A radiation pattern maps how strongly an antenna radiates, and by reciprocity how well it receives, in each direction. Because real antennas are three-dimensional, patterns are drawn as flat slices called polar plots.
Reading a polar plot
Rings are dB below the peak. Read the ring the line crosses, then subtract.
The plot is centred on the antenna. Distance from the centre shows relative strength in that direction, and rings are usually marked in dB, with the outer ring the peak. The strongest direction is the main lobe; weaker ones are side lobes and the dips between are nulls.
Beamwidth is how wide the main lobe is, measured between the points where power has fallen to half (−3 dB). A narrow beam means high gain.
Front-to-back ratio compares the peak to the opposite direction. A high figure means the antenna rejects signals from behind.
Front-to-side ratio does the same for 90° off the peak.
A null is as useful as a lobe: pointing one at a source of local noise can improve reception. See Noise cancelling and phasing.
Two slices of one shape
The wire is the hole of the donut. Signal is strongest straight out from its side, weakest off its ends.
The donut is the whole pattern of a half-wave dipole, but a page can only show a slice. Looking down gives the azimuth cut, the pattern around the compass. Looking from the side gives the elevation cut, the pattern versus angle above the horizon. Slide the elevation below to see how the azimuth cut changes with the angle you slice at.
Same antenna, two views. The elevation cut shows how strength varies with takeoff angle; the azimuth cut shows how it varies around the compass at one chosen takeoff angle. Relative field strength, linear scale; ideal ground.
Strength at this angle0 dB vs peak
For HF this matters a lot. The elevation pattern, shaped by height above ground (see Ground effects and height), decides whether your signal leaves at a low angle for long-distance contacts or a high angle for nearby ones; see Skywave, hops and the skip zone.
Common shapes
Antenna
Azimuth pattern
Notes
Horizontal dipole
figure-8, strongest broadside to the wire
elevation pattern depends on height
Vertical monopole
circle (omnidirectional)
low-angle radiation, none straight up
Small loop
figure-8, deep nulls perpendicular to the loop's plane