— Field Reference
Antenna Gain Explained
dBi — What It Means on the Job
dBi is on every antenna spec sheet, but it's often misread. This guide covers what antenna gain actually means for security alarm, lift emergency phone, and IoT field installs — and how to choose the right gain for the site.
dBi
Gain Unit
2.15
dBi = 0 dBd
6 dBi
D&G Default
Yagi
Fringe Sites
What dBi Actually Measures
dBi = decibels relative to isotropic. An isotropic antenna is a theoretical point source that radiates RF energy perfectly equally in every direction — like a light bulb in a vacuum. No real antenna does this. All real antennas concentrate energy somewhere, reducing it somewhere else.
A 6 dBi omni antenna concentrates its radiation pattern toward the horizon. Compared to the isotropic reference, it delivers 6 dB more signal in the horizontal plane — roughly 4× the power in the useful direction. The tradeoff: it delivers less signal directly above or below.
The practical implication: Higher dBi is not always better. For a rooftop antenna talking to a tower on the same level, high-gain works well. For an antenna in a basement car park where the tower signal arrives at a steep angle from above, a high-gain omni aimed at the horizon may perform worse than a lower-gain omni with a wider vertical pattern.
dBi vs dBd
dBi — Decibels, Isotropic
Referenced against a perfect point source. The most common standard on modern antenna datasheets.
dBd — Decibels, Dipole
Referenced against a half-wave dipole. A dipole already has 2.15 dBi gain, so a 4 dBd antenna = 6.15 dBi.
dBi = dBd + 2.15
Always convert to dBi when comparing antennas from different datasheets.
Gain Selection — Field Reference
Choose gain based on site conditions, not the highest number on the spec sheet.
0–2 dBi
Stub / built-in
Use when
On-device, no external antenna
Limitation
Fails in metal enclosures, basements, fringe sites
3 dBi
Low-gain omni
Use when
Good signal, no obstructions
Limitation
Minimal improvement over stub antenna
5–6 dBi
Standard omni (D&G default)
Use when
Most alarm panel and lift gateway installs
Limitation
Best all-round choice for enclosure exit
8–10 dBi
High-gain omni
Use when
Elevated masts, clear line of sight
Limitation
Narrow vertical beam — do not use on rooftops pointing at adjacent buildings
6–11 dBi
Yagi directional
Use when
Known tower direction, fringe signal sites
Limitation
Must be pointed at tower — wrong aim = worse than omni
12+ dBi
High-gain Yagi / panel
Use when
Rural remote sites, >10 km from tower
Limitation
Very narrow beam, requires alignment. Overkill for most installs.
Three Gain Mistakes on Security & Lift Installs
Buying the highest dBi antenna available
A 12 dBi antenna in an urban environment doesn't mean better performance — it means a very narrow vertical beam pointed at a layer of buildings. A 6 dBi standard omni covers most alarm and lift installs across all signal conditions.
Using a Yagi without knowing the tower direction
A Yagi pointed 30° off-axis from the tower can underperform an omni by 10 dB or more. If you can't identify the tower direction with a field tool (e.g. OpenCelliD or Telstra CoverageChecker), use an omni.
Ignoring cable loss in the gain calculation
A 6 dBi antenna on 20 m of cheap RG58 coax may deliver less signal than a 3 dBi antenna on 2 m of LMR400. For runs longer than 5 m, use LMR240 or LMR400 and recalculate the link budget including cable loss before selecting antenna gain.
D&G Antenna Gain Reference
DG900MB-HG — 5GNR Broadband Omni
Default for alarm panels and lift gateways. Covers all AU 4G bands from a single mount point.
DGY900-6 — Yagi Directional
Fringe signal sites, rural commercial, mining-area premises. Known tower direction required.
2x DG900MB-HG — MIMO Omni
Permaconn PM54 and other MIMO communicators with dual antenna ports for 4G diversity.
Custom Cable Assembly
LMR240 or LMR400, any connector, any length. Keeps cable loss out of the link budget for long runs.