EIRP Calculator
Calculate your effective isotropic radiated power from transmitter output, antenna gain, and feed line loss. See the actual power your station is radiating in watts, dBW, and dBm.
Use the Coax Loss Calculator to find this value.
What Is EIRP?
EIRP (Effective Isotropic Radiated Power) is the total power your station would need to transmit from a perfect, omnidirectional antenna to produce the same signal strength in the direction of your antenna's maximum gain. It combines three factors: your transmitter's output power, the gain of your antenna, and any losses in the feed line between them.
Understanding EIRP matters because it tells you what your station actually puts out in a given direction, not just what the number on your radio says. A 5W handheld with a 10 dBi Yagi produces the same signal as a 50W radio with a rubber duck antenna. The transmitter power is different, but the received signal at the other end is identical.
The EIRP Formula
In decibels, the calculation is straightforward: EIRP (dBW) = TX Power (dBW) + Antenna Gain (dBi) - Cable Loss (dB). To convert transmitter power to dBW, use dBW = 10 × log10(watts). The result converts back to linear watts via watts = 10^(dBW / 10).
Worked example: A 50W mobile radio feeding a 5/8 wave ground plane (5.5 dBi) through 30 feet of RG-8X at 146 MHz (about 1 dB loss). TX power = 16.99 dBW. EIRP = 16.99 + 5.5 - 1.0 = 21.49 dBW = 141W. Your 50W radio is producing the equivalent of 141W into a perfect omnidirectional antenna.
EIRP vs. ERP
ERP (Effective Radiated Power) is the same concept but referenced to a half-wave dipole instead of an isotropic radiator. Since a dipole has 2.15 dBi of gain, ERP is always 2.15 dB less than EIRP: ERP (dBW) = EIRP (dBW) - 2.15. This calculator shows both values. Some regulatory contexts (commercial radio, certain international standards) specify limits in ERP rather than EIRP, so having both is useful.
Typical EIRP Values
Common ham radio setups and their approximate EIRP:
| Setup | TX Power | Antenna | EIRP |
|---|---|---|---|
| Handheld, stock antenna | 5 W | 0 dBi | 5 W |
| Handheld, upgraded whip | 5 W | 3.5 dBi | 11 W |
| Mobile, mag-mount | 50 W | 3.0 dBi | 100 W |
| Base, ground plane | 50 W | 5.5 dBi | 177 W |
| Base, 3-element Yagi | 50 W | 7.0 dBi | 250 W |
| HF station, beam | 100 W | 10 dBi | 1,000 W |
These values assume zero cable loss. Real-world EIRP will be slightly lower depending on feed line length and type. Use the Coax Loss Calculator to find your actual loss figure.
Practical Tips
Upgrade the antenna first. Doubling your transmitter power adds 3 dB. Replacing a stock rubber duck with a half-wave whip adds 3.5 dBi for a fraction of the cost. Antenna upgrades are the most cost-effective way to increase your EIRP.
Minimize feed line loss. Every dB lost in the coax is a dB subtracted from your EIRP. Keep cable runs as short as possible and use low-loss cable for VHF/UHF installations. LMR-400 at 446 MHz loses about 2.4 dB per 100 feet, while RG-58 loses 7.0 dB over the same length.
Check your connectors. A corroded or poorly soldered PL-259 can add 0.5 dB or more of loss per connection. Two bad connectors plus adapter losses can easily eat 1 to 2 dB of your signal. Use quality connectors and weatherproof outdoor connections.
Looking for a radio with higher output power? Browse our handheld radio rankings sorted by TX power, features, and overall score.
Frequently Asked Questions
What is the difference between EIRP and ERP?
EIRP (Effective Isotropic Radiated Power) references a theoretical isotropic antenna that radiates equally in all directions. ERP (Effective Radiated Power) references a half-wave dipole, which has 2.15 dBi of gain. ERP is always 2.15 dB less than EIRP for the same station. FCC Part 97 limits are stated in PEP at the transmitter, but some services (like Part 90 commercial) use ERP limits, so knowing both is useful.
How does antenna gain affect EIRP?
Antenna gain concentrates your signal in a preferred direction rather than creating new power. A 6 dBi antenna does not quadruple your power, it focuses it. In the direction of maximum gain, the effect is the same as quadrupling your transmitter power. This is why upgrading your antenna is often more effective than buying a higher-power radio.
What is a typical EIRP for a handheld ham radio?
A 5W handheld with the stock rubber duck antenna (roughly 0 dBi actual gain, often slightly negative) produces about 3 to 5W EIRP. Replacing the stock antenna with a half-wave whip (3.5 dBi) nearly doubles the EIRP to about 11W, which is the single most impactful upgrade for any handheld.
Does feed line loss reduce my EIRP?
Yes. Every dB of feed line loss directly subtracts from your EIRP. If your coax has 3 dB of loss, half of your transmitter power is being converted to heat in the cable before it ever reaches the antenna. On VHF/UHF where cable losses are highest, using low-loss coax like LMR-400 can make a noticeable difference.
What are the FCC power limits for amateur radio?
FCC Part 97 limits amateur stations to 1,500 watts PEP (peak envelope power) at the transmitter output on most bands. Some bands and segments have lower limits: 200W PEP on 30 meters, and Novice/Technician segments on HF are limited to 200W PEP. These limits are on transmitter output power, not EIRP, so high-gain antennas are allowed.