Antenna Gain (dBi)

Antenna gain describes how effectively an antenna focuses transmitted (or received) radio energy in a particular direction rather than radiating it equally everywhere. It is most commonly expressed in dBi — decibels relative to an isotropic radiator, an idealised point source that radiates uniformly in all directions and is defined as 0 dBi. The crucial physics: antennas do not amplify power. They reshape where the existing power goes, the way a flashlight reflector concentrates the same bulb's output into a beam. Every dB gained toward the horizon is a dB taken from somewhere else — usually straight up and straight down.

dBi versus dBd

A related unit, dBd, references gain against a half-wave dipole instead of an isotropic source. Because a dipole itself has about 2.14 dBi of gain, you convert by adding 2.14 to a dBd figure to get dBi, or subtracting to go the other way. Manufacturers overwhelmingly quote dBi — it produces the bigger number — so a "3 dBi" whip and a "1 dBd" whip are nearly identical antennas. Read spec sheets with that marketing incentive in mind, and treat any compact antenna claiming implausibly high gain with suspicion: physics ties gain to size and pattern, and a pocket-sized "12 dBi" omni is a fiction. See dipole antenna for the reference design itself.

Gain reshapes the doughnut

An omnidirectional vertical radiates a doughnut-shaped pattern centred on the antenna. Raising gain squashes that doughnut flatter: a 2–3 dBi rubber duck radiates a fat, forgiving pattern that tolerates a tilted, handheld, or pocket-carried node. A 5.8 or 8 dBi collinear flattens the pattern toward the horizon — excellent for a fixed rooftop relay reaching distant peers, but it can shoot right over a node in the valley below or a hiker on the ridge above. High-gain omnis also punish tilt: lean the mast a few degrees and the pancake beam aims into the dirt on one side and the sky on the other — a failure mode that shows up constantly on improvised installs, where a zip-tied antenna that "looks straight" costs several dB in the exact direction that mattered. Directional designs such as the Yagi antenna take the same trade further, concentrating energy into a narrow forward lobe for point-to-point links at the cost of coverage everywhere else.

Gain in the link budget

Gain enters the link budget twice — once at each end — and it applies equally to receive, which is why upgrading a single relay's antenna improves every conversation it hears. Decibels are logarithmic: +3 dB doubles effective radiated power in the favoured direction, and on a marginal LoRa link a few dB is routinely the difference between a clean decode and silence. But antenna gain cannot buy back a blocked path: line of sight and a clear Fresnel zone dominate everything, and ten metres of mast height frequently beats ten dB of antenna. Cheap coax quietly eats gains too — a few metres of thin feedline at 900 MHz can impose enough attenuation to cancel an antenna upgrade entirely, so short runs of good cable matter as much as the antenna itself.

Choosing for a mesh node

Match the pattern to the terrain, not the number to your ego. Handheld or mobile node: 2–3 dBi, tolerant of orientation. Fixed rooftop relay serving a flat region: 5–6 dBi omni, mounted plumb. Long point-to-point backbone shot: directional Yagi or panel at both ends. Then verify with data — watch RSSI and SNR before and after the change rather than trusting the label. On a mesh, the craftsman's antenna choice at one well-placed relay lifts the whole neighbourhood's coverage; that is decentralised infrastructure done right.