Quad-ridged conical horn antenna
Image of the Quad-ridged conical horn antenna.

The quad-ridged horn or quadruple-ridge flared horn (QRFH) has two important characteristics: (1) a nearly constant beamwidth over a 5:1 frequency band for nominal 10 dB beamwidths ranging between 50 Ohm and 140 Ohm (2) the input impedance can be designed between 50 Ohm and 100 Ohm. The horn is also popular in weight and cost-sensitive applications where multi-polarisation capable antennas are used, as only one single-ended low-noise amplifier (LNA) is required per polarisation.

The QRFH may be divided into two distinct sections - the coaxial-to-quad-ridged-waveguide transition and the flared ridge section. The basic working of the transition is to convert the input coaxial TEM mode to a TE11 aperture field distribution at the flare-transition intersection (commonly called the throat of the horn). Only the dominant TE11 mode should be present and all unwanted higher order modes should be suppressed. The purpose of the flare of the QRFH is to generate the higher-order modes required to achieve a good distribution in the horn aperture from the dominant TE11 mode incident at the throat.

The operational bandwidth of the QRFH is limited by the point at which the contribution of the secondary mode (TE12) excited by the coaxial feed in the transition becomes considerable enough to impact on the aperture distribution - and therefore the radiation pattern of the horn.

The performance shown below is for a 100 Ohm input resistance and a nominal 10 dB beamwidth of 75 degrees. The nominal beamwidth is taken as the average between the 10 dB beamwidths in the two orthogonal planes. The radiation performance for two different polarisations are plotted: dual-linear and right-hand circularly polarised. The radiation pattern is more symmetrical when excited for circular polarisation.

Total linear gain 3D patterns at (a) 0.5 x f0, (b) f0, and (c) 1.5 x f0
Total circular gain 3D patterns at (a) 0.5 x f0, (b) f0, and (c) 1.5 x f0

The graph below shows the relationship between the flare length and the flare diameter on the nominal beamwidths. For this illustration the flare diameter was set to a wavelength at the minimum frequency (0.5 x frequency centre). The results show that as the flare length becomes longer (ratio becomes larger), the nominal beamwidth at the centre frequency drops and the change in beamwidth response is more gradual versus frequency.

Nominal 10 dB beamwidth for different flare-length-to-flare-diameter ratios