Focal ratio (f/D)

A parabolic dish focuses everything onto a single point, and the feed sits there. The focal ratio f/D tells you the shape: a low f/D is a deep dish with the focus close in; a high f/D is a shallow dish with the focus far out.

What matters for the feed is the angle the dish’s rim subtends at the focus. The feed has to light up the whole surface out to that angle and ideally no further. Too narrow a feed pattern under-illuminates the edges and wastes collecting area; too wide a pattern spills past the rim and picks up warm ground, raising your system temperature. The usual target is about 10 dB down at the rim.

Rim half-angle by f/D

Prime-focus satellite TV dishes, the kind most amateur radio telescopes are built from, are typically deep, in the 0.3 to 0.45 range. Offset dishes have a different geometry and their nominal f/D doesn’t map onto this table directly.

Measuring a dish you found

Lay a straight edge across the rim and measure the depth d at the centre. Then f = D² / (16 d).

Worked example: a 3.0 m dish that’s 0.469 m deep has f = 9 / 7.50 = 1.20 m, so f/D = 0.40. The feed has to cover ±64°, and the phase centre goes 1.20 m above the vertex. Our dish and horn calculator does this from either the depth or the f/D and shows the gain and beam you can expect.

Feeds for deep dishes

A simple open-ended circular waveguide, the cantenna, has a broad pattern that suits f/D around 0.35 to 0.45. Shallower dishes want a more directive feed, often a cantenna with a choke ring or a small horn. LAB-010 compares the options.