Which equation is used for link budgeting in RF communications?

• A. Ohm's law.
• B. Maxwell's equations.
• C. Planck's law.
• D. Friis transmission equation.

Answer: (D)

Link budgeting in RF communications is addressed with a formula that relates transmitted power, antenna gains, distance, and wavelength to the received power. The Friis transmission equation captures this: Pr = Pt Gt Gr (λ/(4πR))^2. Here Pt is the transmitted power, Gt and Gr are the antenna gains in linear (not dB) terms, λ is the wavelength (c/f), and R is the separation between antennas. This equation shows why the received power depends on how effectively the antennas radiate and receive in the direction of each other and why it diminishes with distance as the signal spreads over a sphere (the (λ/(4πR))^2 term). In practice, you incorporate system losses and often work in decibels: Pr(dBm) = Pt(dBm) + Gt(dBi) + Gr(dBi) − Lfs(dB) − Ls(dB), with Lfs(dB) = 20 log10(4πR/λ). The other choices don’t fit link budgeting: Ohm's law is a basic circuit relationship, Maxwell's equations govern electromagnetic fields but aren't a single budgeting tool, and Planck's law deals with blackbody radiation. The Friis equation is the standard for estimating received power in free-space RF links.