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Learn antenna parameters with gain dBi, directivity, radiation pattern, beamwidth HPBW, bandwidth, efficiency, input impedance, VSWR return loss, and polarization explained for engineering students.
In Simple Terms: Several key parameters are used to quantify antenna performance – Gain, Directivity, Beamwidth, Bandwidth, Efficiency, Impedance, Polarization, etc. These parameters are essential for antenna selection and system design.
🎯 Radiation Pattern
In Simple Terms: A radiation pattern is a graphical representation that shows how an antenna distributes energy in space – where the signal is stronger and where it is weaker.
| │ Omnidirectional | Directional: │ |
| │ Key parts of pattern | │ |
| │ • Main lobe | Direction of maximum radiation │ |
| │ • Side lobes | Unwanted radiation in other directions │ |
| │ • Back lobe | Radiation in opposite direction to main │ |
| │ • Nulls | Directions with zero/minimum radiation │ |
Pattern Parameters:
| Parameter | Definition |
|---|---|
| Main lobe | Direction of maximum radiation |
| Half-power beamwidth (HPBW) | Angular width at -3dB points |
| First null beamwidth (FNBW) | Angular width between first nulls |
| Side lobe level (SLL) | Power of strongest side lobe vs main |
| Front-to-back ratio (F/B) | Main lobe / back lobe power |
📐 Directivity
In Simple Terms: Directivity indicates how "focused" an antenna is compared to an isotropic radiator (which radiates equally in all directions).
┌──────────────────────────────────────────────────────┐
│ │
│ D = Maximum radiation intensity / Average intensity│
│ = U_max / U_avg │
│ = 4π × U_max / P_total │
│ │
│ D (dBi) = 10 × log₁₀(D) │
│ │
│ For isotropic antenna: D = 1 (0 dBi) │
│ For half-wave dipole: D = 1.64 (2.15 dBi) │
│ │
│ Higher D = more focused = narrower beam │
│ │
└──────────────────────────────────────────────────────┘
Directivity vs Beamwidth:
Example
D ≈ 41,253 / (θ₁ × θ₂) (approximate)
Where θ₁, θ₂ = half-power beamwidths in two planes (degrees)
Example: 10° × 10° beam → D ≈ 41,253/100 ≈ 413 = 26 dBi
📊 Antenna Gain
In Simple Terms: Gain is directivity × efficiency. It is a practical measure of how effectively an antenna radiates power in a particular direction.
┌──────────────────────────────────────────────────────┐
│ │
│ Gain (G) = Efficiency (η) × Directivity (D) │
│ │
│ G (dBi) = 10×log₁₀(η × D) │
│ = D(dBi) + 10×log₁₀(η) │
│ │
│ dBi = gain relative to isotropic │
│ dBd = gain relative to dipole (dBd = dBi - 2.15) │
│ │
│ If η = 100%: Gain = Directivity │
│ If η < 100%: Gain < Directivity │
│ │
└──────────────────────────────────────────────────────┘
Typical Gain Values:
| Antenna | Gain (dBi) | Pattern |
|---|---|---|
| Isotropic (theoretical) | 0 | Spherical |
| Short dipole | 1.5 | Donut |
| Half-wave dipole | 2.15 | Donut |
| Quarter-wave monopole | 5.15 | Half-donut |
| Yagi (3-element) | 8 | Directional |
| Yagi (10-element) | 12-14 | Narrow beam |
| Patch antenna | 6-9 | Hemisphere |
| Horn antenna | 10-25 | Beam |
| Parabolic dish (1m, 10 GHz) | 35-40 | Pencil beam |
| 5G Massive MIMO (64 elements) | 25-30 | Multiple beams |
📏 Beamwidth
In Simple Terms: Beamwidth is the angular width of the main lobe. Smaller beamwidth = more focused = higher gain.
Half-Power Beamwidth (HPBW or 3-dB beamwidth):
Angle between the two directions where power is half of maximum
Power ↑
│ ╱╲ ← Main lobe
0 dB │────╱──╲──── Peak
│ ╱ ╲
-3 dB │──╱──────╲── Half-power points
│ ╱ ╲
│╱ ╲
┼────────────────▶ Angle
◀─HPBW─▶
| Antenna | Typical HPBW |
|---|---|
| Dipole | 78° (E-plane) |
| Patch | 60-90° |
| Yagi (10-element) | 30-40° |
| Horn (standard) | 10-30° |
| Parabolic dish (1m, 10 GHz) | 2-3° |
| 5G mmWave beam | 5-15° |
📶 Bandwidth
In Simple Terms: Antenna bandwidth is the frequency range over which the antenna provides acceptable performance (VSWR < 2, gain within 3 dB, stable pattern).
| • Narrowband | < 5% (resonant antennas, patch) |
| • Moderate | 5-20% (dipoles, yagis) |
| • Wideband | 20-50% (log-periodic, horn) |
| • Ultra-wideband | > 50% (spiral, Vivaldi) |
| Antenna Type | Typical Bandwidth |
|---|---|
| Patch (single-layer) | 2-5% |
| Dipole (thin) | 5-10% |
| Yagi-Uda | 3-5% |
| Log-periodic | 40-50% (frequency independent!) |
| Horn | 30-50% |
| Spiral | 100%+ (ultra-wideband) |
| Parabolic dish | Wide (limited by feed) |
⚡ Antenna Efficiency
In Simple Terms: Efficiency indicates how much of the input power actually gets radiated by the antenna. The rest is lost to various losses.
