Antennas › Unit Converter

Antenna Unit Converter & Field Calculator

Every conversion you need when moving between a datasheet and a link budget or an EMC report: dBi and dBd, antenna factor, effective aperture, EIRP and ERP, field strength in V/m and dBµV/m, power density, and where the far field of your antenna actually begins.

Inputs

I know the antenna's

Gain

dBi

Frequency

Receiver impedance

Enter any one quantity and the others follow. Antenna factor is defined for the chosen receiver impedance: AF = E / V, the field strength needed to put 1 V at the receiver input.

Transmit power

Cable / feed loss

Antenna gain

Distance

Field values assume free space and a distance inside the antenna's far field. Check the Far field tab if the distance is short compared with the antenna size.

I know the field as

Value

Receive antenna factor

dB/m

Receiver impedance

Plane-wave free-space relations: S = E²/377 and H = E/377. The antenna factor turns the field into the voltage and power at the receiver input.

Largest dimension D

Frequency

D is the largest physical dimension of the antenna (dish diameter, array diagonal, or the dipole length). The far-field boundary is the larger of 2D²/λ, 3D and 3λ.

Results

Diagram

Related antenna tools

Gain & Beamwidth Calculator → Parabolic Dish Calculator → Polarization Loss Calculator → Free Space Path Loss →

dBi, dBd, antenna factor and effective aperture

Gain in dBi is measured against an isotropic radiator, while dBd is measured against a half-wave dipole. A half-wave dipole has 2.15 dBi of gain, so dBi = dBd + 2.15. Gain also fixes the effective aperture, Ae = G·λ²/4π: for the same gain, a lower frequency means a physically larger aperture. EMC engineers prefer antenna factor, AF = E/V in dB/m. For a 50 Ω receiver, AF(dB/m) = 20·log10(f in MHz) − G(dBi) − 29.79, so a 0 dBi antenna at 2.4 GHz has an AF of about 37.8 dB/m.

EIRP, ERP and field strength

EIRP is the power an isotropic radiator would need to radiate to produce the same field in the peak direction: transmit power minus feed loss plus antenna gain in dBi. ERP uses the dipole as reference, so ERP = EIRP − 2.15 dB. In free space the power density at distance d is S = EIRP/4πd², and the electric field is E = √(30·EIRP)/d. To report in EMC units, E in dBµV/m = 20·log10(E in V/m) + 120.

Where does the far field start?

The commonly used boundary is the Fraunhofer distance 2D²/λ, at which the phase error across the aperture is 22.5°. Closer in, the radiating near field (Fresnel region) has a pattern that depends on distance, and very close to the antenna the reactive near field stores energy rather than radiating it. Gain and pattern measurements, and the free-space formulas on this page, are only valid in the far field. For electrically small antennas the practical limit is a few wavelengths rather than 2D²/λ.