How to Calculate Satellite EIRP (Formula + Examples)

Satellite EIRP is a directional transmit quantity, not the power that reaches a receiver. It combines the power supplied to a transmitting antenna with that antenna's gain in a specified direction. The receiving end still has to account for free-space loss, atmospheric attenuation, polarization loss, receive antenna gain, and system noise.

This distinction matters in real link budgets. A BUC power rating is not terminal EIRP, a satellite beam's total EIRP is not automatically a single carrier's EIRP, and rain attenuation along the propagation path does not reduce the EIRP leaving the antenna.

Link Budget Guide | Antenna Types | Terminal Architecture

What EIRP Means

ITU Radio Regulations 2024, Article 1 No. 1.161 defines equivalent isotropically radiated power, or e.i.r.p., in a given direction as the product of the power supplied to the antenna and its gain relative to an isotropic antenna.

In logarithmic units:

EIRP(direction) = Pantenna + Gi(direction)                 dBW

• Pantenna is the carrier power supplied to the antenna, in dBW.
• Gi(direction) is antenna gain in the direction being evaluated, in dBi.

The direction is part of the definition. Boresight EIRP uses boresight gain. Off-axis EIRP uses gain at the specified off-axis angle. If pointing error is represented as a separate loss, start with boresight gain and subtract the pointing loss once; do not also use an already-degraded directional gain.

Choose and State the Power Reference Plane

Power is often known at the amplifier output rather than at the antenna input. In that case:

Pantenna = Pamplifier - Lbetween                         dBW
EIRP     = Pamplifier - Lbetween + Gi(direction)         dBW

Lbetween includes only losses between those two reference planes, such as the relevant cable, waveguide, filter, diplexer, connector, or rotary-joint loss. Use a measured loss at the operating frequency when available. A generic loss per metre is not a substitute for the installed RF path.

Also distinguish a BUC or HPA's rated maximum power from the actual carrier power at the chosen plane. Operating back-off, multiple carriers, gain settings, temperature, and compression can make those values different.

Convert Watts, dBW, and dBm

P(dBW) = 10 log10(Pwatts)
P(dBm) = P(dBW) + 30

For example, 4 W is 10 log10(4) = 6.02 dBW. Gain and loss in dB can then be added to or subtracted from that dBW value.

Reproducible 1.2 m Uplink Example

The following is a hypothetical design calculation, not a product specification or an authorization to transmit. It uses an antenna model included in ITU-R S.1712-0, whose Recommendation has a specific 13.75–14 GHz scope. The example frequency therefore stays inside that band.

Inputs and Boundary Conditions

Use the exact SI value c = 299,792,458 m/s from the BIPM SI Brochure, Section 2.2 and the model:

λ  = c / f
Gi = 10 log10[η(πD/λ)²]

λ  = 299,792,458 / 13,875,000,000
   = 0.0216067 m

Gi = 10 log10[0.65 × (π × 1.20 / 0.0216067)²]
   = 42.964 dBi

The directional EIRP under those assumptions is:

EIRP = 6.0206 + 42.9640 - 0.30 - 0.20
     = 48.4846 dBW
     ≈ 48.5 dBW

The result is reproducible, but its accuracy is limited by the inputs. A commissioning calculation should replace model efficiency, assumed path loss, assumed pointing loss, and nominal power with manufacturer data and measurements referenced to the same planes. Operator access limits and national licensing conditions remain separate checks.

What Changes EIRP?

Antenna Diameter, Frequency, and Efficiency

For the aperture model above, and only when frequency and efficiency remain unchanged, the gain change caused by diameter is:

ΔG = 20 log10(D2 / D1)                                  dB

Moving from 1.2 m to 1.8 m gives 20 log10(1.8/1.2) = 3.52 dB. Moving from 1.2 m to 2.4 m gives 6.02 dB. A larger dish also narrows the main beam, so installation and tracking requirements must be checked rather than inferred from gain alone.

The aperture formula also predicts higher gain at a higher frequency for unchanged diameter and efficiency. Real antennas may not preserve efficiency, surface accuracy, feed performance, radome loss, or pointing performance across bands, so a universal Ku-to-Ka gain increment is not valid.

Actual Carrier Power

If the carrier power at the same reference plane rises from 4 W to 16 W, the power term rises by 10 log10(16/4) = 6.02 dB. This does not mean a nominal 16 W amplifier always supplies 16 W of linear carrier power. Confirm rated-power definitions, back-off, compression, and multi-carrier operation.

