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.
| Parameter | Low Gain | Medium Gain | High Gain |
|---|---|---|---|
| Gain Range | 2-6 dBi | 6-15 dBi | 15-45 dBi |
| Beamwidth | 60-360° | 15-60° | 1-15° |
| Typical Types | Dipole, monopole, patch | Yagi, helical, horn | Parabolic, array, Cassegrain |
| Bandwidth | Narrow to wide | Moderate | Narrow to moderate |
| Complexity | Low | Medium | High |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
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.