Calculate antenna EIRP from transmitter power, cable loss, and antenna gain, or solve for the required transmit power or maximum antenna gain, with results in dBm, dBW, watts, and ERP.
Antenna EIRP Formula
EIRP (effective isotropic radiated power) is calculated in the decibel domain, so gains are added and losses are subtracted:
EIRP = Pt - Lc + Ga
Variables:
- EIRP is the effective isotropic radiated power, in dBm
- Pt is the transmitter output power, in dBm
- Lc is the total loss between the transmitter and the antenna (cable plus connectors), in dB
- Ga is the antenna gain, in dBi
The calculator’s default mode applies this formula directly. The two other solve-for modes rearrange the same equation. “Required transmitter power” solves for Pt when you know the EIRP you need to reach or stay under, and “Maximum antenna gain” solves for Ga when the transmitter power is fixed and you must not exceed an EIRP limit:
Pt = EIRP - Ga + Lc Ga = EIRP - Pt + Lc
Three supporting conversions are used for the unit options and the ERP output. Antenna gain given in dBd (relative to a half-wave dipole) is converted to dBi, ERP is derived from EIRP, and dBm values are converted to watts:
Ga(dBi) = Ga(dBd) + 2.15 ERP = EIRP - 2.15 P(W) = 10^((P(dBm) - 30)/10)
If you enter transmitter power or target EIRP in watts or milliwatts, the calculator first converts it to dBm, runs the formula, then reports the result in dBm, dBW, and watts.
Common EIRP Limits by Band and Region
EIRP is the number regulators actually limit, not raw transmitter power. This table summarizes widely cited limits for common unlicensed bands. Rules change and contain exceptions, so verify the current text of the regulation that applies to your installation before transmitting.
| Band and use | Region | Typical max EIRP |
|---|---|---|
| 2.4 GHz Wi-Fi, point to multipoint | US (FCC 15.247) | 36 dBm (4 W) |
| 2.4 GHz Wi-Fi, fixed point to point | US (FCC 15.247) | Above 36 dBm allowed: Tx power drops 1 dB for every 3 dBi of gain over 6 dBi |
| 2.4 GHz Wi-Fi | EU (ETSI) | 20 dBm (100 mW) |
| 5 GHz U-NII-1 (5150 to 5250 MHz) | US | 36 dBm for access points |
| 5 GHz U-NII-3, fixed point to point | US | Up to 53 dBm (200 W) |
| 5 GHz (5470 to 5725 MHz) | EU (ETSI) | 30 dBm (1 W) |
| 902 to 928 MHz ISM (LoRa, meshtastic, RFID) | US | 36 dBm (30 dBm Tx plus 6 dBi) |
| 868 MHz SRD | EU | 14 dBm ERP (about 16.2 dBm EIRP) |
Typical Coax Cable Loss for the Lc Input
Cable loss is the input most people guess at. These approximate manufacturer figures are per 100 feet (30.5 m) of cable, so scale them to your run length. A 50 ft run of LMR-400 at 2.4 GHz, for example, contributes about 3.4 dB.
| Cable | Loss at 900 MHz | Loss at 2.4 GHz | Loss at 5.8 GHz |
|---|---|---|---|
| RG-58 | about 14.5 dB | about 30 dB | not recommended |
| RG-8X | about 12.8 dB | about 23 dB | not recommended |
| LMR-240 | 7.6 dB | 12.7 dB | 20.4 dB |
| LMR-400 | 3.9 dB | 6.8 dB | 10.8 dB |
| LMR-600 | 2.5 dB | 4.4 dB | 7.3 dB |
Antenna EIRP Example Problems
Example 1: A 1 W (30 dBm) radio feeds an antenna through a cable with 3 dB of total loss. The antenna has a gain of 9 dBi. EIRP = 30 – 3 + 9 = 36 dBm, which is about 3.98 W. The ERP is 36 – 2.15 = 33.85 dBm. In the default mode, enter 30 dBm, 3 dB, and 9 dBi and the calculator returns 36 dBm.
Example 2: Your band limits you to 36 dBm EIRP, your radio puts out 27 dBm, and the feedline plus connectors lose 2 dB. The largest legal antenna gain is Ga = 36 – 27 + 2 = 11 dBi, or 8.85 dBd. Select “Maximum antenna gain,” enter a target EIRP of 36 dBm, a transmitter power of 27 dBm, and 2 dB of loss to confirm the result.
FAQ
What is the difference between EIRP and ERP?
Both describe the same radiated signal against a different reference antenna. EIRP compares your system to an ideal isotropic radiator, while ERP compares it to a half-wave dipole. Because a dipole already has 2.15 dBi of gain, ERP is always 2.15 dB lower than EIRP for the same setup. FCC rules for unlicensed bands are usually written in EIRP, while some broadcast and European short-range-device rules use ERP, so check which one your regulation names before comparing numbers.
Why is the EIRP higher than the transmitter’s output power?
Antenna gain is not amplification. A directional antenna concentrates the same total power into a narrower beam, so a receiver in that beam sees the signal level that a much stronger transmitter would need with an isotropic antenna. EIRP reports that equivalent power. No extra energy is created, which is why EIRP can legitimately exceed the radio’s output rating without violating physics.
How do I lower my EIRP to stay under a legal limit?
You have three levers. First, reduce the transmitter output power in the radio’s settings, which lowers EIRP dB for dB. Second, switch to a lower-gain antenna. Third, add attenuation between the radio and antenna, either with an inline attenuator or simply by accepting more cable loss. Use the calculator’s “Required transmitter power” mode to find the exact power setting that lands on the limit for your antenna and cable.
