Effective Isotropic Radiated Power
Apparent power a transmitter would need if it radiated equally in all directions; combines transmit power, antenna gain and cable loss.
Formula
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Example
10 dBW tx, 15 dBi antenna, 2 dB loss: EIRP = 23 dBW
How it works
Effective isotropic radiated power (EIRP) tells you how strong a transmitter would appear if its energy were spread uniformly in every direction. It is found by starting with the transmitter output, adding the antenna’s directional gain in dBi, and subtracting losses in the feed line or cables, all in decibels so the terms combine by simple addition and subtraction.
When to use it
Use this when checking whether a radio link or transmitter meets regulatory power limits, coverage targets, or interference constraints. It is especially useful in avionics and RF system design when comparing different antenna and cabling choices on a common power basis.
Worked example, step by step
Given: P_t = 8 dBW G_t = 12 dBi L_c = 3 dB Step 1: Start with transmitter power P_t = 8 dBW Step 2: Add antenna gain 8 + 12 = 20 dBW Step 3: Subtract cable/feed loss 20 - 3 = 17 dBW EIRP = 17 dBW
Common mistakes to avoid
- Adding cable loss instead of subtracting it
- Using antenna gain in dB without confirming it is referenced to isotropic gain (dBi)
- Mixing watts and decibel units in the same calculation
Did You Know?
A directional antenna does not create energy — it concentrates it, so a modest transmitter behind a high-gain dish can produce an EIRP of thousands of watts along the beam.
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