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Lt. Commander

[import] Antenna gain - What's the difference between dBd, dBi and just plain dB ?

>>Message imported from previous forum - Category:Trio Data Radios<<
User: joelw, originally posted: 2018-10-17 18:29:18 Id:72
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum.

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**_jweder:
There's an important difference between dBd and dBi. For dBd the reference antenna is a dipole (1/2 wavelength), while for dBi the reference is a theoretical isotropic radiator. (a point source) An isotropic radiator doesn't really exist. The sun radiates spherically (sort of) but is far from being a point source. The definition is that it must radiate equally in a spherical pattern._**

At 2.4 GHz and above some people use the dBi reference point, eg an 8 dBi omni or a 14 dBi yagi. At 900 MHz and below more commonly dBd is used, for example a 6 dBd omni or a 10 dBd yagi.

There is a simple way to convert between dBi and dBd. A real-world dipole antenna has 2.14 dB of gain relative to an isotropic radiator. So a 10 dBd yagi has 12.14 dBi of gain. Most people would just round that off to a 2 dB difference.

Link budget AKA fade margin will definitely increase if a higher gain antenna is used, or decrease with a lower gain antenna. (or more cable loss for example) If you added an 8 dBi omni antenna on a 2.4 GHz system in place of a 2 dBi internal whip, you'd increase the link budget by 6 dB.

Here's where it gets important: If you're comparing two antennas, one of which is specified in dBd and the other in dBi, make sure you convert one of the numbers first, or the results won't make sense.

Also, you will find that some manufacturers state antenna gain simply in "dB." This is bogus and a real pain to work with. It's totally meaningless. Before you work with such an antenna, contact the manufacturer and insist they tell you if they mean dBd or dBi. Some will try to sell you a "10 dB" antenna which is really 10 dBi (8 dBd) but they're hoping you'll be impressed with the bigger number!