About this tool
A link budget adds up what is gained and lost between transmitter and receiver: transmit power plus antenna gains, minus cable losses and minus the path loss. If the received power is above the receiver sensitivity, the link works – the difference is the link margin.
The link budget calculator uses the free-space path loss 20·log10(4π·d/λ) and computes received power, margin and the maximum range for a fade margin of your choice. Presets for Wi-Fi, 5 GHz directional links, LoRa, PMR446 and 2 m radio give typical starting values. Work out cable losses with the Coax Cable Loss Calculator and convert between dBm and watts with the dBm Converter.
The advanced parameters add cable losses, other losses, a path loss exponent for built-up areas and buildings, and the antenna heights. From them the calculator checks whether 60 % of the first Fresnel zone is clear over flat ground, accounts for earth curvature with a k-factor and shows the radio horizon. Everything is calculated locally in your browser.
How it works
- Pick a preset or enter frequency, distance, transmit power (dBm or W), antenna gains and the receiver sensitivity.
- Read the received power, link margin and maximum range – the colour shows whether the link works with fade margin.
- Turn on the advanced parameters for cable losses, walls, path loss exponent, antenna heights and the Fresnel check if you need them.
Frequently asked questions
How is the free-space path loss calculated?
The free-space path loss is 20·log10(4π·d/λ). With d in km and f in MHz this becomes the rule of thumb 32.45 + 20·log10(d) + 20·log10(f). 1 km at 2.4 GHz is therefore about 100 dB. Every doubling of the distance costs 6 dB, and so does every doubling of the frequency – for the same antenna gains in dBi.
How much margin does a radio link need?
The link margin absorbs fading from reflections, weather and interference. 10 dB is common for simple links, 20 to 30 dB for reliable directional links. The calculator uses 10 dB by default; change the fade margin in the advanced parameters and the maximum range follows it.
What is the Fresnel zone and why does it need to be clear?
The first Fresnel zone is an ellipsoid around the direct line of sight that carries most of the energy. Obstacles reaching into it increase the loss even if the antennas can still “see” each other. As a rule of thumb at least 60 % of its radius should be clear. In the middle of a 5 km link the radius is about 8 m at 5.6 GHz and about 21 m at 868 MHz.
What does the path loss exponent mean?
In free space the power falls with the square of the distance, i.e. with exponent 2. Towns and buildings add shadowing and reflections; the log-distance model reflects this with a larger exponent, typically 2.7 to 3.5 in built-up areas and 4 to 6 inside buildings. The calculator assumes free space up to 1 m and continues with 10·n·log10(d) beyond. It is a rough estimate, not a measurement.
Why is the real range often shorter?
The calculation assumes a clear line of sight over flat ground. Hills, buildings, trees, bodies and vehicles add losses, as does rain at high frequencies. Enter such losses under “Other losses” or raise the path loss exponent. For real links, check the terrain profile.