What is the difference between EIRP and ERP and when do I use each one in a link budget?
EIRP and ERP
EIRP represents the total power that would need to be radiated by an isotropic antenna to produce the same signal strength in the direction of the main beam. It is the product of the transmitter output power and the antenna gain: EIRP (dBm) = Pt (dBm) + Gt (dBi). This is the standard metric for determining how much signal arrives at a distant receiver.
ERP uses the half-wave dipole as the reference: ERP (dBm) = Pt (dBm) + Gt (dBd), where dBd is the gain relative to a dipole. Since a dipole has 2.15 dBi of gain: dBd = dBi - 2.15 dB. ERP is commonly used in broadcast engineering where dipole antennas are the standard reference.
Using the wrong reference in a link budget introduces a 2.15 dB error, which is significant for many systems. Always explicitly state whether gains are in dBi or dBd and whether power is EIRP or ERP. Some regulatory limits are specified as ERP (particularly for broadcast), while satellite and wireless standards typically use EIRP.
Frequently Asked Questions
How do I include cable and connector losses?
EIRP = Pt - cable_loss - connector_loss + Gt. The cable and connector losses reduce the power reaching the antenna, so they subtract from the transmitter output before adding the antenna gain. Always account for all losses between the transmitter output and the antenna input.
What about EIRP for a phased array?
For a phased array: EIRP = P_per_element + 10·log10(N) + G_element, where N is the number of elements and G_element is the element gain. The 10·log10(N) term accounts for the coherent power combining. A 64-element array with +10 dBm per element and 5 dBi element gain: EIRP = 10 + 18 + 5 = 33 dBm (2W EIRP).
When is the distinction critical?
When the link budget margin is tight (<3 dB) or when comparing specifications from different sources. A 2.15 dB error can mean the difference between meeting and missing a link budget requirement, especially for long-range satellite and microwave links.