Ian ClearyRF Systems Engineer

Ian ClearyRF Systems Engineer

Link Budgets


A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the loss introduced by components of the transmission system such as connectors, splitters, and cable loss. Additional gains and losses from the use of antennas are also accounted for in the link budget.

Link budgets are often used to predict and model the performance of a communications system, such as a satellite link, prior to its construction.

Within that goal, a designer will create a link budget to achieve some specified performance criteria, with examples include:

What about a gain lineup?

This contrasts with a gain lineup, which is the accounting of all of the gains and losses inside a system (or partition of a system). A gain lineup generally doesn't include free space path loss nor wireless losses.

For example, a gain lineup for a satellite transponder would include the gains and losses of the transponder itself, but not the losses from the propagation through free space.

Friis Transmission Equation

Pr=Pt+Gt+Gr+20log10(λ4πd)P_r = P_t + G_t + G_r + 20 \log_{10} \left( \frac{\lambda}{4 \pi d} \right)


Free Space Path Loss

Influence of distance and frequency

In free space the intensity of electromagnetic radiation decreases with distance by the inverse square law, because the same amount of power spreads over an surface area proportional to the square of distance from the source.

The free-space loss increases with the distance between the antennas and decreases with the wavelength of the radio waves due to these factors:[6]

Signal to Noise Ratio

The signal-to-noise ratio (SNR) is the ratio of the power of a signal (meaningful information) to the power of the noise (unwanted signal).

The SNR is usually measured in decibels (dB). If the incoming signal strength in microvolts is VsV_s and the noise level, also in microvolts, is VnV_n, then the signal-to-noise ratio, S/NS/N, in decibels is given by: 20log10(Vs/Vn)20 \log_{10} (V_s/V_n)

Expressed in terms of power, the signal-to-noise ratio is: 10log10(Ps/Pn)10 \log_{10} (P_s/P_n).


To be continued...

© 2023-present Ian Cleary.All Rights Reserved.
© 2023-present Ian Cleary.
All Rights Reserved.