Antenna for the AN/URN-3 Tacan beacon

A detailed description of the aerial used in the Tacan navigation system operating in the 1000 Mc/s frequency band for ship-or shore-based installation. The main radiating element is a fixed central array composed of 7 biconical dipoles mounted within a fibre-glass cylinder 4 in. in diameter and 48 in. long. A harmonic-frequency system is used i.e. the aerial radiates a 9th harmonic modulation component as well as the fundamental, giving increased accuracy. By suitable phasing of the feeds the vertical lobe of the radiation pattern is tilted upwards thereby reducing the effect of ground reflections. Modulation is purely mechanical and is provided by 2 coaxial cylinders fitted with parasitic wires which rotate together round the centre array. The inner cylinder provides 15 c/s modulation and the outer 135 c/s modulation, the composite pattern rotating at 15 rev/sec. The amplifiers and servo-systems for pitch, roll and azimuth stabilisation of ship based aerials are described.