ERP ⇄ EIRP
Radiated power against the dipole or isotropic reference — the 2.15 dB everyone flips.
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The engineering
Two reference antennas, one eternal 2.15 dB of confusion. EIRP compares your radiated power to a fictional isotropic radiator; ERP compares it to a real half-wave dipole, which itself has 2.15 dBi of gain. Same transmitter, same antenna — EIRP is always the bigger-sounding number, and regulatory paperwork mixes them freely enough to keep this card employed.
Which one you're bound by depends on the rulebook: FCC Part 15 unlicensed bands speak EIRP-equivalents (via conducted power + antenna gain), broadcast and land-mobile licenses speak ERP, and European ETSI rules lean EIRP. Antenna datasheets play the other side: dBd makes a gain figure look modest, dBi makes it look generous — check the suffix before you buy.
Where this math comes from
The half-wave dipole was the reference because it was the antenna — Hertz's 1887 radiator, and the thing every practical gain measurement was compared against for the first half-century of radio. The isotropic radiator can't be built, but as a mathematical reference it makes link-budget algebra clean, so both conventions survived.
The split hardened institutionally: broadcast regulation (FCC, established 1934) had written ERP into licenses long before satellite and microwave engineering standardized on EIRP for Friis-style budgets — leaving the 2.15 dB as a permanent translation toll between the two literatures.
- 1887Heinrich HertzThe dipole — radio's first and reference antenna.
- 1934FCCUS licensing regime; ERP becomes broadcast's yardstick.
- 1946Harald FriisIsotropic-referenced link budgets — EIRP's home turf.
See the full timeline of the math behind every calculator →
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