How to Calculate EIRP and Stay Within FCC, ETSI, and ISM Regulatory Limits

Why EIRP Matters More Than You Think

You've designed your RF system, picked a nice antenna, and everything works on the bench. Then someone asks: *"Are we compliant?"* And suddenly you're digging through FCC Part 15.247 or ETSI EN 300 328 trying to figure out whether your effective radiated power is legal.

The reality is that regulatory bodies don't care about your transmitter output power in isolation. They care about what's actually being radiated into free space — and that means accounting for every dB of gain and loss between your PA output and the antenna's far field. That's where EIRP and ERP come in.

Let's break down the math, walk through a real-world example, and show you how to quickly determine your compliance margin — or figure out the maximum antenna gain you're allowed to use.

EIRP vs. ERP: Getting the Definitions Straight

These two terms are often confused, and mixing them up can cost you 2.15 dB of margin — which matters when you're right at the limit.

EIRP (Effective Isotropic Radiated Power) is the total power that would need to be radiated by an isotropic antenna to produce the same peak power density as your actual antenna in its direction of maximum gain: ERP (Effective Radiated Power) uses a half-wave dipole as the reference instead of an isotropic radiator. Since a dipole has 2.15 dBi of gain:

Or equivalently:

where $G_{antenna}$ in dBd is the gain relative to a dipole. Most datasheets specify gain in dBi, so the first form is usually more convenient.

Key point: FCC Part 15 limits are specified in EIRP (referenced to isotropic), while some older regulations and broadcast standards use ERP. Always check which reference your regulatory body requires.

A Worked Example: 2.4 GHz Wi-Fi Access Point Under FCC Part 15

Let's say you're designing a 2.4 GHz access point for the U.S. market. FCC Part 15.247 allows a maximum EIRP of $36 \text{ dBm}$ (4 W) for frequency-hopping and digitally modulated systems in the 2.4 GHz ISM band.

Here's your system:

• TX Power: $20 \text{ dBm}$ (100 mW) at the radio IC output
• Cable and connector losses: $2.5 \text{ dB}$ (short pigtail + U.FL connector + SMA bulkhead)
• Antenna gain: $9 \text{ dBi}$ (a modest panel antenna)

Step 1 — Calculate EIRP: Step 2 — Calculate ERP: Step 3 — Determine regulatory margin:

You're well within the FCC limit with 9.5 dB to spare. That's a comfortable place to be.

Step 4 — Find the maximum permitted antenna gain:

If you wanted to push to the limit (say, for a point-to-point link), what's the biggest antenna you could legally use?

So you could use up to an 18.5 dBi antenna — a small dish or a high-gain sector panel — and still remain compliant.

Navigating Different Regulatory Regimes

Here's where things get interesting if you're designing for international markets. The same hardware can be legal in one region and wildly non-compliant in another.

Consider the same system ($P_{TX} = 20 \text{ dBm}$, $L_{cable} = 2.5 \text{ dB}$, $G = 9 \text{ dBi}$, EIRP = 26.5 dBm) under different regulations:

\* Assuming a hypothetical 433 MHz version of the same power budget.

For ETSI compliance, you'd need to either reduce TX power to $13.5 \text{ dBm}$ or drop to a $2.5 \text{ dBi}$ antenna (like a simple PCB antenna). For the 433 MHz ISM band, you're looking at $7.5 \text{ dBi}$ max antenna gain — or more realistically, a much lower TX power.

This is exactly the kind of analysis you need to do early in a design, before you've committed to an antenna or RF front-end architecture. Discovering you're 6 dB over the limit at the certification lab is an expensive lesson.

Common Pitfalls

Forgetting cable loss works in your favor. Losses between the transmitter and antenna reduce your EIRP. This means longer cable runs or additional connectors actually give you room for a higher-gain antenna. It sounds counterintuitive, but it's a legitimate design lever — just don't add loss on purpose if you can avoid it, since it also degrades your receive sensitivity. Confusing dBi and dBd. A "6 dB gain" antenna could be $6 \text{ dBi}$ or $6 \text{ dBd}$ ($= 8.15 \text{ dBi}$). That 2.15 dB difference can push you over a limit. Always confirm the reference. Ignoring antenna gain tolerances. If your antenna datasheet says $9 \pm 1 \text{ dBi}$, your worst-case EIRP calculation should use $10 \text{ dBi}$. Certification bodies test worst-case. Not accounting for TX power tolerance. Similarly, if your radio's output power can vary by $\pm 1.5 \text{ dB}$ over temperature, use the upper bound.

Try It

Don't do this math on the back of a napkin when compliance is on the line. Open the EIRP / ERP Regulatory Calculator to plug in your TX power, cable losses, and antenna gain — and instantly see your EIRP, ERP, regulatory margin, and maximum permitted antenna gain for FCC, ETSI, and ISM limits. It's the fastest way to sanity-check your RF chain before you get anywhere near a test chamber.