Losses Before Radiation and Directional Gain

Cable, waveguide, filters, connectors, and other components between the power reference plane and antenna subtract from EIRP. Pointing error, platform motion, structural deformation, blockage, and a wet radome can change the gain or loss seen in the intended direction. These effects need site- and equipment-specific evidence; fixed generic penalties create false precision.

Rain Along the Path Is Not an EIRP Term

Atmospheric rain attenuation occurs after the wave leaves the transmit antenna. It belongs in the propagation section of the link budget, not in the transmitter EIRP definition. ITU-R P.618-14 provides Earth-space propagation prediction methods; the required fade margin depends on frequency, geometry, polarization, location, and target time percentage.

A wet feed or radome can reduce actual radiated performance before the free-space path begins. That equipment effect is distinct from atmospheric path attenuation. Uplink power control may deliberately raise transmitted EIRP during fading, but the fade itself is still a separate path loss.

Uplink EIRP, Downlink Contours, and Per-Carrier Power

On an uplink, terminal EIRP describes the earth station's transmission toward the satellite. On a downlink, a coverage map may show spacecraft EIRP across a beam. Before using either value, confirm what the operator means:

• boresight, local contour, or edge-of-coverage value;
• total beam or transponder EIRP versus a particular carrier allocation;
• saturated, backed-off, nominal, minimum, or measured performance;
• co-polar or cross-polar direction and the stated reference bandwidth;
• clear-sky value and any operational tolerances.

Do not assign a transponder's saturated total EIRP to one carrier in a multi-carrier plan. Per-carrier power depends on the operating point and allocation method.

Hypothetical Beam-Contour Example

Assume a map defines 58 dBW at beam center and 54 dBW at a site, for the same carrier and operating condition. The site is 4 dB below the center:

ΔEIRP = 54 - 58 = -4 dB

If path loss, receive G/T, polarization, interference, bandwidth, and all other noise terms are unchanged, received carrier power and C/N0 are also 4 dB lower. That arithmetic alone does not identify a MODCOD or throughput reduction. Those outcomes require the modem's thresholds, implementation margin, occupied bandwidth, traffic policy, and the complete link budget.

Contour spacing is also a map-specific publishing choice. Read the legend instead of assuming a universal 1 dB or 2 dB interval. Terminal sizing should use the EIRP specified for the actual service location and availability objective, not a blanket rule that every design uses an edge-of-coverage value.

EIRP Density and Off-Axis Limits

EIRP density adds a bandwidth reference to EIRP. For a simplified carrier whose average power is uniformly distributed across an explicitly defined bandwidth B in hertz:

EIRP density = carrier EIRP - 10 log10(B)                dBW/Hz

Using the 48.4846 dBW example and an assumed 2,000,000 Hz uniform reference bandwidth:

10 log10(2,000,000) = 63.0103 dB-Hz
EIRP density        = 48.4846 - 63.0103
                    = -14.5257 dBW/Hz
                    ≈ -14.5 dBW/Hz

This is a normalization example, not a spectral mask assessment. A real modulated signal is not necessarily flat, and a rule or operator may specify another reference bandwidth, detector, averaging method, antenna angle, or power definition.

Regulatory limits are service- and band-specific. For example, ITU-R S.524-9 addresses maximum permissible off-axis e.i.r.p. density from earth stations in geostationary-satellite-orbit fixed-satellite-service networks in specified parts of the 6, 13, 14, and 30 GHz bands. It is not a universal mask for every satellite terminal.

ITU-R S.465-6 provides a reference FSS earth-station radiation pattern from 2 to 31 GHz for coordination and interference assessment. A reference pattern is not proof that an installed antenna complies. Use the applicable Radio Regulations, national authorization, coordination agreement, satellite operator access plan, and measured antenna performance for the specific network.

EIRP vs ERP, Antenna Gain, and G/T

ITU Radio Regulations Article 1 Nos. 1.161–1.162 uses an isotropic reference for EIRP and a half-wave dipole reference for ERP. For the same direction and power:

Gi(dBi) = Gd(dBd) + 2.15 dB
EIRP    = ERP + 2.15 dB

Antenna gain alone omits transmit power. G/T describes the receive side and cannot replace EIRP. In a link calculation, EIRP, propagation losses, receive G/T, and bandwidth-related terms combine to determine received performance; see the reproducible link budget workflow.

