What is the difference between ERP and EIRP

Effective Isotropic Radiated Power (EIRP) and Effective Radiated Power (ERP) are antenna performance metrics used to measure the effective transmitted power of wireless communication systems. EIRP references an ideal isotropic antenna, while ERP references a half wave dipole antenna. Understanding the difference between EIRP and ERP is essential for RF system design, regulatory compliance, antenna selection and wireless network planning. This page explains purpose of EIRP and ERP terms particularly within the context of LoRaWAN.

EIRP (Effective Isotropic Radiated Power)

EIRP represents the total power radiated by a hypothetical isotropic antenna in a single direction. Think of it as the amount of power that would be radiated if the antenna radiated equally in all directions (which isn’t physically possible).

EIRP can be calculated as follows, and is typically expressed in dBm:

Where:

• Transmit power is in dBm
• Antenna gain is in dBi
• Cable loss is in dB

Example (EIRP Calculation):

Let’s say we have:

• TxP (Transmit Power) = +20 dBm
• L (Cable Loss) = -5 dB
• Gt (Antenna Gain) = +10 dBi

Then, EIRP in dBm = +20 - 5 + 10 = 25 dBm

ERP (Effective Radiated Power)

ERP represents the total power radiated by an actual antenna relative to a half-wave dipole antenna, rather than a theoretical isotropic antenna. So, instead of comparing to a perfect, omnidirectional radiator, we’re comparing to a standard, real-world antenna.

The ERP calculation is similar to EIRP, but with one key difference: the antenna gain is expressed in dBd (decibels relative to a dipole) rather than dBi (decibels relative to an isotropic antenna).

Example (ERP Calculation):

Let’s say we have:

• TxP (Transmit Power) = +20 dBm
• L (Cable Loss) = -5 dB
• Gt (Antenna Gain) = +7.85 dBd

Then, ERP in dBm = +20 - 5 + 7.85 = 22.85 dBm

Relation between EIRP and ERP

The following expression depicts the relationship between EIRP and ERP:

As shown, the EIRP value is approximately 2.15 dB greater than the ERP value. This difference stems from the gain difference between an isotropic antenna and a dipole antenna.

Purpose of EIRP and ERP

Both EIRP and ERP serve as crucial metrics for electromagnetic (EM) or radio signal transmitting devices.

• RF transmitting systems must adhere to rules set by regulatory bodies like the FCC (in the US) and ETSI (in Europe).
• Radio devices must not exceed certain EIRP and ERP values established by these bodies. These limits ensure that devices operate within acceptable power levels, minimizing interference and complying with regulations.