Microwave Link
Calculate microwave point-to-point link feasibility from frequency, distance and antenna specs.
About Microwave Link
Free-space path loss (FSPL) is the signal attenuation that occurs simply from a radio wave spreading out over distance in an unobstructed path, with no absorption or reflection involved. The standard formula, FSPL(dB) = 20·log10(distance_km) + 20·log10(frequency_GHz) + 92.45, shows loss increases logarithmically with both distance and frequency — doubling either one adds a fixed ~6 dB of additional loss. This figure feeds directly into a full link budget (see the RF Link Budget or Wireless Link Budget calculators) to determine whether a given link will close with the available transmit power and antenna gains.
Formula
Free-space path loss
FSPL(dB) = 20·log10(distance_km) + 20·log10(frequency_GHz) + 92.45
- d
- Distance in kilometers
- f
- Frequency in GHz
Worked example
With the default values below, Microwave Link returns Free-Space Path Loss of 136.80. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Link Distanceminimum 0.001 km
- 15 km
- Frequencyminimum 0.01 GHz
- 11 GHz
Results
- Free-Space Path Loss
- 136.80 dB
- Additional Loss if Distance Doubled
- 6.02 dB
Step by step
- 1Values used
- 2Free-space path loss
- 3Free-Space Path Loss= 136.80 dB
- 4Additional Loss if Distance Doubled= 6.02 dB
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why does doubling distance add about 6 dB of loss?
Because the formula uses 20·log10(distance), and log10(2) ≈ 0.301, so 20 × 0.301 ≈ 6.02 dB — this holds regardless of the starting distance, which is why the 'doubling adds ~6 dB' rule of thumb is a constant, not distance-dependent.
Why does higher frequency cause more path loss at the same distance?
The formula's frequency term behaves the same way as its distance term — 20·log10(frequency) — so doubling frequency also adds about 6 dB of loss, reflecting that a fixed-size antenna captures proportionally less of a shorter-wavelength wave's energy at a given range.
Does FSPL account for rain, fog, or atmospheric absorption?
No — free-space path loss models only the geometric spreading loss in a vacuum/clear atmosphere. Higher-frequency microwave links (especially above ~10 GHz) need separate rain fade margin added to the link budget, since precipitation attenuation becomes significant at those frequencies.
Why do carriers choose different microwave bands (6, 11, 18, 38 GHz) for different links?
Lower bands (6-11 GHz) suffer less path loss and rain fade, suiting longer links, but require larger antennas and face more spectrum congestion. Higher bands (23-38 GHz) support smaller, cheaper antennas and wider channels for higher capacity, but need shorter hops due to higher path loss and rain sensitivity.