SCR-584 Technical Description
Image – ‘Electronics’; Fig 2, Dec ’45, p.104
The Verlort Plan Position Indicator (PPI) was driven by a mechanical computer. Once target acquisition occurred, the radar operator needed to keep a ‘hairline’ indicator manually centred on the target’s echo to maintain track.
Even more intriguing were the target acquisition techniques.
For targets where only crude position information was available, a helical scan was used to acquire the target. The dish was swung in a circle at 6 rpm along as it moved with the reflector tilting to and fro about 4° per dish revolution to create a helical spiral about 20° wide along the expected track path - see 'Helical scan mechanism' diagram on right.
Where there was more certainty about the target path, a nutating scan mode was used. The slightly offset dipole feed was rotated rapidly to produce off-axis radiation increasing the effective beam width from 2.5° to about 5.5° . Nutation was switched off when the target was acquired. See the ‘conical scan beam’ diagram below left and the ‘nutating antenna dipole feed’ below right.
Image – ‘Electronics’; Fig 1, Nov ’45, p.104
Image – ‘Electronics’; Fig 5, Dec ’45, p.107
