EIRP Calculator — Effective Isotropic Radiated Power

Effective

This word signifies that EIRP is not a direct measurement of the transmitter's raw output. Instead, it is a calculated, "effective" value that represents the power level *after* accounting for all the gains and losses in the system, particularly the focusing effect of the antenna.

Isotropic

This is the theoretical heart of the concept. An **isotropic radiator** is a perfect, hypothetical point-source antenna that radiates energy equally in all directions, creating a perfect sphere of signal. Think of it like a bare lightbulb with no reflector. It's a theoretical baseline against which all real-world antennas are measured.

Radiated

This simply means we are concerned with the power that is actually being sent out—or radiated—into the air from the antenna, not the power being consumed by the device.

Power

The fundamental quantity of energy transferred per unit of time, typically measured in Watts (W) or milliwatts (mW).

Step 1: Input Transmitter Power (Tx Power)

Enter the raw power output of your radio transmitter. You can enter this value in Watts (W), milliwatts (mW), or the logarithmic unit dBm. The calculator can seamlessly convert between them. This value is typically found in the device's technical specifications.

Step 2: Input Cable & Connector Loss

Every foot of cable and every connector between your transmitter and your antenna introduces a small amount of signal loss. This energy is dissipated as heat. Enter the total loss of your transmission line in decibels (dB). This is always a positive value representing a loss, which the formula will subtract.

Step 3: Input Antenna Gain

Enter the gain of your antenna in dBi (decibels relative to isotropic). This value, found on the antenna's datasheet, quantifies how well the antenna focuses energy in a particular direction.

Step 4: Calculate and Analyze

The tool will instantly compute the final EIRP in both dBm and Watts, giving you a complete picture of your system's effective power output.

1. Regulatory Compliance

Government bodies like the FCC in the United States and ETSI in Europe set strict limits on how much power a device can radiate. These limits are almost always specified in EIRP. This is to prevent a single device from "shouting" so loudly that it interferes with other nearby services. For example, standard Wi-Fi in the 2.4 GHz band is often limited to a maximum EIRP of 36 dBm (4 Watts). Using a high-gain antenna without reducing your transmitter power can easily make your system illegal.

2. Accurate System Design and Link Budgets

When engineers design a wireless link (e.g., a microwave link between two buildings or a satellite connection), they create a "link budget." This is a detailed accounting of all the gains and losses the signal will experience from the transmitter to the receiver. EIRP is the starting point—the total effective power leaving the transmitting antenna. Without an accurate EIRP, the entire link budget is meaningless.

3. Meaningful Performance Comparison

How do you compare a system with a low-power transmitter and a high-gain antenna against a system with a high-power transmitter and a low-gain antenna? You compare their EIRP. It provides a level playing field, telling you which system will deliver a stronger signal in its intended direction, regardless of its individual components.

Q: What is the difference between EIRP and ERP?

This is a classic question. **EIRP** (Effective Isotropic Radiated Power) uses the theoretical isotropic antenna as its baseline (gain in dBi). **ERP** (Effective Radiated Power) uses a half-wave dipole antenna as its baseline (gain in dBd). A dipole antenna naturally has a gain of 2.15 dBi. Therefore, the relationship is simple:

EIRP (dBm) = ERP (dBm) + 2.15 dB

ERP is more commonly used in land mobile radio and broadcast applications. EIRP is dominant in satellite and Wi-Fi/WLAN contexts.

Q: Can my EIRP be higher than the legal limit for my transmitter's power output?

Yes, absolutely! This is a critical point. A regulator might limit transmitter power to 1 Watt (30 dBm), but the EIRP limit might be 4 Watts (36 dBm). This means you are legally allowed to have a system gain (Antenna Gain - Cable Loss) of up to 6 dB. If your antenna has a 10 dBi gain and your cable loss is 1 dB, your system gain is 9 dB. If you use a 30 dBm transmitter, your EIRP would be 39 dBm—which would be illegal. You would need to reduce your transmitter power to 27 dBm to stay compliant.

Q: Is a higher EIRP always better?

Not necessarily. A very high EIRP is achieved with a very high-gain, directional antenna. This creates a very narrow "pencil beam" signal. This is excellent for a fixed point-to-point link but terrible for providing general mobile coverage, where you want a broad signal pattern. The "best" EIRP is one that is tailored to the specific application while remaining within legal limits.