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Point-to-point radio link

Enter frequency, distance, elevations and the obstacle to see whether the 1st Fresnel zone stays clear, how tall the towers must be and how much margin the signal has at the far end.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Link
Path profile

Obstacle elevation = ground plus trees or buildings, above the same datum as sites A and B. Earth bulge is added by the calculator.

Link budget
Availability
—Fade margin
—

Path profile with line of sight and 1st Fresnel zone

Terrain simplified from the three elevations, with Earth bulge for the k-factor added. The shaded band is the 1st zone; the dashed line marks 60% of it.

Tap the chart to see the values.

Received level versus distance

Same radios and antennas, no obstruction. The gap between the lines is the fade margin.

Tap the chart to see the values.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Free space (ITU-R P.525): FSPL = 92.45 + 20 log f(GHz) + 20 log d(km) [dB]
Fresnel: r1 = √(λ · d1 · d2 ÷ d) = 17.32 √(d1 d2 ÷ (f d)) [m; km; GHz]; bulge: h = d1 · d2 ÷ (2 k R), R = 6,371 km
Clearance: c = LOS(d1) − (obstacle elevation + h); minimum antennas: increase = 0.6 r1 − c
Knife-edge diffraction (ITU-R P.526): ν = −c √(2 d ÷ (λ d1 d2)); J(ν) = 6.9 + 20 log[√((ν − 0.1)² + 1) + ν − 0.1] for ν > −0.78
Budget: Prx = Ptx − La + Ga − FSPL − J − other + Gb − Lb; margin = Prx − sensitivity
Unavailability (Barnett–Vigants): U = 6 × 10⁻⁷ · a · b · f · d³ · 10^(−margin/10), a = terrain (4; 1; 0.25), b = climate (0.5; 0.25; 0.125)

Assumptions and limits

  • Profile reduced to one dominant obstacle treated as a knife edge; paths with several obstacles or ground reflection need a full survey (ITU-R P.526, Deygout or Bullington methods).
  • Availability uses the empirical Barnett–Vigants formula for multipath fading on non-diversity paths; ITU-R P.530 gives the detailed method using the local refractivity gradient. Rain and gases are not included except under “other losses”.
  • 60% of the 1st zone with k = 4/3; ITU-R P.530 also recommends checking clearance with the local minimum k (0.5 to 1 on long paths).

Standards and technical documents

  • ITU-R P.525-4: Calculation of free-space attenuation
  • ITU-R P.526-16: Propagation by diffraction
  • ITU-R P.530-18: Propagation data and prediction methods required for the design of terrestrial line-of-sight systems
  • ITU-R P.453: The radio refractive index (k-factor and refractivity gradient)
  • Vigants, A. Space-diversity engineering. Bell System Technical Journal, 54(1), 1975

U.S. regulations

  • 47 CFR Part 15 (15.247 and 15.407): unlicensed 2.4 GHz and 5 GHz devices
  • 47 CFR Part 101: fixed microwave services
  • 47 CFR 1.1310: radiofrequency radiation exposure limits
  • OSHA 29 CFR 1926 Subpart M: fall protection; ANSI/ASSP A10.48: communication structures

Frequently asked questions

What is the Fresnel zone and how much must be clear?

It is the ellipsoid around the line of sight that carries most of the signal. The usual rule is to keep at least 60% of the 1st zone radius clear. On a 7-mile link at 5.8 GHz the mid-path radius is about 40 ft, so the beam should pass 24 ft above any obstacle, after adding Earth bulge.

How do I calculate free-space path loss?

With ITU-R P.525: FSPL = 92.45 + 20 log f + 20 log d, with f in GHz and d in km. At 5.8 GHz over 7 miles (11.3 km) the loss is 128.7 dB; doubling the distance adds 6 dB.

What fade margin should I use?

It depends on target availability, distance and climate. For 99.99% over 7 miles at 5.8 GHz in average climate and terrain, about 11 dB is enough; long paths or bands above 10 GHz need 25 to 40 dB.

How much does Earth curvature raise the terrain?

With k = 4/3, the bulge in feet is d1 · d2 ÷ 2, with the distances to each end in miles: 6.1 ft at the middle of a 7-mile link and 24.5 ft at the middle of a 14-mile link.