Antenna gain, dBi and EIRP

Distributing energy in space

A passive antenna does not create extra transmit power. It distributes radiated energy differently across directions. GainGain×Gain describes the ratio of output to input level. In a receiver it can occur before or after digitisation and is often expressed in dB.Example & full explanation → describes radiation intensity in one direction relative to a reference and includes efficiency. High peak gain can therefore come with a narrow main beam and weaker directions elsewhere.

dBi references an ideal isotropic radiator. dBd references an ideal half-wave dipoleDipole×A centre-fed half-wave dipole has two conductor halves. Together they are approximately half a wavelength long; actual dimensions depend on construction and surroundings.Example & full explanation →. The conventional difference is about 2.15 dB: 0 dBd is approximately 2.15 dBi. Compare manufacturer figures only when reference, frequency and direction are clear.

From transmitter power to EIRP

In logarithmic form: EIRP in dBmdBm×dBm expresses an absolute power level relative to 1 milliwatt, combining the logarithmic dB scale with a fixed power reference.Example & full explanation → = transmit power in dBm − feed-line loss in dB + antenna gain in dBi. ERP uses the dipole reference instead. With 5 W, 1 dB line loss and 2.15 dBi gain, the result is about 6.52 W EIRP and 3.97 W ERP.

These numbers describe equivalent radiated power in the direction considered. They do not mean a power meter at the antenna connector would suddenly measure more energy. A radio link also involves receive antenna gain, polarisation, losses and propagation.

What to check when comparing

Look beyond peak gain to the radiation pattern and intended orientation. An antenna with an inconvenient null can perform worse in the desired direction than expected. EIRP calculations replace neither actual field measurements nor checking the requirements applicable to a particular operation.