Effective Isotropic Radiated Power
EIRP = Ptx + Gant − L_cable (dB)
Quick answer
This calculator solves EIRP(dBm) = P_tx(dBm) + G_ant(dBi) − L_cable(dB) for EIRP. With TX Power 0 dBm, Antenna Gain 9 dBi, Cable Loss 20 dB, the result is -11 dBm.
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Formula
Description
Effective Isotropic Radiated Power (EIRP) is the total power that would need to be radiated by an isotropic antenna to produce the same signal strength in the direction of maximum antenna gain. It accounts for the transmitter output power, the antenna gain (which concentrates energy in preferred directions), and the cable losses between the transmitter and antenna. EIRP is the key parameter for link budget calculations, regulatory compliance, and interference analysis. Amateur radio and ISM band regulations often specify maximum EIRP limits.
Variables
- EIRP — Effective isotropic radiated power (dBm)
- P_tx — Transmitter output power (dBm)
- G_ant — Antenna gain relative to isotropic (dBi)
- L_cable — Total feed line loss (dB)
Practical Notes
FCC Part 97 limits amateur radio EIRP to 1500W PEP (about 61.8 dBm) on most bands. ISM devices under Part 15 are limited to much lower EIRP. To convert watts to dBm: P_dBm = 10×log₁₀(P_watts) + 30. A 100W transmitter (50 dBm) with a 10 dBi antenna and 2 dB cable loss produces 58 dBm EIRP (631W equivalent).
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| EIRP | EIRP | dBm | dBm |
| Ptx | TX Power | dBm | dBm |
| G | Antenna Gain | dBi | dBi |
| L | Cable Loss | dB | dB |
Step by step
- 1. What you know: TX Power 0 dBm, Antenna Gain 9 dBi, Cable Loss 20 dB
- 2. The relation: EIRP(dBm) = P_tx(dBm) + G_ant(dBi) − L_cable(dB)
- 3. Substitute: Ptx = 0 dBm, G = 9 dBi, L = 20 dB
- 4. Result: EIRP = -11 dBm
Worked examples
Minimum range
Given: TX Power -4.4 dBm, Antenna Gain 0.2 dBi, Cable Loss 0.3 dB
Result: EIRP = -4.5 dBm
Low range
Given: TX Power -44 dBm, Antenna Gain 2 dBi, Cable Loss 3 dB
Result: EIRP = -45 dBm
Typical
Given: TX Power 0 dBm, Antenna Gain 9 dBi, Cable Loss 20 dB
Result: EIRP = -11 dBm
High range
Given: TX Power 66 dBm, Antenna Gain 20 dBi, Cable Loss 60 dB
Result: EIRP = 26 dBm
Maximum range
Given: TX Power 660 dBm, Antenna Gain 200 dBi, Cable Loss 600 dB
Result: EIRP = 260 dBm
How EIRP changes with TX Power
| TX Power (input) | EIRP (result) |
|---|---|
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
| 0 dBm | -11 dBm |
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<p><a href="https://vbatpower.com/formulas/eirp-ham/">Effective Isotropic Radiated Power</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Effective Isotropic Radiated Power?
Use EIRP(dBm) = P_tx(dBm) + G_ant(dBi) − L_cable(dB). With TX Power 0 dBm, Antenna Gain 9 dBi, Cable Loss 20 dB, EIRP works out to -11 dBm.
What is the formula for Effective Isotropic Radiated Power?
The formula is EIRP(dBm) = P_tx(dBm) + G_ant(dBi) − L_cable(dB). Effective Isotropic Radiated Power (EIRP) is the total power that would need to be radiated by an isotropic antenna to produce the same signal strength in the direction of maximum antenna gain.
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