EIRP Interactive Calculator

When you design a wireless system, you have to balance transmitter power, antenna gain, and cable loss. These three factors set your actual signal reach—and whether your setup will pass regulatory checks. This EIRP calculator lets you figure out Effective Isotropic Radiated Power using whatever you have: transmitter power (in dBm or Watts), antenna gain (in dBi or dBd), and the losses from cables and connectors. Sorting out EIRP is essential for satellite link budgets, long-range wireless bridges, and making sure you don’t accidentally break FCC/ETSI rules. On this page, you’ll get the formula, a worked example, the context behind each term, and a practical FAQ.

What is EIRP?

EIRP (Effective Isotropic Radiated Power) is just a way of saying: “What would the power output of an ideal, perfectly even, in-all-directions antenna need to be to match my setup in its strongest direction?” It combines your transmitter’s output, whatever losses occur before the antenna, and the boost you get from the antenna itself into a single number.

Simple Explanation

EIRP is like the difference between a bare bulb and a flashlight—the bulb’s light goes everywhere, but the flashlight is focused and looks brighter in the direction you care about. EIRP tells you how “bright” your focused beam is compared to that all-directions bulb. Turn up your antenna gain, and you get more EIRP, even if your base transmitter power doesn’t change.

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How to Use This Calculator

1. Select your calculation mode — calculate EIRP, required transmitter power, required antenna gain, or maximum allowable cable loss.
2. Enter your transmitter power, antenna gain, and cable/connector loss values, and choose the appropriate units for each field.
3. If solving for a target EIRP (modes 2–4), enter the target EIRP value and unit.
4. Click Calculate to see your result.

System Diagram

EIRP Calculator

Equations & Formulas

To get EIRP, you combine transmitter power, subtract total cable/connector loss, and add the antenna gain. Here’s the math for the various unit systems:

Simple Example

Transmitter power: 30 dBm (1 W)
Antenna gain: 15 dBi
Cable loss: 2 dB
EIRP = 30 − 2 + 15 = 43 dBm (20 W)

Theory & Practical Applications

EIRP is the best way to describe what an antenna system is really *radiating* in its best direction, after taking into account losses and where power is being focused. Unlike just quoting transmitter specs, EIRP reflects what gets through the whole RF path—transmitter, cables, connectors, then antenna. This matters because regulations and system designers both care about radiated field strength, not just what comes out of your PA.

Physical Interpretation and Isotropic Reference

An isotropic radiator is a made-up idea—a point that sends out energy equally in every direction. Real antennas don’t do this: they focus power in certain directions, and the gain in dBi tells you, “How much stronger is the field in the main direction, compared to isotropic?” For instance, 15 dBi means the antenna creates 31.6 times more power density forward than an isotropic source with the same input power.

EIRP is useful because you can compare two very different setups: for example, a small transmitter with a big dish, and a big transmitter with a simple whip, might actually produce the same field strength in their main direction if the EIRP works out. The formula lets you swap between transmitter power, cable loss, and antenna gain depending on what's easy, legal, or cost-effective in your application.

Cable Loss Mechanisms and Frequency Dependence

Cable loss often gets ignored at first but is a major culprit in signal problems. Loss goes up with frequency: cheap RG-58 loses about 0.8 dB/m at 1 GHz but twice as much at 4 GHz. A 10-meter run at 2.4 GHz on mid-grade coax can dump three-quarters of your power before it even gets to the antenna. Connectors add more loss—each one, even when new, will typically add 0.1–0.3 dB, and old or corroded connectors can get much worse. Five connector pairs can easily mean 2 or 3 dB lost, especially outside. That’s why pro installs use short runs, big low-loss cable, and as few connectors as possible.

Don't overlook cable and connector condition. Water and salt just eat connectors, and losses will creep up without warning. The effect is much larger at high frequencies or long runs—always check a cable's datasheet for frequency, and leave room in your budget for loss to grow over time.

Antenna Gain and Pattern Considerations

Antenna gain is measured only in a specific direction (typically the main lobe axis). So, a directional Yagi that says 15 dBi forward might have -10 dBi to the rear—most energy is just redirected, not multiplied. The total radiated power is always less than or equal to the transmitter output, accounting for loss, regardless of gain; the pattern just changes. For high gain, you generally need a larger and more focused antenna (narrower beamwidth). For example, a 20 dBi dish may only have a 10–15° beam, but an omnidirectional antenna at 6 dBi is radiating power all around horizontally. Orientation errors (such as with mobile units) can kill gain, and the EIRP only applies in the direction with the stated gain. In practice, you may see much less in other directions, which is critical for moving devices.

