How is EIRP calculated?

EIRP(dBm) = P_Tx(dBm) + G_antenna(dBi) - L_cable(dB) - L_connector(dB). Example: P_Tx = +30 dBm (1W), antenna gain = +12 dBi, cable loss = 2 dB, connector losses = 0.5 dB. EIRP = 30 + 12 - 2 - 0.5 = +39.5 dBm. In linear: EIRP(W) = P_Tx(W) * G_antenna(linear) = 1W * 15.85 = 15.85W (after accounting for cable/connector loss). EIRP is always specified in dBm (referenced to isotropic). The related metric ERP (Effective Radiated Power) uses a dipole reference instead of isotropic: ERP(dBm) = EIRP(dBm) - 2.15 dB. Some regulations use EIRP; others use ERP. Always check which reference is used.

What are the FCC EIRP limits?

FCC Part 15.247 (2.4 GHz ISM): max conducted power = 1W (+30 dBm), max EIRP = +36 dBm (4W) for point-to-multipoint. For point-to-point fixed links: for every 3 dB of antenna gain above 6 dBi, conducted power must decrease by 1 dB, allowing higher EIRP with directional antennas. FCC Part 15.407 (5 GHz UNII): UNII-1 (5.15-5.25 GHz): max EIRP = +36 dBm indoor only. UNII-3 (5.725-5.850 GHz): max EIRP = +53 dBm (200W) for PtP with directional antennas. FCC Part 96 (CBRS 3.5 GHz): max EIRP = +47 dBm for Category B devices (outdoor fixed). These limits are designed to prevent interference while maximizing spectrum utility.

How does EIRP relate to link budget?

The received power is: P_Rx = EIRP + G_Rx - FSPL - L_additional, where FSPL is free-space path loss and L_additional includes rain, atmospheric absorption, and multipath fading. For a link to close (work reliably), P_Rx must exceed the receiver sensitivity (MDS) by the required fade margin. Example: EIRP = +36 dBm, G_Rx = +12 dBi, FSPL at 2.4 GHz over 1 km = 100 dB. P_Rx = 36 + 12 - 100 = -52 dBm. Receiver sensitivity = -90 dBm. Fade margin = -52 - (-90) = 38 dB (excellent). Higher EIRP directly extends link range, but is limited by regulation and interference considerations.

Link Budget

EIRP (Effective Isotropic Radiated Power)

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EIRP = P_Tx + G_antenna(dBi) - L_cable. The power an isotropic antenna would need to match the actual system's peak radiation. 1W Tx + 12 dBi antenna = +42 dBm EIRP. Determines link range, interference footprint, and regulatory compliance. Related: ERP uses dipole reference (ERP = EIRP - 2.15 dB). FCC Part 15.247: max +36 dBm EIRP for 2.4 GHz PtMP.

Formula: P_Tx+G-L

ERP: = EIRP - 2.15 dB

FCC 2.4 GHz: +36 dBm max

Understanding EIRP

EIRP is the single most important number in a link budget. It captures both the transmitter power and antenna gain in one figure, representing the effective power radiated toward the receiver. Regulatory agencies limit EIRP to control interference between systems. Higher EIRP means longer range but also greater interference potential. Balancing EIRP against regulatory limits, antenna size, and PA cost is a core RF system design tradeoff.

EIRP Calculations

EIRP:
EIRP = P_Tx + G_ant − L_cable dBm
= P_Tx × G_ant / L_cable (linear)

Received power:
P_Rx = EIRP − FSPL + G_Rx

ERP (dipole ref):
ERP = EIRP − 2.15 dB

EIRP Limits by Regulation

Regulation	Band	Max EIRP	Condition
FCC Part 15.247	2.4 GHz ISM	+36 dBm (4W)	PtMP, FHSS/DSSS
FCC Part 15.407	5.8 GHz UNII-3	+53 dBm (200W)	PtP directional
FCC Part 96	3.5 GHz CBRS	+47 dBm	Cat B outdoor
ETSI EN 300 328	2.4 GHz	+20 dBm	Europe, general
FCC Part 30	28/39 GHz 5G	+75 dBm	Base station

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

System	EIRP	P_Tx	Antenna	Regulatory
WiFi AP	36 dBm	20 dBm	16 dBi	FCC Part 15
4G macro	63–68 dBm	43–46 dBm	18–21 dBi	Licensed
5G mmW	55–65 dBm	30–35 dBm	25–30 dBi	3GPP
Satellite (GEO)	50–60 dBW	10–20 dBW	35–45 dBi	ITU
Radar	90–100 dBW	60–70 dBW	30–40 dBi	Gov/Mil

Common Questions

Frequently Asked Questions

Calculation?

EIRP = P_Tx + G(dBi) - L_cable. 1W + 12 dBi - 2 dB cable = +40 dBm. In linear: EIRP(W) = P_Tx * G_linear. ERP = EIRP - 2.15 dB. Always check which reference (dBi vs. dBd) regulations use.

FCC limits?

2.4 GHz: +36 dBm PtMP. 5.8 GHz PtP: +53 dBm with directional antenna. CBRS 3.5 GHz: +47 dBm Cat B. Limits vary by band, application, and antenna type. PtP links get higher EIRP allowances.

Link budget?

P_Rx = EIRP + G_Rx - FSPL - L_misc. Fade margin = P_Rx - sensitivity. Need 10-20 dB FM for reliable link. Higher EIRP = longer range but more interference and stricter regulation.

Related Terms

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