Commissioning Checklist

Before accepting an EIRP value, record:

1. Frequency, polarization, carrier bandwidth, and operating mode.
2. Whether the value is total, per carrier, or a density with a stated reference bandwidth.
3. The power measurement plane and whether the figure is rated, set, or measured power.
4. Every loss between that plane and the antenna input, measured at the operating frequency where practical.
5. Antenna gain source, frequency, polarization, and direction.
6. Pointing or tracking loss and whether it is already included in directional gain.
7. Amplifier back-off, compression, multi-carrier loading, and control-loop state.
8. The applicable operator access plan, coordination conditions, and national authorization.
9. Calibration dates, measurement uncertainty, and environmental state.

Common Calculation Errors

1. Calling amplifier watts EIRP. Convert actual carrier power to dBW, establish the reference plane, then combine it with directional gain and intervening loss.
2. Subtracting a loss twice. Do not subtract pointing loss if the gain figure already represents the actual direction; do not subtract feed loss from power already measured at the antenna input.
3. Mixing dBm and dBW. A 30 dB unit offset becomes a 30 dB EIRP error.
4. Using theoretical dish gain as certified gain. Aperture efficiency is an assumption until supported by the antenna's validated data or measurement.
5. Treating rain-path loss as transmitter EIRP loss. Keep propagation attenuation separate from the radiated-power reference plane.
6. Using total beam or saturated transponder EIRP for one carrier. Confirm allocation, back-off, and map definitions.
7. Applying one off-axis mask to every service and band. Check the exact rule, scope, authorization, and operator plan.

Frequently Asked Questions

What EIRP should a VSAT terminal have?

There is no universal good or typical value that can size a terminal responsibly. The required value comes from the satellite operator's access plan and a link budget using the carrier, bandwidth, satellite receive performance, propagation objective, antenna pattern, interference constraints, and regulatory authorization.

Is higher EIRP always better?

No. More EIRP can improve a link only when another limit does not dominate. Excessive or incorrectly directed power can violate operator or regulatory constraints and cause interference. Spacecraft power, amplifier linearity, thermal limits, spectral density, and adjacent-system coordination also constrain operation.

Can a larger antenna increase EIRP?

Yes, if actual gain in the intended direction increases and the other terms remain unchanged. Under the same-frequency, same-efficiency aperture model, the gain difference is 20 log10(D2/D1). Larger antennas generally have narrower beams and may impose stricter pointing and structural requirements.

How does rain affect EIRP?

Rain in the atmosphere attenuates the propagation path; it does not redefine the EIRP leaving the antenna. Wet hardware before radiation and rain-induced pointing or tracking effects can change actual radiated performance. An uplink power-control system can also change EIRP intentionally during a fade.

How do I read a satellite EIRP map?

Locate the service position, read the contour value and legend, then verify whether the value is total or per carrier, saturated or backed off, nominal or minimum, and tied to a specific polarization or operating condition. Use the site value in the complete downlink budget; do not infer throughput from the contour alone.

Key Takeaways

• EIRP is power supplied to the antenna combined with gain in a specified direction.
• When starting at an amplifier flange, subtract only the losses before the antenna reference plane.
• State whether an EIRP value is boresight, off-axis, total, per carrier, saturated, backed off, or bandwidth-normalized.
• Keep atmospheric attenuation separate from transmitter EIRP.
• Use explicit inputs and reference planes; do not rely on generic terminal ranges or fixed loss assumptions.
• Treat standards, operator limits, and national authorizations according to their exact service, band, and network scope.

Related Articles

• Satellite Link Budget Calculation — Carry EIRP through free-space loss, receive G/T, and noise calculations
• Satellite Antenna Types Guide — Compare the antenna technologies that determine directional gain
• Satellite Terminal Architecture — Locate the modem, BUC, RF path, feed, and antenna reference planes
• Satellite Frequency Bands Explained — Understand band definitions without assuming identical hardware performance
• HTS Spot Beams and Beamforming Explained — Interpret spot-beam coverage and beam allocation
• Adaptive Coding and Modulation — Relate complete link quality to mode-selection thresholds
• Satellite Interference Explained — Review adjacent-system and cross-polar interference mechanisms