Wireless

Wireless Link

Compute the point-to-point RF link budget: free-space path loss (FSPL), EIRP, received level (RSSI), fade margin and the radius of the first Fresnel zone with the recommended 60 % clearance. All in your browser.

redzilla.cl — rf

Fade margin

—

Received level (RSSI)

—

FSPL

—

free-space path loss

EIRP —

RSSI —

Margin —

Fresnel zone (1st) at the midpoint

Radius (r)

—

max radius of the 1st zone

Recommended clearance (60 %)

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min clear height above obstacles

Keep at least 60 % of the 1st Fresnel zone clear to avoid degrading the link.

How it is calculated · link budget and Fresnel

1. FSPL(dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44. Free-space propagation loss.

2. EIRP(dBm) = Tx + gain_Tx − cable_Tx. Equivalent isotropically radiated power.

3. Prx(dBm) = EIRP − FSPL + gain_Rx − cable_Rx − extra. This is the estimated RSSI at the receiver.

4. Margin(dB) = Prx − sensitivity. >10 dB good, 0–10 dB tight, <0 no link.

5. Fresnel zone (radius at the midpoint): r = 8.66·√(d_km / f_GHz) meters; clearing the 60 % (0.6·r) is recommended.

Runs locally in your browser · no sign-up · nothing leaves your browser.

How it works

The calculator solves the link budget of a point-to-point radio link from the frequency, the distance and the data of each end (Tx power, antenna gains in dBi, cable losses and receiver sensitivity). It returns the free-space path loss (FSPL), the EIRP, the estimated received level (RSSI), the fade margin with a traffic-light verdict and the radius of the first Fresnel zone with the recommended 60 % clearance.

The formulas are the classic RF ones: FSPL(dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44 (the Friis equation in logarithmic form), EIRP = Tx + Tx_gain − Tx_cable, Prx = EIRP − FSPL + Rx_gain − Rx_cable − extra losses and margin = Prx − sensitivity. The verdict rates a margin above 10 dB as good, 0 to 10 dB as tight and below 0 as no link. The mid-path Fresnel radius is r = 8.66·√(d_km / f_GHz) meters.

Example: 5 km link at 5 GHz with 24 dBi antennas

FSPL: 20·log₁₀(5) + 20·log₁₀(5000) + 32.44 ≈ 120.4 dB.
EIRP: 20 dBm + 24 dBi − 1 dB of cable = 43 dBm.
RSSI: 43 − 120.4 + 24 − 1 = −54.4 dBm; with a −78 dBm sensitivity the margin is +23.6 dB: a good link.
Fresnel: r = 8.66·√(5/5) = 8.66 m at the midpoint; you must clear at least 60 %, about 5.2 m above any obstacle.

Frequently asked questions

How much fade margin does a radio link need?

As a rule of thumb, 10 dB or more for a stable link; the calculator flags margins between 0 and 10 dB in yellow. Long or critical links are designed with 15 to 20 dB to absorb rain, multipath fading and aging. With a negative margin the signal arrives below sensitivity and the link does not come up.

What happens if I double the distance or the frequency of the link?

Each doubling adds 6 dB of FSPL, because the loss grows with 20·log₁₀ of both variables: going from 5 to 10 km costs 6 dB of margin, and moving from 2.4 to 5.8 GHz costs about 7.7 dB. That is why long links use higher-gain antennas or lower frequencies.

Why does the Fresnel zone matter if I have line of sight?

Because the RF signal does not travel as a thin ray but as an ellipsoid: if a hill, tree or building intrudes into the first Fresnel zone, part of the energy diffracts and the link loses several dB even though the antennas can see each other. Standard practice is to keep at least 60 % of the first-zone radius clear.

What is the difference between Tx power and EIRP?

Tx power is what the radio delivers at its connector; EIRP adds the antenna gain and subtracts the cable: it is the power effectively radiated in the main direction and the figure regulators cap (for example 36 dBm EIRP in several unlicensed bands). Everything runs in your browser and no data is sent anywhere.

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Disclaimer

We take great care to keep every tool accurate and review it thoroughly; even so, we can't guarantee it is free of errors or take responsibility for how the results are used. We recommend double-checking anything critical.

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