What is an RF Link Budget?
A link budget operates just like a financial budget. You start with the transmitter's output power (your income), add the amplification from your antennas (bonuses), and subtract the massive energy loss as the signal travels through the air and cables (your expenses).
The final result is the Received Signal Level (RSL). For the link to work, the RSL must be stronger than the receiver's minimum sensitivity threshold, with enough 'buffer' leftover to handle weather interference.
Free Space Path Loss (FSPL)
The largest 'expense' in any wireless link is Free Space Path Loss. As a radio wave leaves an antenna, it spreads out like a flashlight beam. The further it travels, the weaker the signal gets. FSPL is dependent on both the distance of the link and the frequency of the radio.
Higher frequencies (like 11 GHz or 24 GHz microwave) suffer significantly higher path loss than lower frequencies (like 900 MHz or 2.4 GHz WiFi), meaning they require larger dish antennas to bridge the same distance.
How to Calculate the Link Budget
Calculations are done using decibels (dB) and decibel-milliwatts (dBm), which allows you to use simple addition and subtraction rather than complex multiplication of massive numbers.
The Formula: Received Power (dBm) = Transmitter Power (dBm) - Cable Losses (dB) + Tx Antenna Gain (dBi) - Path Loss (dB) + Rx Antenna Gain (dBi).
Worked Example: You are setting up a 5 GHz link. The radio transmits at 20 dBm. There is 2 dB of loss in the jumper cables on both sides (4 dB total). You are using two dish antennas that each provide 25 dBi of gain. The path loss calculator determines the FSPL for the distance is 120 dB. Calculation: 20 dBm - 4 dB + 25 dBi - 120 dB + 25 dBi = -54 dBm. The Received Signal Level is -54 dBm.
Fade Margin and Fresnel Zones
If the radio requires a minimum of -74 dBm to operate, and your calculated RSL is -54 dBm, you have a 'Fade Margin' of 20 dB. This margin is crucial; it ensures the link stays up when heavy rain or atmospheric ducting temporarily weakens the signal.
Additionally, the link budget assumes clear Line of Sight. However, RF signals require a three-dimensional elliptical clearance space called the Fresnel Zone. If trees or buildings protrude into the middle of the Fresnel Zone—even if you can visually see the other tower—the signal will diffract and cause heavy losses.
Frequently asked questions
What is the difference between dBm and dBi?
dBm is an absolute measurement of power relative to one milliwatt (used for radio output). dBi is a relative measurement of antenna gain compared to a theoretical isotropic antenna that radiates equally in all directions.
How much Fade Margin is considered safe?
For mission-critical carrier-grade links (five-nines reliability), engineers aim for a fade margin of 25 dB to 30 dB. For non-critical data links, 15 dB is often acceptable.
What is rain fade?
At high microwave frequencies (typically above 10 GHz), the wavelength of the RF signal approaches the physical size of raindrops. The rain absorbs and scatters the signal, causing severe temporary attenuation.
Does higher Tx power always make a link better?
No. Turning up transmitter power increases self-interference, amplifies noise, and often violates FCC/regulatory EIRP limits. It is always better to increase Antenna Gain rather than Tx power, as larger antennas 'listen' better as well as transmit better.