Antenna Gain & EIRP Calculator

Directional antennas with significant gain date to Heinrich Hertz\'s parabolic reflectors of 1888, but the modern concept of antenna gain as a measurable figure of merit took shape in the 1930s. John Kraus at Ohio State University and Sergei Schelkunoff at Bell Labs formalized the relationship between an antenna\'s physical aperture, its radiation pattern, and its gain relative to an isotropic or dipole reference. The decibel scale (developed at Bell Labs in 1924 for telephone-system calculations) made logarithmic gain expression practical.

The FCC and its predecessors have regulated radio power since 1927, but EIRP-based limits as we know them today emerged with the 1985 Part 15 rules that opened the 902-928, 2400-2483.5, and 5725-5875 MHz ISM bands to unlicensed low-power devices. Part 15 specifies maximum EIRP rather than conducted transmitter power precisely because EIRP is what other users see — and it lets regulators limit interference without dictating antenna design.

The dBi vs dBd distinction has a practical consequence: marketing literature loves dBi because the numbers are ~2 dB larger, while amateur radio tradition prefers dBd. Both are correct; just subtract 2.15 from dBi to get dBd. The same antenna, same gain, different reference.

Enter antenna gain in your preferred unit, transmit power in watts, and feedline (coax, waveguide) loss in dB. The tool computes EIRP in dBm and cross-converts the gain value to all three common representations — essential for reading datasheets that mix units and for comparing against regulatory limits expressed in EIRP.

Keep in mind: this calculates main-beam EIRP, not sidelobe or back-lobe radiation. High-gain antennas have most of their energy concentrated in a narrow beam, so off-axis radiation can be 20+ dB lower. All math runs client-side; no values leave your browser.