Free Effective Radiated Power (ERP) and EIRP Calculator with FCC Compliance, Field Strength, and Complete Station Budget

ERP: Referenced to the Half-Wave Dipole

Effective Radiated Power (ERP) is defined by IEEE Standard 149-1979 as the power that would have to be radiated by a half-wave dipole antenna to produce the same signal strength in the direction of maximum radiation as the actual antenna. If your 100-watt transmitter feeds a 3-element Yagi with 7.1 dBd gain and your feedline loses 1.7 dB, the ERP in the beam direction is: ERP(W) = 100W times 10^(7.1/10) / 10^(1.7/10) = 100 times 5.129 / 1.479 = 347W ERP. That means your 100-watt radio station is effectively as loud in the beam direction as a 347-watt station using a dipole. ERP is the standard used for most US broadcast services, GMRS, FRS, and the amateur 60-meter band power limits.

The formula is: ERP(dBW) = P_TPO(dBW) + G_amp(dB) – L_feedline(dB) – L_other(dB) + G_antenna(dBd). Because the reference is a half-wave dipole, antenna gain must be expressed in dBd (decibels relative to dipole). A half-wave dipole has exactly 0 dBd gain, 2.15 dBi gain, and 1.64 linear gain relative to an isotropic radiator. These relationships are exact and defined in IEEE 149-1979. The FCC uses ERP in virtually all its broadcast service rules, in FRS and GMRS power limits, and in the new amateur 60-meter band rules effective February 13, 2026.

EIRP: Referenced to the Isotropic Radiator

Effective Isotropic Radiated Power (EIRP) is referenced to a theoretical isotropic radiator, a point source that radiates equally in all directions. Because an isotropic antenna has 1.64 times less effective aperture than a half-wave dipole in any given direction, EIRP is always exactly 2.15 dB higher than ERP for the same station. The formula is: EIRP(dBW) = ERP(dBW) + 2.15, or in watts: EIRP(W) = ERP(W) times 1.64. EIRP is the standard used in mobile communications (4G LTE, 5G NR), satellite services, the amateur 2200m and 630m bands, and Part 15 unlicensed device rules including WiFi and Bluetooth. When you see an EIRP limit in FCC rules and your antenna spec says dBi, you can use the antenna gain directly without adding 2.15 dB.

The 2.15 dB relationship between ERP and EIRP is derived from the gain of a lossless half-wave dipole relative to an isotropic radiator: 1.64 linear, 2.15 dB. This is a fixed physical constant of antenna physics, not an approximation. The FCC’s own rules at 47 CFR 97.313 state explicitly that EIRP equals ERP multiplied by 1.64 when discussing the 630m and 2200m amateur band limits.

The New 60-Meter Rules: 9.15W ERP Effective February 13, 2026

FCC Report and Order 25-60, released December 9, 2025, expanded US amateur radio privileges on the 60-meter band significantly. Under the new rules effective February 13, 2026, US amateurs with a General class or higher license now have access to four discrete channels (5330.5, 5346.5, 5371.5, and 5403.5 kHz) at 100 watts ERP, plus a new 15-kHz wide segment from 5351.5 to 5366.5 kHz at a maximum of 9.15 watts ERP (equivalent to 15 watts EIRP per WRC-15). The old channel centered on 5358.5 kHz that many operators used for FT8 now falls within the new 15-kHz segment and is therefore subject to the reduced 9.15W ERP limit.

This ERP limit means that high-gain antennas on 60 meters work against the operator. A 100-watt transmitter into a 3-element Yagi with 7.1 dBd gain and 1 dB feedline loss produces 347W ERP, far above the 9.15W limit for the new segment. To stay within 9.15W ERP with a 3-element Yagi and 1 dB line loss, your transmitter output must be limited to: TPO = 9.15W times 10^(1/10) / 10^(7.1/10) = 9.15 times 1.259 / 5.129 = 2.25 watts. This calculator’s inverse function computes exactly this, and the compliance check flags violations with the specific regulation reference.

The ARRL 60-meter FAQ provides the authoritative US amateur operating guidance for these new rules. The old channel at 5371.5 kHz (Channel 3) that previously overlapped with FT8 operations has been renumbered to accommodate the new band plan. Per the FCC rules, General class license holders (and higher) may operate on these allocations.