Designers relying on optimal gain in mobile or outdoor settings often have to increase transmitter power or use more antennas to make up for bad alignments. High-end systems use smart antennas that steer the beam to hold onto gain even when things move.

Regulatory Framework and Compliance

Regulations (FCC, ETSI, etc.) are set using EIRP, not just transmit power. For instance: in the US 2.4 GHz ISM band, you can run 36 dBm (4W) EIRP if you're point-to-point, but only 30 dBm (1W) for omnidirectional or multipoint use. If you crank up both transmit power and antenna gain, you hit the EIRP wall fast—even if each alone looks “legal.” It's common for folks to get tripped up mixing up dBi (isotropic reference) and dBd (dipole reference): 12 dBd is really 14.15 dBi, not just 12. Always check which unit the rules use, or you might wind up breaking the law without knowing it—especially when mixing old and new datasheets.

Link Budget Integration and System Design

EIRP goes directly into your link budget. The classic formula is Friis: P_RX = EIRP + Receiver Gain – Path Loss – Misc Loss. Path loss in free space grows fast (20log₁₀ of distance/wavelength), but real environments can add a lot more loss (multipath, walls, rain, etc.). Boosting EIRP by 3 dB gives you roughly double the range, all else equal. For battery-powered or portable radios, higher EIRP eats up more current unless you’re clever with power control. Designers often make systems “adaptive,” ramping up transmit power only when the path gets tougher, to save life and heat.

Worked Example: Long-Range Wireless Bridge Design

Suppose you need a wireless bridge at 5.8 GHz, spanning 4.7 km, and the radio receiver is good down to -85 dBm. The rules cap EIRP at 36 dBm. You have a 24 dBi dish and 18 meters of LMR-400 (0.22 dB/m) between indoor radio and rooftop antenna:

Step 1: Cable loss.
Cable: 18 m × 0.22 dB/m = 3.96 dB
Connectors: say 3 pairs × 0.3 dB = 0.9 dB
Total Loss: 3.96 + 0.9 = 4.86 dB

Step 2: Required transmit power.
EIRP = TX Power – Loss + Antenna Gain, so:
TX Power = EIRP + Loss – Gain = 36 + 4.86 – 24 = 16.86 dBm (about 48.5 mW)

Step 3: Free-space path loss.
Wavelength: λ = 3×10⁸ / 5.8×10⁹ ≈ 0.0517 m
L_FS = 20log(4π×4700/0.0517) ≈ 121.2 dB

Step 4: Received power at other end.
Assume identical parts on RX; P_RX = EIRP (TX) + Antenna gain (RX) – Cable loss (RX) – Path loss
P_RX = 36 + 24 – 4.86 – 121.2 = -66.1 dBm

Step 5: Link margin.
-66.1 dBm – (-85 dBm) = 18.9 dB
That’s enough margin to ride out typical storms and some misalignment. If you ran short, you could pick up gain with a better antenna, cut cable length, or add a relay. Simple as that—the basic process always comes back to power, loss, gain, and how much field strength you really need.

Advanced Considerations: Polarization and MIMO Systems

Dual-polarized and MIMO systems add extra wrinkles. Each polarization stream in a dual setup has its own EIRP, and regulations generally limit EIRP per polarization, not total combined. True MIMO (multiple antennas, multiple streams) counts EIRP per-channel. Some modern radios can combine streams to point a 'super beam,' which ups the effective EIRP briefly in a narrow angle, but it’s the per-antenna (or per polarization) EIRP that matters for the rules. Always check if regulations address total radiated power (TRP) as well. Testing and verification in MIMO isn’t trivial; beamforming makes the “peak EIRP” point move, so you may need to scan every configuration.

For more linked wireless design calculations—like Friis, path loss, and antenna pattern tools—see the full calculator collection.

Frequently Asked Questions

📹 Video Walkthrough — How to Use This Calculator

📹 Video Walkthrough — How to Use This Calculator