Calculator Inputs
Example Data Table
| Scenario | Antenna 1 | Antenna 2 | k-Factor | Path | Frequency | Mutual Horizon | Status |
|---|---|---|---|---|---|---|---|
| Broadcast relay | 60 m | 45 m | 1.3333 | 52 km | 950 MHz | 57.87 km | Within horizon |
| Coastal link | 25 m | 25 m | 1.20 | 38 km | 450 MHz | 34.71 km | Beyond horizon |
| Rural tower pair | 90 m | 35 m | 1.3333 | 68 km | 1800 MHz | 65.66 km | Beyond horizon |
Formula Used
1) Geometric horizon for one antenna
d = √(2Rh + h²)
2) Radio horizon with refraction
dradio = √(2kRh + h²)
3) Mutual horizon between two antennas
Dmutual = d1 + d2
4) Effective Earth bulge at path midpoint
b = L² / (8kR)
5) First Fresnel radius at midpoint
F1 = √(λL / 4)
6) Recommended Fresnel clearance
Clearance = F1 × (clearance percentage / 100)
Where R is Earth radius, k is the effective Earth radius factor, h is antenna height, L is path length, and λ is wavelength from frequency.
How to Use This Calculator
- Enter both antenna heights in meters or feet.
- Keep Earth radius at 6371 km unless a custom model is required.
- Set the refraction k-factor. A common planning value is 4/3.
- Choose the distance unit for entered path length and reported horizon results.
- Enter frequency to estimate the first Fresnel zone at midpoint.
- Enter desired Fresnel clearance percentage, often 60% for link planning.
- Click the calculate button to display results above the form.
- Review the graph, export CSV or PDF, and compare planning scenarios.
FAQs
1. What does radio horizon mean?
Radio horizon is the farthest theoretical line-of-sight distance between antennas after accounting for Earth curvature and standard atmospheric refraction effects.
2. Why is radio horizon longer than geometric horizon?
Atmospheric refraction bends radio waves slightly downward. That makes the Earth appear effectively larger in radius, which extends the practical horizon distance.
3. What k-factor should I use?
A planning default of 4/3 is common for standard conditions. Lower values represent weaker refraction and reduce predicted range.
4. Does this calculator include terrain and buildings?
No. It estimates Earth-curvature and Fresnel effects only. Real paths also depend on terrain elevations, vegetation, clutter, diffraction, and local atmosphere.
5. Why is frequency included?
Frequency is used to estimate the first Fresnel zone radius. Higher frequencies usually produce smaller Fresnel zones for the same path length.
6. What does clearance margin tell me?
Clearance margin compares midpoint line height against Earth bulge and required Fresnel clearance. Positive margin suggests a cleaner geometric path.
7. Can I use feet and miles together?
Yes. Height units and distance units are handled separately, so you can mix common field units without changing the equations manually.
8. Is a visible horizon enough for a reliable link?
Not always. A reliable link also needs fade margin, acceptable signal level, antenna gain, polarization control, and obstacle-free clearance.