Cobham Antenna Systems - European Antennas

Electronic WarfareCavity-Backed Spiral Antennas - Receive onlyOmni antennas onarmoured vehiclesCavity-Backed Spiral Antennas - Receive onlyThe cavity-backed spiral is the traditionalantenna that provides very wide bandwidth,circular polarization and high fidelity monotonicpatterns and is the mainstay of an EWdirection-finding system for instantaneousthreat analysis.The spiral element comprises two paralleltransmission lines formed into a planar spiralwhich are fed via a broadband balun, usuallyat the centre point of the spiral. The highestfrequency of the spiral is determined by thefineness of the feed structure at the centre ofthe spiral, whilst the lowest usable frequency isdetermined by its diameter.RadomeSpiral elementHousing (cavity)AbsorberBalunConnectorThe spiral is mounted at the front of a cavitywhich is filled with RF absorbing material toensure that internal reflections, which couldcorrupt the forward-looking radiation patterns,are nullified.It is important that cavity–backed spiralantennas provide a high level of fidelity fromone antenna to another and therefore criticalcomponents are sourced from the same batch,with antennas manufactured and tested inbatches.To provide accurate direction finding (DF),an array of spiral antennas (usually 4, 6 or 8),each with 60-90° beamwidths are arranged toprovide 360° coverage.The antennas in a DF system must be phaseand/or amplitude matched across the range offrequencies (for example 2-18GHz). This meansthat all spiral antennas used for this type ofapplication must be fully characterized (at 10to 100 frequencies) and grouped into matchedbatches.High accuracy phase and amplitudemeasurement is carried out in the company’sspherical anechoic test chamber.Many of our standard cavity-backed spirals maybe used in DF systems, covering bands from300MHz to 20GHz, for a variety of platformsincluding UAVs, helicopters, sub-sonic aircraftand terrestrial systems. Special frequencies andfunctions can be incorporated into designs fornew applications.Wide band cavity-backed spirals (10) showing betterthan 0.4dB amplitude matching over wide beamwidthWideband cavity backed spirals (10) at 3.5GHz showingbetter than 10° phase matching over ±30° in azimuth6