Antenna Basics
Antenna fundamentals — gain, pattern, polarization, impedance, and common types.
Reference
Key parameters
- Gain (dBi)
- Relative to isotropic radiator; dBd = dBi − 2.15
- Beamwidth
- Angular width of main lobe at −3 dB
- VSWR
- Standing-wave ratio — 1:1 perfect match, >3:1 poor
- Return loss
- RL (dB) = −20·log₁₀(|Γ|), want > 10 dB
- Polarization
- Vertical, horizontal, or circular (LHCP / RHCP)
- Impedance
- Typically 50 Ω for RF, 75 Ω for TV
- Radiation pattern
- Directional (Yagi, dish) vs omnidirectional (dipole)
Common antenna types
| Type | Gain (typ.) | Pattern | Use |
|---|---|---|---|
| Quarter-wave whip | 2–3 dBi | Omni | FM, 2.4 GHz, simple monopole |
| Dipole | 2.15 dBi | Omni (donut) | Baseline antenna |
| Folded dipole | 2.15 dBi | Omni | 300 Ω — TV |
| Yagi | 10–20 dBi | Directional | Amateur, TV DX |
| Patch / microstrip | 6–9 dBi | Directional | GPS, WiFi, cellular |
| Helix | 8–15 dBi | Circular pol. | Satellite uplink |
| Dish (parabolic) | 20–50 dBi | Narrow beam | Microwave, satellite, radar |
| Log-periodic | 6–10 dBi | Directional | Broadband — HF-UHF |
| PCB chip antenna | 0–3 dBi | Varies | BLE, 2.4 GHz small devices |
Path loss (Free-space, Friis)
- FSPL (dB)
- = 20·log₁₀(d) + 20·log₁₀(f) + 32.45 (d in km, f in MHz)
- Example
- @ 2.4 GHz, 100 m → ~80 dB free-space loss
Notes
- Length: ¼-wave monopole = 300 / (4 · f_MHz) in meters (physical length; velocity factor on wire reduces it ~5%).
- Always terminate with the correct impedance — mismatches reflect power back and can damage transmitters.
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