Enter Link Details
Use heights above local ground. Obstacle checks assume the terrain baseline is level.
Formula Used
Radio horizon: d = 3.57 × √(K × h)
Total LOS path: D = 3.57 × [√(K × h₁) + √(K × h₂)]
Required height: h₁ = [D / 3.57 − √(K × h₂)]² / K
First Fresnel radius: F₁ = √[λ × d₁ × d₂ / (d₁ + d₂)]
Distances use kilometres for horizon calculations. Heights use metres. Fresnel distances are converted to metres internally. K equals 1.333 by default, representing standard atmospheric refraction.
How to Use This Calculator
- Select maximum distance or a required antenna-height mode.
- Enter actual antenna heights above their nearby ground levels.
- Enter the planned route length and a suitable K factor.
- Enter operating frequency to calculate the first Fresnel zone.
- Add an obstacle position and height for a clearance assessment.
- Review the path margin, Fresnel result, and required antenna height.
Example Link Data
| Input | Example value | Purpose |
|---|---|---|
| Transmitter height | 30 m | Defines the first radio horizon. |
| Receiver height | 15 m | Defines the second radio horizon. |
| Path distance | 40 km | Tests the planned route. |
| Frequency | 5800 MHz | Sets Fresnel-zone size. |
| Obstacle distance | 20 km | Places an obstruction near path midpoint. |
Planning a Dependable Antenna Path
Why antenna height matters
Antenna line of sight is the direct radio path between two antennas. Earth curvature limits this path long before maps appear flat. Raising either antenna extends its radio horizon. Raising both stations usually gives the best coverage. A taller tower can solve a difficult path. It cannot always overcome terrain, buildings, trees, or interference. A realistic calculation considers every major limitation before equipment is purchased.
Radio horizon and atmospheric bending
Radio horizon estimates use antenna height above local ground. The common relationship uses Earth radius and an atmospheric refraction factor. Refraction bends radio waves slightly toward Earth. A standard effective Earth factor is four thirds. Dry, unstable, or unusual air can produce different conditions. The calculator lets you adjust this factor. Use local engineering guidance for critical links. The result is a planning estimate, not a site acceptance test. Survey data and field measurements remain essential for important installations.
Why Fresnel clearance matters
A clear geometric path is not enough for dependable microwave or wireless service. The first Fresnel zone surrounds the direct signal path. Obstacles inside this zone can weaken the received signal. Trees may also move in wind or grow seasonally. Buildings may be added after a link begins operation. Good practice often preserves at least sixty percent of the first zone. Enter obstacle position, obstacle height, and frequency to check this clearance.
Choosing the calculation mode
Use maximum-distance mode when both antenna heights are known. It returns each horizon and the estimated combined limit. Compare this value with the intended route length. A positive margin does not confirm path quality. It only shows that curvature should not block the direct path. Use required-height mode when one antenna is fixed. Enter the desired path distance and the other antenna height. The calculator estimates the minimum height before clearance allowances. Add a practical construction margin for terrain uncertainty and future growth.
Entering useful field data
Start with measured heights above surrounding terrain, not tower length alone. Include rooftop elevation only when it represents the antenna position above local ground. Select a refraction factor suitable for your planning rule. Enter frequency in megahertz for the Fresnel calculation. Enter an obstacle only when its distance is inside the route. Review calculated clearance and the required Fresnel percentage. Increase either antenna height when clearance is insufficient. Consider relocating an antenna when height becomes impractical. Obtain terrain profiles for long paths and licensed systems. Verify final installations with alignment, received-signal readings, and fade-margin testing.
Limits of a geometric estimate
LOS calculations describe geometry, not guaranteed communication performance. Rain, foliage, nearby transmitters, receiver sensitivity, and antenna gain still matter. A long route may meet geometric requirements yet deliver poor availability. Link-budget calculations estimate the power available at the receiver. They should accompany this height calculation for serious projects. Also examine legal limits, tower loading, grounding, and lightning protection. Use qualified professionals for structures and regulated radio services. Careful planning prevents expensive revisions after construction. It also strongly improves reliability for users who depend on the connection.
Frequently Asked Questions
What does LOS mean for antennas?
LOS means line of sight. It describes an unobstructed direct route between transmitting and receiving antennas. Radio LOS also considers Earth curvature, which can hide one antenna below the other station’s horizon.
Does a visible path guarantee a reliable link?
No. A visible path may still have Fresnel blockage, weak signal level, interference, foliage loss, or poor antenna alignment. Use this calculation with a full link budget and site survey.
What antenna height should I enter?
Enter the height of each antenna above nearby local ground. Do not enter tower length alone when a building, ridge, or raised platform changes the actual antenna elevation.
Why is the K factor set to 1.333?
A value near 1.333 represents common atmospheric refraction. It models radio waves bending slightly toward Earth. Conditions vary, so critical systems may need a different design factor.
What is the first Fresnel zone?
It is an ellipsoidal region surrounding the direct radio path. Obstacles entering it can cause diffraction and reduce received signal strength, even when the antennas appear visually clear.
Why use 60 percent Fresnel clearance?
Sixty percent is a common planning target for many point-to-point links. It gives a practical buffer against diffraction. Your equipment, path reliability target, and local obstacles may justify more clearance.
Can I use this for cellular or broadcast sites?
It can provide an early geometric estimate. Professional cellular and broadcast planning also needs terrain models, clutter data, propagation modelling, antenna patterns, regulations, and measured coverage verification.
What happens when the obstacle is at the path midpoint?
The first Fresnel zone is usually widest near the middle of a path. A midpoint obstacle therefore often requires the greatest clearance and deserves careful field measurement.
Does this calculator include hills and terrain profiles?
No. The obstruction check assumes a level local terrain baseline. For long routes or uneven ground, use surveyed elevations or a terrain-profile tool before choosing final tower heights.
Why can required height calculate as zero?
The fixed antenna may already have enough horizon distance for the entered route under this geometric model. It does not mean ground-level hardware will clear terrain, Fresnel requirements, or nearby structures.
Should I add extra height above the result?
Usually, yes. Add engineering margin for terrain uncertainty, antenna mounting, vegetation growth, construction tolerance, weather effects, and future obstacles. Verify the final design with a complete site survey.