Field Strength: Connecting ERP to Real-World Coverage

ERP and EIRP are transmitter-side metrics. The metric that determines actual coverage is field strength at the receiver location, expressed in microvolts per meter or millivolts per meter. The relationship between ERP and field strength in free space follows from electromagnetic theory: E(V/m) = square root of (30 times ERP_W) divided by d_meters, where d is the distance from the transmitting antenna. This formula, referenced in ITU-R Recommendation P.525 and the ARRL Antenna Book, assumes free-space propagation with no ground reflections. Real-world propagation adds terrain effects, atmospheric absorption, and multipath, but the free-space formula provides the upper bound on signal strength at any given distance for planning purposes.

For US FM broadcast stations, the FCC FM service contour calculator uses the F(50,50) and F(50,90) propagation curves from 47 CFR 73.313 rather than the free-space formula, because FM broadcasts operate near the ground where terrain effects are dominant. The free-space formula in this calculator is most accurate for elevated antenna systems, VHF/UHF links, and first-order coverage planning. For ARES/RACES communications planning and repeater coverage estimation, the free-space field strength result combined with a terrain analysis tool gives a practical coverage estimate.

Stage 1: Transmitter Power Output (TPO)

The starting point is the transmitter’s rated power output in watts. For amateur radio, this is the peak envelope power (PEP) at the transceiver output connector. For commercial FM broadcast, it is the transmitter output power after internal filtering. If an external linear amplifier is used, its gain in dB is added first. All internal arithmetic uses decibels with high-precision logarithmic conversion: P_dBW = 10 times log10(P_watts).

Stage 2: Feedline Loss

Feedline loss can be entered as a known dB value, or calculated from the cable type using the K1/K2 formula (Loss_dBper100ft = K1 times sqrt(f_MHz) plus K2 times f_MHz) with manufacturer K1/K2 constants for 8 common US cable types. The detailed coaxial cable comparison tool (Tool 4 in this hub) provides a full 18-cable database with SWR mismatch, duty cycle, and temperature derating for stations that need precise feedline analysis.

Stage 3: Additional Loss Stages

Duplexers used in repeater systems introduce 0.5 to 2 dB of insertion loss depending on the frequency separation between transmit and receive paths. Lightning arresters add typically 0.1 to 0.3 dB. Combiners used when sharing an antenna between multiple transmitters introduce 3 dB for a 2-way split plus any additional combiner insertion loss. Connectors add approximately 0.05 to 0.15 dB per pair depending on type. All stages are summed as: Total_Loss = feedline_dB + duplexer_dB + arrester_dB + combiner_dB + connector_loss_dB.

Stage 4: Power at Antenna Input

The power reaching the antenna input connector equals the transmitter output minus all losses (plus amplifier gain if used): P_antenna = TPO_W times 10^(amp_gain/10) times 10^(-total_loss/10). This is the actual power fed into the antenna’s feedpoint, which is a useful intermediate result for verifying that the antenna is not being overdriven beyond its power handling specification.

Stage 5: ERP via Antenna Gain in dBd

ERP is computed by multiplying the power at the antenna input by the antenna’s gain relative to a half-wave dipole: ERP_W = P_antenna_W times 10^(G_dBd/10). If the antenna gain is specified in dBi, the calculator automatically subtracts 2.15 dB to convert to dBd before the calculation. The 15 antenna preset buttons provide standard gains for common ham radio and commercial antenna types. Custom gain values can be entered directly in either dBd or dBi.

Stage 6: EIRP and Field Strength

EIRP is always ERP times 1.64 (equivalently, ERP_dBW plus 2.15 dB). This is a fixed relationship independent of frequency, antenna type, or any other parameter. Field strength at the entered distance is computed using the IEEE 149-1979 and ITU-R P.525 formula: E(V/m) = square root of (30 times ERP_W) divided by d(m). Results are shown in mV/m, µV/m, and dBµV/m simultaneously. The inverse calculator reverses stages 4 and 5 to compute the required TPO for a target ERP, given antenna gain and total system loss.