Antenna gain and aperture efficiency

Gain is a ratio: how much more signal an antenna collects from the direction it’s pointed than an imaginary antenna that listens equally in all directions. Expressed in decibels relative to that isotropic antenna, it’s written dBi.

For a parabolic dish the gain is set by its diameter in wavelengths, multiplied by an aperture efficiency η. Efficiency captures everything that keeps a real dish from behaving like a perfect one: the feed spilling energy past the rim, uneven illumination across the surface, the feed and its supports blocking the middle, and surface errors. Amateur dishes with a decent feed land somewhere around 50 to 65%; our calculator defaults to 55%.

Gain at 1420 MHz

With η = 0.55, from our dish and horn calculator:

Doubling the diameter quadruples the area and adds 6 dB. A 1.0 m dish at 20.9 dBi is only about 4 dB ahead of a 17 dBi hand-built horn, which is one reason a horn is a sensible first antenna (see pyramidal horn).

Effective area is the useful number

For a point source, the power you collect is flux density times effective area, so A_e is what sets sensitivity to anything compact (worked through in brightness temperature). For Galactic hydrogen, which fills the beam, more gain mostly buys a narrower beam and finer detail rather than a stronger line. LAB-010 works through when the extra aperture is worth the trouble.

Getting the efficiency you paid for

The biggest lever on η after the dish itself is the feed: it has to illuminate the dish out to the rim and no further. That depends on the dish’s focal ratio and the feed’s pattern. A cantenna matched to a deep TVRO dish is the usual amateur answer.