Wireless Link Budget & Fresnel Zone Calculator
Enter a point-to-point wireless path to check both the radio budget and the first-Fresnel clearance at a limiting obstruction. The result shows whether the link meets your target fade margin and how much antenna height would be needed if clearance is short.
Link assessmentPasses the target fade margin and Fresnel-clearance checks
- Free-space path loss (dB)
- 121.7
- Predicted received power (dBm)
- -71.7
- Available fade margin (dB)
- 18.3
- Fade-margin surplus or deficit (dB)
- 3.3
- Maximum free-space distance (km)
- 7.31
- First Fresnel-zone radius at obstacle (m)
- 8.04
- Effective-Earth curvature bulge (m)
- 0.37
- Available Fresnel clearance
- 119.83%
- Clearance surplus or deficit (m)
- 4.81
- Required equal antenna raise (m)
- 0
- Required transmitter-only raise (m)
- 0
- Required receiver-only raise (m)
- 0
Assumes free-space propagation, one entered obstruction, and endpoint and obstruction elevations on the same vertical datum.
Required antenna raises are zero when the entered obstruction already meets the selected Fresnel-clearance target.
How to use this calculator
- Enter the frequency, path distance, transmit power, antenna gains, fixed losses, receiver sensitivity, and target fade margin.
- Enter both endpoint ground elevations and antenna heights using one vertical datum.
- Enter the distance and top elevation of the most limiting obstruction on the path.
- Adjust the effective Earth-radius factor and required Fresnel clearance if your planning standard calls for different values.
- Read the link assessment, margin surplus, clearance surplus, maximum free-space range, and antenna-height changes.
How the wireless link is calculated
The radio side uses the free-space attenuation equation from ITU-R P.525. With frequency in MHz and path distance in km, the path loss is:
FSPL = 32.45 + 20 log10(f) + 20 log10(D)
Predicted received power is transmitter power plus both antenna gains, minus fixed system losses and the free-space path loss:
Pr = Pt + Gt + Gr - L - FSPL
The available fade margin is received power minus receiver sensitivity. The calculator compares that margin with your target margin, then reverses the same free-space equation to estimate the maximum distance that would still meet the requested margin.
How Fresnel clearance is checked
The first Fresnel radius at the entered obstruction is based on wavelength and the two path segments from the transmitter and receiver:
r1 = sqrt(lambda d1 d2 / D)
The distances in that expression are meters. Wavelength is 299.792458 / f meters when frequency is in MHz. The straight radio path height at the obstruction is found by linear interpolation between the two antenna centerlines. The effective-Earth bulge is then subtracted from that line:
B = d1 d2 / (12.742 k)
Here d1, d2, and D are in km, and k is the effective Earth-radius factor. The available vertical clearance is the interpolated path height minus the obstacle top and the curvature bulge. The required clearance is your selected percentage of the first Fresnel radius, commonly 60% for planning.
What moves the result most
Frequency and distance drive free-space path loss. Higher antenna gain, more transmit power, lower cable loss, or a more sensitive receiver improve fade margin. Clearance is controlled by endpoint height, obstruction position, obstruction elevation, frequency, and the Earth-radius factor. An obstruction near one endpoint has little leverage from raising the far endpoint, so the transmitter-only or receiver-only raise can become large.
What this calculator leaves out
This is a single-obstruction planning check, not a full terrain profile. It does not model foliage loss, buildings, rain fade, atmospheric absorption, interference, multipath, diffraction from a blocked zone, antenna pattern details, or regulatory limits such as EIRP and licensing. Treat survey error, tree growth, mount tolerances, and antenna alignment as additional margin rather than as part of the displayed result.
Worked example
For a 5 km link at 5800 MHz with 20 dBm transmitter power, two 16 dBi antennas, 2 dB of system loss, -90 dBm receiver sensitivity, and a 15 dB target fade margin, the free-space path loss is 121.70 dB. Predicted received power is -71.70 dBm, so the available fade margin is 18.30 dB and the fade-margin surplus is 3.30 dB.
With both antenna centers 10 m above 100 m ground elevations, an obstruction halfway along the path at 100 m elevation, a 1.333 Earth-radius factor, and a 60% Fresnel target, the first Fresnel radius is 8.04 m. Curvature bulge is 0.37 m, available clearance is 119.83% of the first Fresnel radius, and the clearance surplus is 4.81 m. The link passes both checks, so the required antenna raises are 0 m.
Common questions
What fade margin should a point-to-point wireless link have?
A common starting point is 10 to 20 dB, with 15 dB often used for basic outdoor planning. Critical links, long paths, noisy bands, wet climates, or links expected to carry high modulation rates may need more margin.
Why should about 60% of the first Fresnel zone be clear?
The direct visual line of sight can be open while nearby terrain still intrudes into the Fresnel zone and causes diffraction loss. Clearing about 60% of the first Fresnel zone is a common planning rule because it avoids most of that loss without requiring the full zone to be empty.
How do I find receiver sensitivity for a specific MCS and channel width?
Use the radio vendor data sheet for the exact modulation, coding, channel width, and target packet error rate. Receiver sensitivity can change by many decibels between a robust low-rate mode and a high-throughput mode.
Does higher antenna gain increase maximum link distance?
Yes, antenna gain adds directly to the link budget, so more gain increases received power and maximum free-space distance. It also narrows the beam, which makes alignment and mounting stability more important.
How does Earth curvature affect long wireless links?
Curvature makes the terrain appear to rise into the path as distance increases. The effective Earth-radius factor adjusts that bulge for atmospheric refraction, but it is a planning assumption rather than a fixed site condition.
Why can a link budget pass while the Fresnel-clearance test fails?
The link budget only checks signal strength under a free-space assumption. A terrain, tree, or building obstruction can still intrude into the Fresnel zone and add diffraction or blockage loss that the free-space budget does not include.