RFEIRP vs ERP: Understanding the Difference in LoRaWAN
This page compares Effective Isotropic Radiated Power (EIRP) and Effective Radiated Power (ERP), highlighting their differences and uses, particularly within the context of LoRaWAN. We’ll cover their purpose and the relationship between them.
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.
