Glossary

Plain-language definitions of the terms you will meet on every antenna page.

Gain (dBi / dBd)

How strongly an antenna concentrates energy in its best direction compared with a reference. dBi compares to a theoretical isotropic (all-directions) radiator; dBd compares to a dipole (add 2.15 to convert dBd → dBi). Higher gain means a narrower beam — antennas don't create power, they focus it.

Directivity

How focused an antenna's radiation pattern is, ignoring losses. Gain is directivity minus the antenna's internal losses.

Polarization

The orientation of the radio wave's electric field — vertical, horizontal, or circular (rotating). Two stations communicate best when their antennas share polarization; a mismatch can cost 20 dB or more.

Feedpoint impedance

The electrical load (in ohms) an antenna presents to its feedline. A resonant half-wave dipole is about 73 Ω; common coax is 50 Ω. A close match lets power flow with little reflected back.

SWR (Standing Wave Ratio)

A measure of how well the antenna matches the feedline. 1:1 is perfect (no reflected power); 2:1 is usually fine; 3:1 and up is increasingly poor. See Antenna Fundamentals for more.

Resonance

The frequency where an antenna naturally "rings" and accepts power most readily, presenting a purely resistive feedpoint. Resonant length is set by the wavelength — roughly half a wavelength for a dipole.

Wavelength & frequency band

Wavelength (m) ≈ 300 ÷ frequency (MHz). Antenna size scales with wavelength, which is why low bands need big antennas. See the frequency bands reference.

Radiation pattern

A plot of how strongly an antenna radiates in each direction — omnidirectional (even around the horizon), a dipole's figure-eight, or a focused beam.

Balun

A device (balanced-to-unbalanced) that connects a balanced antenna to unbalanced coax and stops RF flowing back down the feedline. Some baluns also transform impedance (e.g. 4:1, 49:1).