Ham Amateur Radio Technician Practice Exam

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How does antenna gain in dBi compare to gain stated in dBd for the same antenna?

Gain in dBi is 2.15 dB higher
Gain in dBi is 2.15 dB lower
There is no difference
Gain in dBi equals gain in dBd

The correct answer is: Gain in dBi is 2.15 dB higher

Antenna gain is measured in different ways, with dBi and dBd being two common units. The “dBi” measurement refers to the gain of an antenna compared to an isotropic radiator, which radiates power uniformly in all directions. In contrast, the “dBd” measurement refers to the gain of an antenna compared to a dipole antenna, which is a more focused radiator. Since a dipole antenna has a well-defined gain of approximately 2.15 dB compared to an isotropic radiator, when you convert gain from dBd to dBi, you must add that 2.15 dB. Therefore, for any antenna, if you have a gain measured in dBd, to convert it to dBi, you would take the dBd value and add 2.15 dB to arrive at the dBi equivalent. This is why gain in dBi is considered to be 2.15 dB higher than the gain stated in dBd for the same antenna.