Link Budget
Calculator
A multi-input tool that brings together transmit power, antenna gains, cable losses, free space path loss, and system noise figure to determine whether a communications link closes with adequate margin.
How a Link Budget Works
A link budget is the RF engineer's fundamental tool for determining whether a wireless communications link will work. It is an accounting of every gain and loss in the system, from the transmitter amplifier to the receiver demodulator. If the signal arriving at the receiver exceeds the receiver's sensitivity by the required margin, the link "closes" and will operate reliably.
The Link Budget Equation
EIRP = TX Power (dBm) - TX Cable Loss (dB) + TX Antenna Gain (dBi)
Step 2: Path Loss
FSPL (dB) = 20×log₁₀(d_km) + 20×log₁₀(f_GHz) + 92.45
Total Path Loss = FSPL + Rain + Atmospheric + Other Losses
Step 3: Received Power
P_received = EIRP - Total Path Loss + RX Antenna Gain - RX Cable Loss
Step 4: Link Margin
Margin = P_received - RX Sensitivity
If Margin ≥ Required Margin → Link Closes ✓
Worked Example: Ka-Band Point-to-Point Link
28 GHz link over 1 km with 30 dBm TX, 20 dBi antennas on both ends, 2 dB cable loss on each side, and -80 dBm receiver sensitivity:
FSPL = 20×log₁₀(1) + 20×log₁₀(28) + 92.45 = 121.39 dB
Received Power = 48 - 121.39 + 20 - 2 = -55.39 dBm
Margin = -55.39 - (-80) = 24.61 dB
Result: Link closes with 24.61 dB of margin ✓
How Much Link Margin Do You Need?
| Application | Typical Margin | Key Fade Mechanisms |
|---|---|---|
| Indoor short-range (lab) | 3 – 5 dB | Multipath, cable aging |
| Fixed terrestrial, clear air | 5 – 10 dB | Atmospheric, aging |
| Terrestrial, rain regions | 10 – 20 dB | Rain fade (at Ka-band+) |
| Satellite downlink | 3 – 6 dB | Rain fade, scintillation |
| Satellite uplink | 6 – 10 dB | Rain fade, uplink power control |
| Military / high reliability | 10 – 15 dB | Jamming, all-weather operation |
Tips for Closing a Difficult Link
- Increase antenna gain on either end. Every 3 dB of additional gain effectively doubles the link range for the same margin.
- Reduce cable losses by placing the transmitter/LNA closer to the antenna (feed-mounted equipment).
- Use a lower noise figure receiver to improve sensitivity. Every 1 dB improvement in NF gives 1 dB more margin.
- Increase TX power as a last resort. Regulatory limits (EIRP caps) often constrain this option.
- Use adaptive coding and modulation (ACM) to trade data rate for link robustness during fade events.
Frequently Asked Questions
What is a link budget?
A link budget is an accounting of all gains and losses from transmitter to receiver. It determines whether the received signal power exceeds receiver sensitivity by enough margin for reliable communication. Every wireless system design starts with a link budget.
What is EIRP?
EIRP (Effective Isotropic Radiated Power) is the total effective radiated power in the antenna's main beam direction. EIRP = TX Power - TX Cable Loss + TX Antenna Gain (all in dB). It represents what an isotropic antenna would need to radiate to produce the same field strength.
How much link margin is enough?
It depends on required reliability and fade mechanisms. 3-6 dB for controlled indoor environments. 10-15 dB for outdoor links subject to rain fade at Ka-band and above. Military systems may require 10-15+ dB to handle adverse conditions.
What factors are not included in FSPL?
FSPL only accounts for geometric spreading. Additional real-world losses include: rain attenuation (major above 10 GHz), atmospheric gas absorption (peaks at 22 GHz and 60 GHz), foliage, building penetration, multipath fading, pointing losses, and polarization mismatch. Add these as "Additional Path Losses" in the calculator.