Building a radio link budget

Use a common reference

Start with transmit power in dBmdBm×dBm expresses an absolute power level relative to 1 milliwatt, combining the logarithmic dB scale with a fixed power reference.Example & full explanation →. Add transmit and receive antenna gains in dBi, then subtract cable and path losses in dB. The result is a modelled receive level. Every value must refer to the same frequency and actual antenna direction; using peak gainGain×Gain describes the ratio of output to input level. In a receiver it can occur before or after digitisation and is often expressed in dB.Example & full explanation → from another direction would be incorrect.

Free-space loss is FSPL = 20·log₁₀(4πdf/c), using metres and hertz. At 145 MHz over 10 km it is about 95.7 dB. Doubling distance adds roughly 6 dB, as does doubling frequency when isotropic-referenced antenna gains stay unchanged.

Margin above receiver threshold

With 37 dBm transmit level, 2 dBi at each antenna and 3 dB total additional loss, the example gives about −57.7 dBm at the receiver. Against an assumed −110 dBm threshold, calculated margin is about 52.3 dB. That threshold is an example and must be based on an appropriate quality criterion for a real receiver.

Negative margin means the model falls below the selected threshold. Positive margin is still not a guarantee of success. Unmodelled blockage, polarisation, multipath and interference can substantially alter the outcome. Record each additional loss assumption explicitly instead of hiding it behind a seemingly precise number.

When free space is insufficient

The free-space model assumes suitable far-field geometry without obstacles. It models neither an ionospheric HF link nor a path through several mountain ranges. Check horizon and Fresnel clearance and use terrain and propagation data for real site planning. The calculator primarily helps explain the effect of individual assumptions.