EIRP (Effective Isotropic Radiated Power) is the standard way to describe how strong a transmitter “looks” in the direction of maximum antenna gain. In satellite uplinks, EIRP is the number that ties together transmit power, antenna gain, and losses—and it’s a primary driver of whether the satellite can reliably receive your signal.
This guide explains what EIRP means, how to estimate it, and what usually limits uplink power in real ground stations and gateways.
Table of contents
- What Is EIRP?
- Why EIRP Matters for Uplinks
- How to Calculate EIRP
- EIRP vs Tx Power vs EPFD
- What Limits EIRP in the Real World
- Uplink Power Control and ACM
- How Frequency Band and Antenna Size Change EIRP
- EIRP and Licensing Compliance
- Best Practices for Setting Uplink EIRP
- EIRP FAQ
- Glossary
What Is EIRP?
EIRP is the effective radiated power you would need from an ideal isotropic antenna (one that radiates equally in all directions) to produce the same signal strength in the direction of your antenna’s main beam. Because real antennas concentrate power into a beam, EIRP can be far higher than the transmitter’s raw output power.
In satellite links, EIRP is usually expressed in dBW (decibels relative to one watt) or dBm (decibels relative to one milliwatt).
Why EIRP Matters for Uplinks
The satellite’s receiver does not “care” how you achieve your uplink strength—whether you use a bigger amplifier, a bigger dish, or lower losses. It cares about the signal level it receives. EIRP is how you express that uplink strength in a way that is comparable across different stations and hardware designs.
If your EIRP is too low, the satellite sees a weak carrier: you may get low throughput, high error rates, or loss of lock. If EIRP is too high, you can saturate satellite transponders, create adjacent-channel interference, or violate regulatory limits.
How to Calculate EIRP
At a practical level:
EIRP (dB) = Tx Power (dBW) + Antenna Gain (dBi) − Tx Losses (dB)
Where:
Tx Power: the RF output power at the amplifier (often at the HPA/SSPA output).
Antenna Gain: the gain of the antenna in the direction of peak radiation (main lobe).
Tx Losses: waveguide loss, filter loss, switch loss, radome loss, pointing loss, polarization mismatch, and any other losses between amplifier and free space.
In system design, you often compute EIRP at the point where the signal leaves the antenna, then compare it to required uplink performance in the link budget.
EIRP vs Tx Power vs EPFD
Tx power is just what the amplifier outputs. EIRP includes how well the antenna focuses that power and how much is lost in the transmit chain.
EPFD (Equivalent Power Flux Density) is a different concept used in coordination and interference protection, especially when considering how much energy reaches a victim receiver over an area or across angles. You can think of EPFD as an interference protection metric, while EIRP is the main engineering metric for how strong your uplink is in the intended direction.
What Limits EIRP in the Real World
In real ground stations, EIRP is limited by a mix of physics, equipment, regulations, and operational constraints:
Amplifier capability: Antenna size and efficiency: Waveguide and RF losses: Linearity and spectral regrowth: Thermal and power constraints: Pointing accuracy: Regulatory license limits: Satellite payload constraints:
For high-capacity gateways, the practical limit is often not “maximum power,” but “maximum clean power” while staying linear and compliant.
Uplink Power Control and ACM
Many modern networks don’t run a fixed uplink EIRP all the time. They use:
Uplink power control: ACM (Adaptive Coding and Modulation): Carrier management policies:
These tools let you meet availability targets with less overbuild, but they require strong monitoring and disciplined operations.
How Frequency Band and Antenna Size Change EIRP
Frequency affects antenna gain for a given physical size. Higher frequencies can produce higher gain with the same dish diameter, which can raise EIRP without changing amplifier power. This is one reason Ku/Ka systems can deliver high capacity using relatively compact antennas.
The tradeoff is that higher frequencies are more sensitive to pointing, surface accuracy, and weather. In practice, you can achieve impressive EIRP in Ku/Ka—but you must keep it stable through calibration and fade mitigation.
EIRP and Licensing Compliance
EIRP is frequently a regulated parameter. Licenses may specify:
Maximum EIRP: Power spectral density limits: Emission masks: Antenna pattern constraints:
Staying compliant often means operating below hardware maximums to preserve linearity, keep spurious emissions low, and ensure worst-case conditions still meet limits.
Best Practices for Setting Uplink EIRP
Strong uplink engineering is as much operational as it is technical:
Calibrate regularly: Measure at the right point: Account for backoff: Monitor spectrum continuously: Use envelopes: Coordinate with the satellite operator:
EIRP FAQ
Why can EIRP be much higher than amplifier power?
Because antenna gain concentrates the transmitted energy into a narrow beam. A 100 W amplifier (20 dBW) paired with a high-gain dish can produce EIRP tens of dB higher than the amplifier’s raw output.
Is more EIRP always better?
No. Excess EIRP can create interference, violate licensing limits, and reduce overall network performance by saturating payload components or increasing adjacent-channel noise. The goal is the minimum clean EIRP that meets performance targets with sufficient margin.
What’s the most common reason a station can’t reach its expected EIRP?
Hidden losses and operational issues: underestimated waveguide loss, mispointing, polarization mismatch, radome losses, or amplifier backoff requirements that weren’t included in early calculations.
How does rain fade relate to EIRP?
During rain fade (especially Ku/Ka), a network may increase uplink EIRP within licensed and hardware limits. If you can’t increase EIRP enough, the system must fall back to more robust modulation/coding or accept reduced throughput.
Glossary
EIRP: Effective Isotropic Radiated Power—Tx power plus antenna gain minus transmit-chain losses.
dBW / dBm: Logarithmic power units relative to 1 W (dBW) or 1 mW (dBm).
Antenna gain (dBi): How much an antenna focuses energy relative to an isotropic radiator.
Tx losses: Losses between the amplifier and free space (waveguide, filters, switches, radome, pointing, polarization).
HPA / SSPA: High Power Amplifier / Solid State Power Amplifier used to generate uplink RF power.
Backoff: Operating an amplifier below its maximum output to preserve linearity and reduce spectral regrowth.
Power spectral density (PSD): Power per unit bandwidth (e.g., dBW/MHz); often regulated for wideband carriers.
EPFD: Equivalent Power Flux Density—an interference protection metric used in coordination and regulatory contexts.
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