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).

Antenna Gain	Approx. Vertical Beamwidth	Radiation Elevation Angle
2 dBi (dipole)	~75°	Broad; works at steep angles
5 dBi collinear	~35–40°	Slightly elevated; works for nearby nodes
8 dBi collinear	~15–20°	Near-horizontal; close nodes may be in null
10 dBi collinear	~10–12°	Essentially horizontal; nodes must be far away to be in the beam

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:

Gain Choice	Use When	Avoid When
2–3 dBi (whip, dipole, GP vertical)	Indoor node; node surrounded by other nodes at similar elevation; portable device; building where nodes are on every floor	Outdoor exposed relay where range to distant nodes is the primary goal
5 dBi (short collinear)	Outdoor rooftop node in urban/suburban area; nodes are within 2–5 km; mixed elevation terrain; best all-around choice for most mesh relay nodes	Indoor use; terrain with significant elevation variation around the node
8 dBi (medium collinear)	High hilltop or tower relay overlooking flat terrain; all served nodes are at roughly the same elevation and 5–20 km distant; rural backbone relay	Urban environment; any situation with nodes at varying elevations; anywhere nodes might be directly below the antenna

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.