Applet Notes:

This applet demonstrates how the major design parameters affect the gain and beamwidth performance of parabolic reflector antennas.
Reflector dish geometry parameters, including, diameter-to-wavelength ratio (Diam/W'Length), focal length-to-dish diameter ratio (F/D Ratio), and the distance of the feed phase centre relative to the focal distance (Feed Dist:/F ratio), can be adjusted using the scrollbars.
The feed beamwidth (Feed BW) is varied by adjusting the relevant scrollbar and there is a choice of either sin(x)/x or Gaussian feed beam shapes from the drop-down menu.
The upper left display models the feed geometry and the feed polar pattern.
The lower left display indicates the dish illumination across the reflector aperture. The main plot (blue) shows the full aperture illumination and the secondary plot (magenta) shows the illumination profile across the central horizontal strip. The latter can more easily be related to the feed pattern.
The third and largest plot on the right displays the resulting beam pattern. All plots are normalised, and the key performance values are updated in text below the beam plot.
Moving the mouse in aperture illumination-aperture position, or beam pattern-azimuth space, displays the plot values related to x-axis positional value.
Default values may be modified and the display updated using the relevant scrollbars.

User Notes:
The applet default values are close to an optimum design. Explore the effect of defocusing, or changing the F/D ratio with a complementary change in the feed beamwidth. The full aperture plot is distorted by phase variation across the reflector aperture plane when the feed is defocused.
The assumptions made in this analysis include an ideal feed antenna with no losses, 100% feed efficiency, and no aperture blockage so the results will be somewhat optimistic by 1 or 2dB.
The results apply to circular dishes fed by antennas with equal azimuth and elevation beamwidths.
The aperture illumination mouse position readout can be used to assess the edge illumination.

Applet Help