Military - Ultra Wide Band

The spiral element comprises two parallel transmission lines formed into a planar spiral, which are fed via a broad-band balun usually at the centre point of the spiral.

The highest frequency of the spiral is usually determined by the fineness of the feed structure at the centre of the spiral whilst the lowest frequency is determined by the outer circumference of the spiral. The spiral is mounted at the front of a cavity which is filled with RF absorbing material. This ensures that internal reflections which could corrupt the forward–looking radiation patterns are nullified. It is especially important that cavity-backed spirals antennas provide a high level of fidelity from one antenna to another and therefore critical components are sourced from the same batch.(see figX)
These antennas are used for receive-only applications and have been the stalwart of EW/ESM systems, for many decades especially airborne direction finding (DF). The benefit of this type of structure is that it provides a very wide bandwidth (such as 2-18GHz), which when coupled with its circular polarisations provides RF for instantaneous threat analysis. To provide direction finding an array of antennas (4,6 or 8) are arranged to provide 360 degree coverage. The phases and/or amplitudes of each antenna are compared from which a direction of arrival (of the threat signal) is determined in real-time. This allows the high value platform to be able to take evasive action.

Depending on the system requirements a set of cavity-backed spiral antennas must be phase and/or amplitude balanced over the range of frequencies. This is a service that European Antennas can provide, using the anechoic test chamber to measure patterns in both phase and amplitude with spherical coordinates.

European Antennas provides many standard cavity-backed spirals covering bands from 500MHz to 20GHz, for a variety of applications including for sub-sonic aircraft. Special frequencies and functions can be incorporated into new designs.