Link Budget

Calculate signal power from a satellite transmitter to a ground receiver. A typical Ku-band link budget helps design dish antennas and LNA specifications.

Core Concept

A link budget accounts for all gains and losses from the transmitter to the receiver. It determines the final signal power available at the receiving antenna. Engineers use this to size components like a 4 GHz C-band transponder.

The transmitted power gets amplified by the transmitting antenna gain. The signal then travels through space and suffers free space path loss. This loss happens because the spherical wave spreads out over distance.

At the destination the receiving antenna captures a tiny fraction of this energy. Additional losses occur in cables and waveguides. The final received power must stay above the threshold of a low noise amplifier like the MAX2659.

Key Equations

The Friis transmission equation calculates the received power in a free space link. Pr = Pt * Gt * Gr * (lambda / (4 * pi * d))^2 is the absolute form.

In decibels this formula becomes addition and subtraction. Pr(dBm) = Pt(dBm) + Gt(dB) + Gr(dB) - FSPL(dB) gives the power at the receiver.

Given:\nTransmitter Power (Pt) = 40 dBm\nTx Antenna Gain (Gt) = 35 dB\nRx Antenna Gain (Gr) = 45 dB\nFree Space Path Loss (FSPL) = 205 dB\n\nWhy this formula:\nReceived power is the sum of transmitted power and gains minus path losses in decibels.\n\nFormula:\nPr = Pt + Gt + Gr - FSPL\n\nSubstitution:\nPr = 40 + 35 + 45 - 205\n\nCalculation:\nPr = 120 - 205\nPr = -85\n\nFinal Answer:\n-85 dBm

Exam Tip: Free space path loss increases by 6 dB every time you double the frequency or double the distance. Watch out for questions that change both parameters at once.

Key Properties

• Equivalent Isotropically Radiated Power (EIRP) is the product of transmitted power and antenna gain.
• A typical geosynchronous satellite path loss is around 200 dB at 4 GHz.
• Receiver sensitivity dictates the minimum acceptable power, often around -100 dBm for narrow bandwidths.
• System noise temperature includes sky noise and LNA noise.
• Fade margin provides extra power to combat rain attenuation at high frequencies.

Quick Revision

• Link budget relies on logarithmic additions and subtractions.
• Antenna gain is measured relative to an isotropic radiator.
• Path loss depends heavily on the operating wavelength.
• Cable losses reduce both transmitted power and received sensitivity.
• Higher frequencies suffer more atmospheric absorption.
• Exam trap: Students often mix up Watts and dBm when plugging numbers into the Friis equation.