Calculate EIRP Within FCC, ETSI, ISM 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.

Here's the thing — regulatory bodies don't care about your transmitter output power in isolation. They care about what's actually being radiated into free space. That means accounting for every dB of gain and loss between your PA output and the antenna's far field. Every connector, every inch of coax, every bit of antenna gain. That's where EIRP and ERP come in, and getting them right is the difference between passing certification and redesigning your entire RF chain.

Most engineers I've worked with treat this as a checkbox at the end of a project. Bad idea. You need to run these numbers early — ideally before you've locked down your antenna choice or committed to a specific transmit power level. I've seen teams discover they're 6 dB over the limit during pre-compliance testing, which is about the worst time to find out. At that point, your options are all expensive: redesign the PA, swap the antenna (and probably the enclosure), or add an attenuator that kills your link budget.

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 before you cross into illegal territory.

EIRP vs. ERP: Getting the Definitions Straight

These two terms get confused constantly, and mixing them up can cost you 2.15 dB of margin. When you're right at the limit, that's the difference between compliant and non-compliant.

EIRP (Effective Isotropic Radiated Power) is the total power that would need to be radiated by an isotropic antenna (a theoretical point source that radiates equally in all directions) to produce the same peak power density as your actual antenna in its direction of maximum gain. The calculation is straightforward:

You take your transmitter output power, subtract all the losses between the PA and the antenna feed point, then add your antenna gain referenced to isotropic. Simple enough.

ERP (Effective Radiated Power) uses a half-wave dipole as the reference instead of an isotropic radiator. Since a real dipole has 2.15 dBi of gain over isotropic (it's directional, not a perfect sphere), the relationship is:

Or if you're working directly with antenna gain specified in dBd (gain over dipole):

Most modern antenna datasheets specify gain in dBi, so the first form is usually more convenient. But you'll occasionally run into older specs or broadcast standards that still use dBd, especially in VHF/UHF land.

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. Assuming the wrong one is an easy way to fail certification.

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. That's actually pretty generous — the FCC knows this band is crowded and wants decent range.

Here's your system budget:

• 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)

These numbers are typical for a small enterprise AP or a decent outdoor unit. Nothing exotic. Step 1 — Calculate EIRP: Step 2 — Calculate ERP (for reference, even though FCC uses EIRP):

You don't strictly need this for FCC compliance, but it's useful if you're also targeting markets that specify ERP limits, or if you're comparing to older equipment specs.

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 — enough headroom that component tolerances and measurement uncertainty won't bite you during certification testing.

Step 4 — Find the maximum permitted antenna gain:

Now let's flip the question. If you wanted to push to the limit (say, for a point-to-point link where you need every dB of range), what's the biggest antenna you could legally use?

So you could use up to an 18.5 dBi antenna — think a small parabolic dish or a high-gain sector panel — and still remain compliant. That's a substantial antenna. In practice, you'd probably run into mechanical or cost constraints before you hit the regulatory limit here. But it's good to know where the ceiling is.

This calculation is especially useful when you're designing directional links. If you're doing a 5 km point-to-point shot, you want as much antenna gain as the law allows. Knowing your maximum permitted gain lets you shop for antennas intelligently instead of guessing.

Navigating Different Regulatory Regimes

Here's where things get interesting if you're designing for international markets. The same hardware can be perfectly legal in one region and wildly non-compliant in another. I've seen this catch teams off guard more times than I can count.

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

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

Look at that ETSI limit. You're 6.5 dB over. For European compliance, you'd need to either reduce TX power to $13.5 \text{ dBm}$ or drop to a $2.5 \text{ dBi}$ antenna — basically a simple PCB antenna or a short monopole. That's a significant hit to your link budget. Many Wi-Fi products ship with different firmware for different regions specifically to handle this, dialing back the PA output when operating under ETSI rules.

For the 433 MHz ISM band (common for IoT and telemetry), you're looking at $7.5 \text{ dBi}$ max antenna gain with that transmit power. More realistically, you'd run much lower TX power in that band — something like 10 dBm — and use a modest antenna. The 433 MHz band has tighter restrictions because it's more congested and propagates better than 2.4 GHz.

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. I can't stress this enough. Discovering you're 6 dB over the limit at the certification lab is an expensive lesson — you're looking at another board spin, new antennas, recertification fees, and schedule slip. Run the numbers when you're still in the schematic phase.

Common Pitfalls

Forgetting cable loss works in your favor. This one trips people up. Losses between the transmitter and antenna reduce your EIRP. That 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.

That said, don't add loss on purpose if you can avoid it. Yes, it helps your transmit compliance, but it also degrades your receive sensitivity by the same amount. You're better off with low loss and a legal antenna gain than high loss and a massive antenna. Your link budget will thank you.

Confusing dBi and dBd. A "6 dB gain" antenna could be $6 \text{ dBi}$ or $6 \text{ dBd}$ (which equals $8.15 \text{ dBi}$). That 2.15 dB difference can push you over a limit. I've seen this mistake in production designs. Always confirm the reference when reading a datasheet, and if it's not specified, assume dBi since that's the modern standard. 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 scenarios. They're not interested in your typical performance — they want to know what happens when everything stacks up in the wrong direction. Manufacturing variations are real, and that ±1 dB tolerance exists for a reason. Not accounting for TX power tolerance. Similarly, if your radio's output power can vary by $\pm 1.5 \text{ dB}$ over temperature and supply voltage, use the upper bound in your compliance calculations. Your PA might put out 21.5 dBm on a hot day with a fresh battery, and that's what the test lab will catch. I've seen radios that were fine at room temperature fail at temperature extremes because nobody checked the PA's temperature coefficient. Assuming "indoor only" is a loophole. Some teams think they can skirt the limits by marking their product for indoor use only. That's not how it works. The EIRP limits apply regardless of intended use case. The FCC doesn't care if you pinky-swear that nobody will use your high-gain antenna outdoors.

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.

The calculator handles all the conversions and gives you a clear picture of where you stand. Use it early, use it often, and save yourself the pain of discovering compliance issues when it's too late to fix them easily.