EIRP stands for Effective Isotropic Radiated Power. It’s a single number that describes the total signal strength leaving a radio system in its strongest direction, combining the transmitter’s output power, any signal lost in cables and connectors, and the focusing ability of the antenna. Engineers, regulators, and network designers use EIRP as the standard way to compare how “loud” different radio systems are, regardless of the specific equipment involved.
How EIRP Works
Every radio system has three basic parts that determine how much power actually reaches the airwaves: a transmitter that generates the signal, cables and connectors that carry it to the antenna, and the antenna itself. The transmitter produces a certain amount of power. Some of that power is lost as heat in the cable run. Then the antenna focuses whatever power remains into a beam, effectively amplifying it in one direction.
EIRP captures all of this in one formula:
EIRP = Transmitter Power − Cable/Connector Losses + Antenna Gain
All three values are expressed in decibels (dB), a logarithmic scale that makes the math simple: you just add and subtract. Take a real example. A transmitter puts out 1,000 watts (30 dBW). The cable connecting it to the antenna introduces 3 dB of loss, which cuts the power roughly in half. The antenna has a gain of 10 dBi, meaning it focuses energy so its peak direction is 10 times stronger than a perfectly uniform radiator would produce. The EIRP is 30 − 3 + 10 = 37 dBW, equivalent to about 5,011 watts. The system radiates as if it were a 5,000-watt transmitter broadcasting equally in every direction, even though the actual transmitter only produces 1,000 watts.
The Isotropic Reference Point
The “I” in EIRP refers to an isotropic radiator, a theoretical antenna that radiates energy equally in all directions, like a perfect point source of light. No real antenna does this. Every physical antenna concentrates energy more in some directions than others. But the isotropic radiator serves as a universal baseline. By definition, its gain is 0 dB (no focusing at all), so any real antenna’s performance can be measured against it.
Antenna gain is typically expressed in dBi, where the “i” confirms the comparison is against this isotropic reference. You may also see gain listed in dBd, which compares against a dipole antenna instead. A dipole has a gain of 2.15 dBi, so a dBd figure is always 2.15 lower than the equivalent dBi figure. EIRP calculations use dBi.
EIRP vs. ERP
You’ll often see ERP (Effective Radiated Power) alongside EIRP, and the two are easy to confuse. The only difference is the reference antenna. EIRP uses the isotropic radiator as its baseline. ERP uses a half-wave dipole, which already has 2.15 dBi of gain built in. That means EIRP is always 2.15 dB higher than ERP for the same system:
- EIRP (dBm) = ERP (dBm) + 2.15
- EIRP (watts) = ERP (watts) × 1.64
If a regulation or spec sheet gives you one value, you can convert to the other with these formulas. Just pay attention to which one is being quoted, because mixing them up throws your calculations off by a factor of about 1.6.
Why Cable Loss Matters
The cable between a transmitter and antenna is where power quietly disappears. Coaxial cable loss depends on two things: the cable type and the frequency of the signal. Higher frequencies lose more energy over the same length of cable.
A common cable like LMR-400 loses about 1.5 dB per 100 feet at 150 MHz, which is modest. But at 2.4 GHz (Wi-Fi frequencies), that same cable loses 6.8 dB per 100 feet, meaning roughly 80% of the power turns into heat before it ever reaches the antenna. At 5.8 GHz, the loss climbs to 10.8 dB per 100 feet. This is why keeping cable runs short and using low-loss cable is one of the easiest ways to improve a system’s EIRP without buying a bigger transmitter or a higher-gain antenna.
Regulatory Limits on EIRP
Governments regulate EIRP directly because it represents the actual signal strength that could interfere with other users of the radio spectrum. In the United States, the FCC sets maximum EIRP levels for unlicensed devices under Part 15 rules. For Wi-Fi equipment operating in the newer 6 GHz band, the limits break down by device type:
- Standard-power access points (outdoor-capable, controlled by an automated frequency coordinator): 36 dBm (about 4 watts)
- Clients connected to standard-power access points: 30 dBm (1 watt)
- Low-power indoor access points: 30 dBm (1 watt)
- Clients connected to indoor access points: 24 dBm (about 250 milliwatts)
Outdoor devices also face an additional restriction: at elevation angles above 30 degrees from the horizon, maximum EIRP drops to just 21 dBm (125 milliwatts) to protect satellite services from interference. These limits apply to the combined system, not just the transmitter, so swapping in a higher-gain antenna without reducing transmitter power can push you over the legal limit.
Where EIRP Shows Up in Practice
EIRP is central to satellite communications. When engineers design a satellite link, EIRP is the starting point of what’s called a link budget: a ledger of every gain and loss between the transmitter and the receiver. A satellite’s downlink EIRP determines how large a dish you need on the ground to pick up a usable signal. Higher EIRP from the satellite means smaller, cheaper ground antennas.
The same principle applies to cellular towers, point-to-point microwave links, and even your home Wi-Fi router. When a router manufacturer lists a device’s EIRP, they’re telling you the maximum signal strength it produces in its strongest direction. Two routers with identical EIRP will deliver comparable range, even if one achieves it through a powerful transmitter and a modest antenna while the other uses a weaker transmitter paired with a high-gain antenna.
This is the real utility of EIRP: it collapses all the variables of a transmitting system into a single, comparable number. Whether you’re sizing a satellite dish, checking regulatory compliance, or troubleshooting why a wireless link isn’t performing, EIRP gives you the one figure that describes what the system actually puts into the air.