, by special attention to the input match. Other approaches are needed when bandwidths as wide as several octaves are required, taking note that input match and radiation pattern behavior are parameters which usually define UWB operation.
A common multi-octave bandwidth technique is to use a tapered ridge in the horn aperture, to serve as an impedance transition between the ridged waveguide feed and free-space. A double-ridge is used for single linear polarization and a quad-ridge for dual linear or circular polarization. Circular polarization is achieved by feeding the two coaxial inputs in phase quadrature....
The design objective is to taper the ridges to make a smooth transition from the input (usually a coaxial transition to the ridged guide) to the aperture to achieve a good aperture match.
If quad-ridges are used, it is vital to maintain symmetry between the ridges, otherwise cross-polarization will suffer. [Volakis]
While quad-ridge horn patterns may vary significantly over the wide frequency range, an acceptable return loss may be achieved over several octaves.