Example
η_total = η_radiation × η_mismatch × η_polarization
η_radiation = P_radiated / P_input
= R_radiation / (R_radiation + R_loss)
Losses include:
• Ohmic losses (conductor resistance)
• Dielectric losses (substrate)
• Surface wave losses (patch antennas)
• Mismatch losses (impedance mismatch)
| Antenna Type | Typical Efficiency |
|---|---|
| Wire dipole (full-size) | 95-99% |
| Patch antenna | 80-95% |
| Small antenna (< λ/10) | 10-50% |
| Horn antenna | 95-99% |
| Mobile phone antenna | 50-80% |
| Chip antenna (tiny) | 20-50% |
🔌 Input Impedance
In Simple Terms: The input impedance of an antenna (Zin = R + jX) is the impedance that the antenna presents to the transmission line. It must match the source impedance for maximum power transfer.
| For half-wave dipole | Z = 73 + j42.5 Ω (theoretical) |
| For quarter-wave monopole | Z ≈ 36.5 + j21 Ω |
| Standard system impedances | 50 Ω or 75 Ω |
📊 VSWR & Return Loss
In Simple Terms: VSWR (Voltage Standing Wave Ratio) indicates how good the matching is between the antenna and the transmission line.
┌──────────────────────────────────────────────────────┐
│ │
│ VSWR = (1 + |Γ|) / (1 - |Γ|) │
│ │
│ Return Loss (dB) = -20×log₁₀(|Γ|) │
│ │
│ Where Γ = reflection coefficient │
│ = (Z_ant - Z_line) / (Z_ant + Z_line) │
│ │
└──────────────────────────────────────────────────────┘
| VSWR | Return Loss | Reflected Power | Match Quality |
|---|---|---|---|
| 1.0 | ∞ dB | 0% | Perfect ✅ |
| 1.5 | 14 dB | 4% | Excellent |
| 2.0 | 9.5 dB | 11% | Good (typical spec) |
| 3.0 | 6 dB | 25% | Fair |
| 5.0 | 3.5 dB | 44% | Poor ❌ |
| ∞ | 0 dB | 100% | Total mismatch |
In Simple Terms: VSWR < 2 (return loss > 10 dB) is considered acceptable performance.
🔄 Polarization
In Simple Terms: Polarization is the orientation of the electric field in space. The polarization of the transmitter and receiver must match for maximum signal transfer.
| Polarization | E-field | Example |
|---|---|---|
| Vertical | E-field vertical | Mobile tower, AM radio |
| Horizontal | E-field horizontal | TV antenna |
| Circular (RHCP/LHCP) | E-field rotates | GPS, satellite |
| Elliptical | E-field traces ellipse | General case |
Polarization Mismatch Loss:
| TX Polarization | RX Polarization | Loss |
|---|---|---|
| Vertical | Vertical | 0 dB (matched) |
| Vertical | Horizontal | ∞ dB (total loss!) |
| Vertical | 45° tilted | 3 dB |
| Circular | Circular (same) | 0 dB |
| Circular | Linear | 3 dB |
| RHCP | LHCP | ∞ dB (cross-pol) |
📝 Summary Table {#summary-table}
| Parameter | Symbol | Unit | What it Tells |
|---|---|---|---|
| Gain | G | dBi | How much antenna focuses power |
| Directivity | D | dBi | Maximum focusing (lossless) |
| Beamwidth | HPBW | degrees | Width of main beam |
| Bandwidth | BW | MHz or % | Usable frequency range |
| Efficiency | η | % | Power radiated vs input |
| Impedance | Zin | Ω | Matching requirement |
| VSWR | – | ratio | Matching quality |
| Polarization | – | V/H/C | E-field orientation |
| Gain ↔ Beamwidth | Inverse | – | Narrow beam = high gain |
| Gain ↔ Size | Proportional | – | Bigger antenna = more gain |
❓ FAQ
Q: What is the difference between dBi and dBd? A: dBi = gain relative to isotropic antenna. dBd = gain relative to half-wave dipole. Conversion: dBi = dBd + 2.15.
Q: What is the gain of a mobile phone antenna? A: Typically 0-3 dBi (nearly omnidirectional). A phone antenna is small and involves compromises (for size, multi-band support, and aesthetics).
Q: What is the disadvantage of a high gain antenna? A: Narrow beamwidth – if the user moves outside the beam, the signal drops. Plus it is physically larger. Low-gain omnidirectional is better for mobile devices, while high-gain is better for fixed links.
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