Simplifications needed

The free space loss is easily calculated using Friis transmission equation which states that the loss is proportional to the square of the distance and the square of the frequency. Additionally losses are incurred in most radio links, including atmospheric attenuation by gases, rain, fog and clouds. Fading due to variations of the channel, multipath losses and antenna misalignment. In non line of sight links, diffraction and reflection losses are the most important since the direct path is not available.

Transmission line and polarization loss

In practical situations (deep space telecommunications, weak signal DXing etc.) other sources of signal loss must also be accounted for

• The transmitting and receiving antennas may be partially cross-polarized.
• The cabling between the radios and antennas may introduce significant additional loss.
• Fresnel zone losses due to a partially obstructed line of sight path.
• Doppler shift induced signal power losses in the receiver.

Endgame

If the estimated received power is sufficiently large (typically relative to the receiver sensitivity), which may be dependent on the communications protocol in use, the link will be useful for sending data. The amount by which the received power exceeds receiver sensitivity is called the link margin.

Equation

A link budget equation including all these effects, expressed logarithmically, might look like this:

${\displaystyle P_{\text{RX}}=P_{\text{TX}}+G_{\text{TX}}-L_{\text{TX}}-L_{\text{FS}}-L_{M}+G_{\text{RX}}-L_{\text{RX}}\,}$

where:

${\displaystyle P_{\text{RX}}}$, received power (dBm)

${\displaystyle P_{\text{TX}}}$, transmitter output power (dBm)

${\displaystyle G_{\text{TX}}}$, transmitter antenna gain (dBi)

${\displaystyle L_{\text{TX}}}$, transmitter losses (coax, connectors...) (dB)

${\displaystyle L_{\text{FS}}}$, path loss, usually free space loss (dB)

${\displaystyle L_{\text{M}}}$, miscellaneous losses (fading margin, body loss, polarization mismatch, other losses, ...) (dB)

${\displaystyle G_{\text{RX}}}$, receiver antenna gain (dBi)

${\displaystyle L_{\text{RX}}}$, receiver losses (coax, connectors, ...) (dB)

The loss due to propagation between the transmitting and receiving antennas, often called the path loss, can be written in dimensionless form by normalizing the distance to the wavelength:

${\displaystyle L_{\text{FS}}{\text{(dB)}}=20\log _{10}\left(4\pi {{\text{distance}} \over {\text{wavelength}}}\right)}$ (where distance and wavelength are in the same units)

When substituted into the link budget equation above, the result is the logarithmic form of the Friis transmission equation.

In some cases, it is convenient to consider the loss due to distance and wavelength separately, but in that case, it is important to keep track of which units are being used, as each choice involves a differing constant offset. Some examples are provided below.

${\displaystyle L_{\text{FS}}}$ (dB) ≈ 32.45 dB + 20 log10[frequency (MHz)] + 20 log10[distance (km)]^[1]

${\displaystyle L_{\text{FS}}}$ (dB) ≈ −27.55 dB + 20 log10[frequency (MHz)] + 20 log10[distance (m)]

${\displaystyle L_{\text{FS}}}$ (dB) ≈ 36.6 dB + 20 log10[frequency (MHz)] + 20 log10[distance (miles)]

These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (c, approximately 3×10⁸ m/s) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and (1/s).

Earth–Moon–Earth communications

Link budgets are important in Earth–Moon–Earth communications. As the albedo of the Moon is very low (maximally 12% but usually closer to 7%), and the path loss over the 770,000 kilometre return distance is extreme (around 250 to 310 dB depending on VHF-UHF band used, modulation format and Doppler shift effects), high power (more than 100 watts) and high-gain antennas (more than 20 dB) must be used.

• In practice, this limits the use of this technique to the spectrum at VHF and above.
• The Moon must be above the horizon in order for EME communications to be possible.

Voyager program

The Voyager program spacecraft have the highest known path loss (308 dB as of 2002^[4]: 26) and lowest link budgets of any telecommunications circuit. The Deep Space Network has been able to maintain the link at a higher than expected bitrate through a series of improvements, such as increasing the antenna size from 64 m to 70 m for a 1.2 dB gain, and upgrading to low noise electronics for a 0.5 dB gain in 2000–2001. During the Neptune flyby, in addition to the 70-m antenna, two 34-m antennas and twenty-seven 25-m antennas were used to increase the gain by 5.6 dB, providing additional link margin to be used for a 4× increase in bitrate.^[4]: 35