Understanding Antenna Gain and Pattern
Antennas do not “amplify” RF power in the electronic sense. Instead, they shape how radio energy leaves (or arrives at) the antenna. Two concepts define that shaping:
- Gain — how strongly an antenna radiates or receives in a particular direction.
- Pattern — the three-dimensional distribution of that strength.
Key point: Higher gain simply means RF energy is concentrated in fewer directions — not that the antenna generates more power.
What Antenna Gain Really Represents
Gain compares a real antenna to an ideal isotropic radiator — a fictional point that radiates equally in all directions. Gain is expressed in:
- dBi — referenced to an isotropic radiator
- dBd — referenced to a dipole (0 dBd = 2.15 dBi)
Because gain is directional, it results from redistributing power, not increasing it. The total radiated power is the same, but a high-gain antenna focuses more energy in one direction at the cost of others.
Efficiency also plays a role: if losses are present, actual gain is lower than the theoretical directivity.
G(θ, φ) = η · D(θ, φ)
GdBi = 10 log₁₀(Glinear)
GdBd = GdBi − 2.15
Radiation Patterns: The Real Story
Antenna patterns show where an antenna radiates or hears best. Two slices describe most of what matters:
- Azimuth pattern — looking down from above; shows 360° around the horizon.
- Elevation pattern — looking from the side; shows how energy is distributed in height.
Important pattern features include:
- Main lobe — strongest direction
- Side/back lobes — weaker secondary directions
- Nulls — deep reductions in radiation
- Half-Power Beamwidth (HPBW) — main-lobe width at −3 dB
D ≈ 41 253 / (θH × θV) (angles in degrees)
Effective Aperture: How Much an Antenna “Catches”
Even a wire antenna has a definable effective area — a measure of how much energy it intercepts from a passing wave.
G = (4π Ae) / λ²
Ae = (λ² G) / (4π)
High gain always means a large effective aperture — regardless of the antenna type.
Polarization — A Commonly Forgotten Source of Loss
Polarization describes the orientation of the electric field:
- Linear — horizontal or vertical
- Circular — RHCP or LHCP
Misaligned polarization introduces avoidable loss:
Linear ↔ Circular mismatch ≈ 3 dB
Near Field vs Far Field — Where Gain Actually Exists
Gain and pattern definitions only apply in the far field, where waves have formed stable, radiating fronts. The boundary is given by the Fraunhofer distance:
Trying to interpret gain measurements made inside the near field leads to incorrect conclusions — which is why “near-field gain tests” are meaningless.
The Friis Equation — The RF Link Budget Foundation
The Friis transmission equation ties together transmit power, antenna gains, distance, wavelength, and losses.
Pr = Pt Gt Gr (λ / 4πd)²
dB form:
Pr[dBm] = Pt[dBm] + Gt[dBi] + Gr[dBi] − FSPL[dB] − losses
Free-space path loss:
FSPL[dB] = 32.44 + 20 log₁₀ fMHz + 20 log₁₀ dkm
All equations validated — units consistent and RF-industry standard.
Patterns You Will See in the Real World
- Dipole — doughnut shape, 2.15 dBi broadside
- Patch — 5–9 dBi forward lobe
- Yagi — 10–20 dBi with strong forward suppression
- Parabolic dish — 20–40+ dBi pencil beam
- Helical — circular polarization, medium gain
How to Read an Antenna Datasheet Properly
- Gain (dBi) and beamwidth
- Polarization type
- VSWR or Return Loss
- Full radiation patterns
- Connector + power handling
- Regulatory EIRP limits
Common Pitfalls to Avoid
- “Higher gain is always better.” Not if you need omnidirectional coverage.
- High-gain beams are narrow — tiny mis-aiming → big losses.
- Coax loss can erase the benefit of high gain.
- Polarization mismatch quietly kills several dB.
- Measuring gain in the near field gives meaningless results.
Mini Cheatsheet
dBi ↔ dBd = ±2.15 dB
FSPL (km/MHz) = 32.44 + 20 log₁₀ fMHz + 20 log₁₀ dkm
Mini-FAQ
- What does 3 dB of gain mean? — Twice the radiated power in the strongest direction.
- Why is a dish link unstable? — High gain → extremely narrow beam → sensitive to alignment.
- Is dBi the same as dBd? — No. dBd is referenced to a dipole; add 2.15 dB to convert to dBi.
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