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i have a doubt about converting dBd to dBi.

for exmple if i have 10dBd and i want to know how many dBi is, i will add 10+2.14 and the result in dBi is 12.14dBi?

is that correct????

Hi ZURC,

Yes, your right.

Antenna gain is a measure of the amount of focus that an antenna can apply to the incoming signal relative to one of two reference dispersion patterns.

dBi is the amount of focus applied by an antenna with respect to an "Isotropic Radiator" A dispersion pattern that radiates the energy equally in all directions onto an imaginary sphere surrounding a point source.

dBd refers to the antenna gain with respect to a reference dipole antenna. A reference dipole antenna is defined to have 2.14 dBi of gain.

So converting between dBi and dBd is adding or subtracting 2.14 according to these formulas:

dBi = dBd + 2.14
dBd = dBi - 2.14


When the radio is in transmit the higher gain antennas achieve extra power (apparent increase in power) by focusing the power coming from the intentional radiator in to a tighter pattern (this is passive gain). Remember that the greater the gain, the smaller the area covered as measured in degrees of beam width both vertical and horizontal. So in some cases a high gain antenna could degrade the overall system performance due to loss of total area coverage.

Be careful not to confuse the "azimuth" antenna gain pattern (the pattern in the horizontal plane) and the "elevation" gain pattern (the pattern in the vertical plane) with the patterns for horizontally and vertically polarized signals.

When the radio is in receive the same electromagnetic properties of the antenna that created transmit gain will create receive gain by gathering in more signal from the air (? or ether ?). Many references will suggest thinking of the transmit antenna as a variable focus flashlight or a prism glass used in a lighthouse, on the receive side I like to think of the antenna creating gain in the same way a telescope or binoculars creates magnification by gathering in more light.

73 mike

Zurck,

Your calculation is correct.

Mike,

Your answer about passive gain is somehow confusing...

Passive gain is the focus of energy ( Same power ) just focused !

Active gain is the increase of energy ( extra power ) by equipments like amplifiers...

Cheers !

Thank you

dBd (dB dipole) is the gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the smallest, least gain practical antenna that can be made. The term dBd is used to describe antenna gain for antennas that operate at less than 1GHz.

ERP (Effective Radiated Power) is defined as the transmitter output power in dBm, minus the power loss in the coaxial cable and connectors in dB, plus the power gained from the directivity of the antenna in dBd.



dBi (dB isotropic) is the gain an antenna has over a theoretical isotropic (point source) antenna. An isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and System Operating Margins. The gain of Microwave antennas (above 1 GHz) is generally given in dBi.

EIRP (Effective Isotopic Radiated Power) is defined as the transmitter output power in dBm, minus the power loss in the coaxial cable and connectors in dB, plus the power gained from the directivity of the antenna in dBi.


dBd is to ERP as dBi is to EIRP

The reason why the gain of many antennas, especially VHF/UHF antennas, is measured in dBd is because antenna manufacturers calibrate their equipment using a simple dipole antenna as the standard. Then they replace it with the antenna they are testing. The difference in gain in dB is referenced to the signal from the dipole.

A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. So if an antenna gain is given in dBd, not dBi, add 2.14 to it to get the dBi rating.

If an omni antenna has 5 dBd gain, it would have 5 + 2.14 = 7.14 dBi gain.

73 mike