EIRP
Understanding EIRP
EIRP simplifies link budget calculations by combining the transmitter power and antenna gain into a single number. Instead of tracking power and gain separately, the link budget starts with EIRP and subtracts only path loss, atmospheric effects, and receive-side parameters.
EIRP Calculation
In linear terms, EIRP = P_tx x G_antenna. In dB, EIRP (dBm) = P_tx (dBm) + G (dBi). Note that the gain must be in dBi (relative to isotropic), not dBd (relative to dipole). If gain is given in dBd, add 2.15 dB before calculating EIRP.
EIRP in Link Budgets
The Friis transmission equation relates received power to EIRP through free-space path loss: P_rx = EIRP + G_rx - FSPL - L_misc, where all values are in dB. Higher EIRP means stronger received signal, which allows longer links, smaller receive antennas, or higher data rates.
Regulatory Limits
- Wi-Fi (2.4 GHz): 36 dBm (4W) EIRP in most countries.
- 5G mmWave: Up to 75 dBm (30W) EIRP for base stations.
- Satellite uplinks: Coordinated per-carrier EIRP to avoid interference with adjacent satellites.
- Radar: EIRP can exceed 100 dBW (10 GW) for high-power military radar.
EIRP (dBm) = P_tx (dBm) + G (dBi)
EIRP (dBW) = P_tx (dBW) + G (dBi)
Example: 10W transmitter (+40 dBm) with 30 dBi dish:
EIRP = 40 + 30 = 70 dBm = 40 dBW = 10 kW equivalent
Friis link budget:
P_rx = EIRP + G_rx - 20log10(4πd/λ) - L_atm
Typical EIRP Values
| System | TX Power | Antenna Gain | EIRP |
|---|---|---|---|
| Wi-Fi router | 20 dBm (100 mW) | 3 dBi | 23 dBm |
| Cell phone | 23 dBm (200 mW) | 0 dBi | 23 dBm |
| Satellite earth station | 40 dBm (10W) | 45 dBi | 85 dBm |
| Weather radar | 64 dBm (2.5 MW pk) | 40 dBi | 104 dBm |
| Deep space (DSN 70m) | 50 dBm (100W) | 74 dBi | 124 dBm |
Frequently Asked Questions
What is EIRP in simple terms?
EIRP tells you how strong a transmitter appears in its best direction. It combines the actual transmitter power with the antenna gain. A 1-watt transmitter with a high-gain antenna can have the same EIRP as a 1000-watt transmitter with an omnidirectional antenna.
How do you calculate EIRP?
Add the transmitter power in dBm to the antenna gain in dBi. For example, a 10W transmitter (40 dBm) connected to a 30 dBi dish antenna has EIRP = 40 + 30 = 70 dBm. This is equivalent to 10,000 watts radiated from an isotropic antenna.
What is the difference between EIRP and ERP?
EIRP uses an isotropic radiator as the reference. ERP (Effective Radiated Power) uses a half-wave dipole as the reference. EIRP = ERP + 2.15 dB. Regulatory specifications may use either convention, so always check which reference is assumed.