How the link budget is calculated
A link budget adds every gain and subtracts every loss between the transmitter and the receiver, then compares the received power with the receiver's noise. The difference between the SNR you get and the SNR your modulation needs is the link margin.
- Free-space path loss:
FSPL = 20·log₁₀(d km) + 20·log₁₀(f MHz) + 32.44 dB - EIRP:
P_tx − L_cable + G_tx - Received power:
EIRP − FSPL − other losses + G_rx − L_rx - Noise floor:
−174 dBm/Hz + 10·log₁₀(B) + NF - Link margin:
SNR − required SNR
Worked example: CubeSat UHF downlink
A 1 W (30 dBm) CubeSat transmits at 437 MHz through a 0 dBi antenna with 1 dB of feed loss, 1,500 km from a ground station with a 14 dBi Yagi. Path loss is about 148.8 dB, so after 3 dB of pointing and polarization loss the station receives roughly −110 dBm. In a 9.6 kHz bandwidth with a 1 dB noise figure the noise floor is about −133 dBm, giving an SNR near 23 dB. BPSK needs about 10.5 dB, which leaves roughly 12 dB of margin.
Good to know
- Aim for at least 3 dB of margin on a fixed link and 6–10 dB where fading, rain or pointing vary.
- Above about 10 GHz, rain fade usually decides the design. Pro uses ITU-R P.838 and P.618 to size it for your availability target.
- Terrestrial links also need a clear first Fresnel zone. Pro draws the path profile and adds knife-edge diffraction loss.