EIRP
Effective Isotropic Radiated Power: Tx power plus antenna gain minus cable losses, representing total radiated power in the direction of maximum gain.
EIRP is the number that says how loud your transmitter actually is in its best direction, accounting for the antenna. Raw transmit power out of the amplifier isn't the whole story — by the time the signal reaches the antenna it's lost a bit in the feeder cable and connectors, then the antenna's gain focuses what remains. EIRP rolls all of that up: transmit power, minus cable/connector losses, plus antenna gain, expressed as the equivalent power an idealised isotropic (omnidirectional) antenna would need to radiate to match your peak.
It matters in two everyday contexts. For coverage, EIRP is the headline output term in the link budget — it's what your path loss eats into. For compliance, regulators cap EIRP per band to control interference and exposure, so a planner can't just bolt on a higher-gain antenna without checking the limit. The key distinction to keep straight: a 40 W transmitter into a high-gain antenna can have a far higher EIRP than a 40 W transmitter into a modest one. Power tells you what the amplifier produces; EIRP tells you what actually goes out the front toward the user.
Frequently asked questions
- What's the difference between EIRP and transmit power?
- Transmit power is what the power amplifier produces, measured at the radio. EIRP is the effective radiated power in the antenna’s direction of maximum gain, after you subtract feeder and connector losses and add the antenna gain. So two sites with identical transmit power can have very different EIRP if their antennas differ. EIRP is the figure that matters for coverage and for regulatory limits, because it reflects what actually leaves the antenna toward the user, not just what the amplifier generated.
- What's the difference between EIRP and ERP?
- They measure the same thing — radiated power in the peak direction — but against different reference antennas. EIRP is referenced to an isotropic antenna (a theoretical point that radiates equally in all directions). ERP is referenced to a half-wave dipole, which itself has about 2.15 dB of gain. So EIRP is always about 2.15 dB higher than ERP for the same setup. Cellular work and regulatory limits usually quote EIRP; older broadcast and some regulatory contexts use ERP, so it pays to check which reference a figure uses.
Related terms
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