Learn to read an antenna datasheet: the azimuth and elevation plots, beamwidth, gain and front-to-back ratio. Then mount the antenna and see where its beam actually lands on the floor.
Gain is focus, not power
An antenna adds no energy. A higher-gain antenna takes the same power and squeezes it into a narrower beam, so it reaches further in some directions and less far in others. A datasheet shows this with two cuts through the three-dimensional pattern: the azimuth plot (seen from above) and the elevation plot (seen from the side). The beamwidth is the angle between the points where the signal is 3 dB below the peak, and the front-to-back ratio is how much weaker the signal is straight behind.
1 · The antenna
Type
Plot scale
2 · The datasheet view
View
Move over a plot to read the pattern at any angle. Red marks are the −3 dB points that define the beamwidth.
−40 dB to the peakAxes: front (0°), side (90°), up
Azimuth: seen from above
0° is where the antenna points
Elevation: seen from the side
0° is the horizon, 90° straight up
−3 dB points
Where the gain comes from
3 · On the floor
Mount
From above: signal at desk height
From the side: where the beam goes
−90 to −35 dBm−67 dBm, a common targetWhere the antenna points
Definitions
↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.
Notes
Generic models, not products: the dipole uses the exact half-wave dipole pattern; the others use smooth textbook shapes (a cosn main lobe sized to the beamwidth, and a flat back lobe at the front-to-back ratio), with the horizontal and vertical shapes multiplied together. Real antennas have side lobes, nulls and ripples, so always read the manufacturer's datasheet.
Gain is worked out from the pattern itself (the peak divided by the average over all directions), assuming a lossless antenna. Real antennas lose a little in the materials, so their gain is slightly lower. IEEE 802.11 does not define antennas; beamwidth and front-to-back ratio are standard antenna-engineering terms.
The floor views use 5.5 GHz, a client antenna of 0 dBi at desk height (1 m), and path loss that follows free space for the first metre and then an indoor exponent of 3, a common rule of thumb for an office. There are no walls and no reflections; indoors, reflections fill in the nulls, so real coverage is smoother.
Beware the dB plot: datasheets plot patterns in dB, which makes beams look fatter than they are. Switch to linear power to see how quickly the signal really falls away from the peak.