EIRP vs ERP
EIRP (Effective Isotropic Radiated Power) and ERP (Effective Radiated Power) both quantify the apparent transmit power in a given direction, but they reference different antennas. EIRP references an isotropic antenna; ERP references a half-wave dipole. The 2.15 dB difference between them causes confusion in regulatory filings and link budgets.
EIRP (Effective Isotropic Radiated Power)
EIRP is the power that a theoretical isotropic antenna would need to radiate to produce the same field strength as the actual antenna in its direction of maximum radiation. EIRP (dBm) = P_tx (dBm) + G_antenna (dBi) − Cable losses (dB).
Advantages
- Universal reference — isotropic antenna is a well-defined theoretical standard
- Used in international regulations (ITU, 3GPP, WiFi)
- Directly used in Friis link budget equation
- No ambiguity — the reference is unambiguous
Disadvantages
- The isotropic reference is theoretical — no real antenna has 0 dBi gain
- Requires knowing antenna gain in dBi
- Not always used in legacy US FCC regulations (which use ERP)
When to use
Use EIRP for satellite communications, cellular link budgets, WiFi (IEEE 802.11 regulatory limits are in EIRP), and international regulatory compliance. The Friis equation uses EIRP.
ERP (Effective Radiated Power)
ERP is the power that a half-wave dipole would need to radiate to produce the same field strength as the actual antenna. ERP = EIRP − 2.15 dB. ERP is used in FCC rules for AM/FM broadcasting and some legacy Part 15 regulations.
Advantages
- Used in US FCC broadcast regulations — required for AM/FM/TV filings
- Half-wave dipole is a practical, realizable reference antenna
- Legacy systems and older regulatory documents use ERP
Disadvantages
- Ambiguous: sometimes confused with EIRP in documentation
- The 2.15 dB offset from EIRP causes errors in link budgets if mixed
- Less used in modern wireless standards (which use EIRP)
- Requires knowing antenna gain in dBd instead of dBi
When to use
Use ERP when filing with the FCC for broadcast (AM, FM, TV), and when working with legacy Part 15 devices that specify ERP limits. Convert to EIRP for modern wireless link budgets.
Key Differences
- ▸EIRP references an isotropic antenna (0 dBi); ERP references a half-wave dipole (2.15 dBi)
- ▸ERP = EIRP − 2.15 dB (dipole has 2.15 dBi gain over isotropic)
- ▸WiFi, cellular, and satellite regulations use EIRP; FCC broadcast uses ERP
- ▸Friis transmission equation uses EIRP directly
- ▸Mixing EIRP and ERP without conversion introduces a 2.15 dB error in link budgets
Summary
EIRP and ERP differ by exactly 2.15 dB — the gain of a half-wave dipole over an isotropic antenna. Use EIRP for modern wireless systems (WiFi, cellular, satellite, link budgets). Use ERP when filing FCC broadcast applications or referencing legacy Part 15 limits. Always clarify which reference is used to avoid 2.15 dB errors.
Frequently Asked Questions
What is the difference between EIRP and ERP?
EIRP = ERP + 2.15 dB. ERP references a half-wave dipole (2.15 dBi); EIRP references a lossless isotropic antenna. Both describe the same physical radiation — just measured against different reference antennas.
Which do WiFi regulations use — EIRP or ERP?
WiFi regulations use EIRP. For example, the FCC limits 2.4 GHz WiFi to 30 dBm EIRP for point-to-multipoint and 36 dBm EIRP for point-to-point. ETSI (Europe) uses similar EIRP limits. Always verify with current regulatory documents.
How do I convert ERP to EIRP?
EIRP (dBm) = ERP (dBm) + 2.15 dB. Alternatively, EIRP (W) = ERP (W) × 1.64 (the numeric gain of a half-wave dipole over isotropic). This conversion is exact and defined in IEEE standards.
Why does the FCC use ERP for broadcast?
The FCC adopted ERP for broadcast regulations (AM, FM, TV) decades ago when half-wave dipoles were the reference antennas used in the field. Modern wireless standards shifted to EIRP for universality. Both systems coexist, which is why engineers must be careful to check which reference applies.