The depth of a parabolic dish is a measure of the distance from the dish to the feed/LNB structure. Deep dishes have an f/D, the ratio of focal length to diameter, of 0.25 to 0.31 while shallow dishes range from 0.37 to 0.45. When f/D equals 0.25 the feed sits along the line between the edges of the dish. In general, everything else being equal, deeper dishes have smaller side lobes because the feed and LNB are closer to the reflective surface and thus are better screened from the surrounding environment.

Determining the focal length of a parabolic dish (axi-symmetric, circular)

Focal length = f
Depth = c
Diameter = D

f = ( D * D ) / ( 16 * c )

Measure the depth using a tight fishing line across the dish and a rule to measure depth c.

If the f/D ratio is low, say 0.25 to 0.35 then the feed will be close to the dish and needs to spread its power at a wide angle to efficiently illuminate the dish. The feed therefore needs to be of small diameter. If the f/D is 0.25 the feed is level with the dish aperture, which may make it difficult to make a satisfactory feed.

If the f/D is large like 0.5 to 0.75 then the feed will be further away from the dish and needs to project its power into a narrower angle. The feed needs to be of larger diameter.

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.