Also known as: ERP, EIRP, effective radiated power, effective isotropic radiated power
ERP (effective radiated power) and EIRP (effective isotropic radiated power) state how much power a transmitter appears to radiate in its strongest direction, folding the transmitter output, feedline loss, and antenna gain into a single number.1 The two differ only in their reference antenna: ERP is referenced to a half-wave dipole (gain in dBd), while EIRP is referenced to an ideal isotropic radiator (gain in dBi). Because a lossless dipole already has 2.15 dBi of gain over isotropic, EIRP = ERP + 2.15 dB for the same system.
How it works
Both quantities are built by adding decibels along the transmit chain. Start with the power delivered by the transmitter (say +40 dBm), subtract the loss of the feedline and connectors, then add the antenna gain:
- EIRP (dBm) = P_tx − L_feed + G (dBi)
- ERP (dBm) = P_tx − L_feed + G (dBd), with G(dBd) = G(dBi) − 2.15
The key idea is that antenna gain is not amplification — a passive antenna radiates no more total power than it receives. Gain describes directivity: energy is concentrated into a narrower beam, so along the main lobe the field is stronger than an omnidirectional reference would produce. ERP and EIRP capture exactly that peak-direction equivalent power. Away from the main lobe the radiated power is lower, which is why a high-EIRP directional link can be invisible off to the side.
Because the reference antennas differ by a fixed 2.15 dB, the two scales are trivially converted. Broadcasters and land-mobile regulators traditionally quote ERP (dipole reference is natural for VHF/UHF whip and dipole systems); satellite, microwave, and radar engineers quote EIRP (isotropic reference is cleaner for link budgets and antenna-theory math).
In practice
Regulators cap EIRP or ERP, not raw transmitter power, because what matters for interference and exposure is the field actually radiated. A Wi-Fi rule of “36 dBm EIRP” lets you trade a bigger antenna against a smaller amplifier as long as the product stays under the limit. A broadcast license of “50 kW ERP” fixes the effective coverage regardless of how much of that comes from transmitter power versus antenna gain. This is also why EIRP is the natural starting point for a link budget: it is precisely the term you plug in before subtracting free-space path loss on the way to the receiver.
Relevance to SDR
ERP and EIRP are transmit-side concepts, so a receive-only SDR never radiates them — but they govern the signals it hears. The EIRP of a trunking control channel, a broadcast tower, an ADS-B transponder, or a satellite downlink sets how strong that signal arrives after path loss, and hence whether it clears your noise floor and decodes. Estimating a distant site’s EIRP (published ERP for licensed land-mobile and broadcast transmitters is often on record) lets you sanity-check expected receive levels and coverage.
GopherTrunk is a receiver and does not transmit, so it has no ERP/EIRP of its own. The concept still matters for planning: a site whose EIRP and distance imply a weak arriving signal will need a better antenna, low-noise amplifier, or antenna gain on the receive side to decode reliably.
Sources
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Effective radiated power — Wikipedia, definitions of ERP and EIRP, dipole vs isotropic references, and the 2.15 dB relationship. ↩