Line-of-Sight Clearance & RF Propagation
Fresnel Zone Calculator
Calculate 1st Fresnel zone radius, the mandatory 60% clearance threshold, and midpoint maximum expansion for point-to-point microwave and wireless links.
Carrier
GHz
TX to RX
km
TX to Obstacle
km
1st Fresnel Radius (r₁)
11.37 m
37.3 ft
60% Required Clearance
6.82 m
22.4 ft (Mandatory)
Midpoint Max Radius
11.37 m
At d₁ = 5.0 km
RF Wavelength (λ)
5.17 cm
Speed of light ≈ 3×10⁸ m/s
Line-of-Sight Clearance Rule
To ensure zero diffraction fade margin loss, maintain at least 6.82 meters of unobstructed space above all terrain, buildings, and tree canopies at the obstacle point. For paths over water or smooth desert, 80% to 100% clearance (11.37 m) is recommended to avoid destructive ground reflections.
Engineering Reference
Fresnel Zone Clearance Benchmarks at Link Midpoint
| Frequency | Link Distance | Wavelength (λ) | 100% Radius (r₁) | 60% Clearance Req | Typical Application |
|---|---|---|---|---|---|
| 915 MHz (ISM) | 5.0 km | 32.8 cm | 20.25 m (66.4 ft) | 12.15 m (39.9 ft) | Long-range LoRaWAN & SCADA telemetry |
| 2.4 GHz (Wi-Fi) | 5.0 km | 12.5 cm | 12.50 m (41.0 ft) | 7.50 m (24.6 ft) | Outdoor 802.11b/g/n rural Wi-Fi bridges |
| 5.8 GHz (UNII) | 10.0 km | 5.17 cm | 11.37 m (37.3 ft) | 6.82 m (22.4 ft) | WISP backbone & PtP Gigabit wireless |
| 11 GHz (Microwave) | 15.0 km | 2.73 cm | 10.12 m (33.2 ft) | 6.07 m (19.9 ft) | Licensed telecom cellular backhaul |
| 24 GHz (ISM) | 5.0 km | 1.25 cm | 3.95 m (13.0 ft) | 2.37 m (7.8 ft) | Short-hop high-capacity urban links |
| 60 GHz (V-Band) | 1.5 km | 0.50 cm | 1.37 m (4.5 ft) | 0.82 m (2.7 ft) | Multi-gigabit mmWave mesh & metro backhaul |
Knowledge Base
Frequently Asked Questions
Common questions about RF power conversions, negative dBm, and voltage calculations.
In metric units, the first Fresnel zone radius is calculated as: r₁ (meters) = 17.32 × √((d₁ × d₂) / (f × D)), where d₁ and d₂ are distances from the obstacle to TX and RX in kilometers, D = d₁ + d₂ is total link distance in km, and f is frequency in GHz. At link midpoint, this simplifies to r₁ = 8.656 × √(D / f).
Radio waves propagate as an expanding ellipsoid. If obstacles penetrate within 60% of the 1st Fresnel zone radius (0.6 × r₁), diffraction and phase cancellation occur, inducing severe signal attenuation exceeding free space path loss. Keeping 60% of the zone free of buildings, trees, and terrain maintains near-free-space propagation conditions.
Higher frequencies have shorter wavelengths, resulting in a smaller (tighter) Fresnel zone radius. For example, over a 10 km link at midpoint: a 2.4 GHz signal requires a 17.7 meter radius (10.6 m at 60%), while a 5.8 GHz signal requires 11.4 meters (6.8 m at 60%), and a 60 GHz V-band link requires only 3.5 meters (2.1 m at 60%).
Optical Line of Sight (LOS) only requires a clear visual path between antennas. Radio Line of Sight (RF LOS) requires both visual clearance AND an unobstructed 1st Fresnel zone (at least 60% clearance). An optical path can be perfectly visible through binoculars while suffering 20 dB+ diffraction attenuation if tree canopies infringe upon the Fresnel ellipsoid.
Over distances exceeding 5 km (3 miles), the curvature of the Earth bulges upward into the link path. The bulge height is approximately h (meters) = (d₁ × d₂) / (12.75 × k), where k = 4/3 is standard atmospheric refraction. Antenna tower heights must accommodate both the Earth bulge and the 60% Fresnel zone clearance.