Understanding Antenna Gain and Pattern
Antennas don’t amplify power like electronic amplifiers — they shape where radio energy goes. Two ideas describe that shaping:
- Gain — how strongly an antenna sends or receives in a particular direction.
- Pattern — the map of where that strength goes in space.
Key takeaway: the more an antenna focuses its energy, the stronger it becomes in that direction — but usually weaker elsewhere.
What “Gain” Really Means
Imagine a magical point antenna that radiates equally in all directions — the isotropic reference. Real antennas are compared to that ideal.
Gain in dBi tells you how much stronger your antenna is in its best direction compared to that isotropic yardstick. In dBd, it’s referenced to a half-wave dipole (2.15 dBi).
- 3 dB ≈ 2× power
- 10 dB = 10× power
- 0 dBd = 2.15 dBi
Gain happens because the antenna concentrates energy. Total power stays the same, but it’s “pushed” into fewer directions. Real antennas also lose some power as heat — that’s where efficiency enters:
G(θ, φ) = η · D(θ, φ)
GdBi = 10 log₁₀(Glinear)
GdBd = GdBi − 2.15
What a Radiation Pattern Shows
Radiation patterns are typically plotted as 2D slices that show how the antenna radiates in space. Two main views are used:
- Azimuth pattern — like looking down from above; shows strength around the horizon (0° elevation).
- Elevation pattern — shows signal strength as you look up and down in height (from horizon to zenith).
Together, they show where the antenna sends or hears best.
- Main lobe: strongest direction
- Side/back lobes: smaller lobes to sides or rear
- Nulls: deep dips with weak radiation
- Half-power beamwidth (HPBW): width of the main lobe at −3 dB points
A rule of thumb for narrow beams:
D ≈ 41 253 / (θH × θV) (angles in degrees)
Higher directivity → higher gain (if efficiency is good).
Effective Aperture — the “Catch Area”
Every antenna, even if not a dish, has an effective capture area Ae that tells how much power it collects from a passing wave.
G = (4π Ae) / λ² ⇄ Ae = (λ² G)/(4π)
Example: 2.4 GHz (λ ≈ 0.125 m), 8 dBi → G ≈ 6.31
Ae ≈ (0.125² × 6.31)/(4π) ≈ 0.00785 m² = 78.5 cm² effective area.
Polarization — Don’t Leave dB on the Table
Waves have an electric-field direction:
- Linear (horizontal or vertical)
- Circular (right-hand or left-hand)
Mismatched polarizations cost signal. Two linear antennas misaligned by angle ψ lose:
Loss = 10 log₁₀(cos² ψ) dB
Linear ↔ Circular mismatch ≈ 3 dB.
Near Field vs Far Field
Gain and patterns are defined in the far field, where the wavefront is stable. The boundary is called the Fraunhofer distance:
Example: 10 cm antenna at 2.4 GHz (λ ≈ 12.5 cm) → RFF ≈ 0.16 m.
Larger antennas push the far-field boundary farther out.
The Friis Equation — Power at the Receiver
This is the “gravity equation” of radio links — it ties together transmitter power, antenna gains, distance, and wavelength.
Pr = Pt Gt Gr (λ / 4πd)²
dB form:
Pr[dBm] = Pt[dBm] + Gt[dBi] + Gr[dBi] − FSPL[dB] − losses
FSPL[dB] = 32.44 + 20 log₁₀(fMHz) + 20 log₁₀(dkm)
Wi-Fi / 2.4 GHz: d = 100 m → FSPL ≈ 80 dB
Pt = 20 dBm (100 mW), Gt = Gr = 8 dBi, 1 dB cable loss each →
Pr ≈ 20 − 1 + 8 + 8 − 1 − 80 = −46 dBm — a healthy Wi-Fi-level signal.
UHF / 433 MHz: λ ≈ 0.69 m, d = 1 km → FSPL ≈ 91.2 dB
Pt = 30 dBm (1 W), Gt = Gr = 2 dBi →
Pr ≈ 30 + 2 + 2 − 91.2 = −57.2 dBm — still good for LoRa or telemetry.
VHF / 144 MHz: λ ≈ 2.08 m, d = 10 km → FSPL ≈ 92.6 dB
Pt = 43 dBm (20 W), Gt = Gr = 6 dBi →
Pr ≈ 43 + 6 + 6 − 92.6 = −37 dBm — very strong local repeater signal.
HF / 3.7 MHz (80 m): λ ≈ 81 m, d = 100 km (NVIS) → FSPL ≈ 77 dB
Pt = 47 dBm (50 W), Gt = Gr ≈ 0 dBi (dipoles) →
Pr ≈ 47 − 77 = −30 dBm — very comfortable HF contact strength.
EIRP[dBm] = Pt − Ltx + Gt
ERP uses dBd → EIRP = ERP + 2.15 dB.
Typical Pattern Shapes
- Half-wave dipole: donut-shaped; strong broadside, weak off ends (~2.15 dBi)
- Patch: broad forward lobe, 5–9 dBi, linear polarization
- Yagi-Uda: strong forward lobe, small back lobe, 10–20 dBi
- Parabolic dish: narrow pencil beam, 20–40 dBi+
- Helical: circular polarization, moderate gain, broad beam
How to Read an Antenna Datasheet
- Gain (dBi) and HPBW — beam tightness
- Polarization — match it or expect loss
- VSWR / Return Loss — matching quality (lower VSWR = better)
- Radiation patterns — side lobes, nulls, main-lobe direction
- Power handling and connector type/loss
- Regulatory limits — EIRP caps combine gain + TX power
VSWR = (1 + |Γ|)/(1 − |Γ|)
Common Pitfalls & Quick Fixes
- “More gain is always better.” Not if you need coverage all around.
- High-gain dishes lose many dB with tiny mispointing — use fine mounts.
- Coax losses eat gain — keep cables short and low-loss.
- Polarization mis-twist: 20° → ~ 1.2 dB loss.
- Don’t measure far-field performance in the near field.
Mini Cheatsheet (dB Side)
Double power → +3 dB • Half power → −3 dB
dBi ↔ dBd = ± 2.15 dB
FSPL (km/MHz) = 32.44 + 20 log₁₀ fMHz + 20 log₁₀ dkm
Scientists & Pioneers to Know
- James Clerk Maxwell — predicted radio waves via Maxwell’s equations.
- Heinrich Hertz — first to generate and detect them experimentally.
- Guglielmo Marconi — pioneer of long-distance wireless links.
- Harald T. Friis — developed the Friis transmission equation.
- Hidetsugu Yagi & Shintaro Uda — invented the Yagi-Uda antenna.
- John D. Kraus — invented the helical antenna; wrote Antenna Theory.
- Harold A. Wheeler — work on antenna impedance and small antennas.
- L. J. Chu — defined limits for electrically small antennas.
- Constantine A. Balanis — modern Antenna Theory textbook author.
- David M. Pozar — author of Microwave Engineering; RF design reference.
Mini-FAQ
- What does 3 dB of gain mean? — It means double the power in that direction compared to the reference.
- Why is my dish link unstable? — High-gain antennas have very narrow beams; tiny misalignments cause big drops.
- Is dBi the same as dBd? — No. dBd is referenced to a dipole; dBi is isotropic. Add 2.15 dB to convert dBd → dBi.
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