Q2Electromagnetics Waves

• Gain (G): the ratio of the radiation intensity in the direction of maximum radiation to the radiation intensity that would result from an isotropic radiator fed with the same total input power (including antenna losses); dimensionless, usually expressed in decibels (dBi relative to an isotropic source, or dBd relative to a half-wave dipole). Gain = ηrad × Directivity, where ηrad is the radiation efficiency.
• Directivity (D): the ratio of the radiation intensity in the direction of maximum radiation to the average radiation intensity over all directions (i.e., to the intensity of a hypothetical isotropic radiator emitting the same total radiated power); dimensionless (often expressed in dB). Directivity depends only on the pattern shape, not on antenna losses, unlike gain.
• Beamwidth (half-power beamwidth, HPBW): the angular separation, in degrees, between the two directions on the main lobe of the radiation pattern at which the radiated power falls to half (−3 dB) of its maximum value; a narrower beamwidth generally corresponds to a higher directivity/gain.
• Radiation resistance (Rrad): the equivalent resistance (in ohms) at the antenna's feed point that would dissipate, as ordinary I²R loss, exactly the power the antenna actually radiates; together with the loss resistance it determines radiation efficiency and impedance matching to the feed line.

Other commonly cited parameters include input impedance (Ω, for matching), polarization (linear/circular/elliptical), bandwidth (Hz or %, the frequency range over which performance criteria are met), and effective aperture (m², relating received power to incident power density) — the four described above are the ones most directly tied to how strongly and in which directions an antenna concentrates radiated energy.