Antenna Concepts
What gain really means
An antenna is passive: it cannot amplify. Gain (dBi) is directivity - energy focused into a narrower pattern at the expense of coverage elsewhere. About 6 dB of additional gain roughly halves beamwidth in each plane. Always read both azimuth and elevation charts: an omni is wide in azimuth, a patch or panel is semi-directional, a Yagi or parabolic dish is highly directional.
Key specifications
Polarization (linear vs circular), front-to-back ratio, sidelobes and nulls, downtilt (mechanical or electrical), and VSWR. VSWR of 1.5:1 or better is healthy; 2:1 signals a mismatch (roughly 11% reflected power) and usually points to a cable, connector or lightning-arrester fault. Reciprocity means a higher-gain antenna also hears better.
System math
EIRP = radio power + antenna gain − cable loss − connector loss. Regulators usually cap EIRP, so adding antenna gain may force a reduction in radio power (commonly 1 dB of power reduction for every 1 dB of gain above a 6 dBi reference). MIMO and transmit beamforming raise SNR and throughput using multiple antennas rather than one larger antenna.
Traps
Reporting dBd as dBi; assuming a higher-gain antenna always improves a short indoor link (it may create nulls where clients sit); forgetting circular polarization costs roughly 3 dB against a linear client; and mounting an omni flat against metal.
Worked example
A 20 dBm radio feeds a 6 dBi antenna through 2 dB of cable and connector loss: EIRP = 20 + 6 − 2 = 24 dBm (about 250 mW).