Antenna Gain and Coverage Tradeoffs

Antenna Gain and Coverage Tradeoffs

Antenna gain is not free - it is always traded against something else. Understanding what gain costs you is essential before choosing an antenna for a mesh deployment. The fundamental law of antenna physics is conservation of energy: an antenna cannot create power, only redistribute it.

How Gain Concentrates Signal

Consider a theoretical isotropic antenna radiating 1 watt equally in all directions. At 1 km, that power is spread over a sphere of area 4π(1000)² = 12.57 million square meters. A 5 dBi antenna (3.16× linear gain) compresses its radiation into a narrower cone, delivering 3.16× more power density in its peak direction. From the perspective of a receiver in the main beam, it is equivalent to the transmitter having 3.16× the power.

This is the core of EIRP (Effective Isotropic Radiated Power):

EIRP (dBm) = Transmit Power (dBm) + Antenna Gain (dBi) − Feedline Loss (dB)

FCC Part 15.247 limits EIRP to +30 dBm (1 watt) for spread spectrum systems in the 902 - 928 MHz band when operating with a fixed, point-to-point link with directional antennas. For point-to-multipoint operation, the limit is effectively lower. Most LoRa nodes run 17 - 20 dBm transmit power, leaving 10 - 13 dB of "antenna budget" before hitting the legal limit.

Elevation Angle and Radiation Pattern Compression

As gain increases, the radiation pattern in the vertical plane becomes flatter - more like a pancake and less like a donut. This is measured as the vertical beamwidth (the angle between the −3 dB points above and below the horizon).

Dead Zones Below High-Gain Antennas

This is the most commonly overlooked problem with high-gain omnidirectional antennas in mesh networks. When you mount a 10 dBi collinear antenna on a rooftop, the signal goes predominantly outward - not down. Nodes directly beneath the tower, or on the same city block, may receive weaker signal than nodes kilometers away.

The approximate dead zone radius under a vertical omni antenna can be estimated as:

Dead Zone Radius ≈ h / tan(θ/2)

Where:
 h = antenna height above nodes (meters)
 θ = vertical beamwidth (degrees)

Example: 10 dBi antenna at 30 m height, 10° vertical beamwidth:
Dead Zone Radius ≈ 30 / tan(5°) ≈ 30 / 0.0875 ≈ 343 meters

In this example, any node within 343 meters of the tower base would be in the side lobe or null region and might receive 10 - 20 dB less signal than a node 2 km away. In a dense urban mesh, this is disastrous.

The 3 / 5 / 8 dBi Decision Guide

Use this framework when selecting omni antenna gain for a fixed node:

Rule of thumb: When in doubt, choose 5 dBi for any outdoor fixed node. It provides meaningful gain improvement over a whip without creating serious dead zone problems. Reserve 8+ dBi for well-planned backbone relay sites with known terrain profiles.

Directional antennas: When gain beyond 8 dBi is needed, switch to a directional antenna (panel or Yagi) aimed at the intended coverage direction. You gain range in the beam, and the dead zone problem is inherent to the design intent - it only covers one sector anyway.