Jul 30, 2018 · Measure the diameter of the beam at various positions around the focal point: take at least 10 data points, approximately half within the first Rayleigh distance of the focal point, and the other half beyond 2 Rayleigh lengths. Use the regression equations detailed in the norm to fit a hyperbola to your data points for both the X and Y axes. With a open circular waveguide antenna feed (scalar feed) the focal length will be the distance from the reflector to a phase centre point just inside the circular feed wave guide aperture. If the feed is a horn, the phase centre (i.e. the apparent origin point of the rays) of the feed may be further back inside the horn.