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Discussion in 'Amateur Radio Antennas' started by Need2Know, Feb 18, 2012.
Quality ham radio antennas built to last a lifetime
VHF: 3.4 dBi UHF: 4.3 dBi
Here is my stab at the meaning of dbi:
The "i" stands for isotropic.
Isotropic means "identical in all directions."
dBi equals the decibels over isotropic. This is the power in the strongest direction divided by the power that would be transmitted by an isotropic antenna emitting the same total power.
That defintion makes little sense for an antenna that is supposed to be omnidirectional.
Is there a better way to describe dbi?
"Isotropic figures" vs. what other method? Is there a better/more meaningful way of specifying antenna gain?
There are two other dB references in the two-way field: dBd (dB reference to a ½ wave dipole) and a lesser known dBq (dB referenced to a ¼ wave vertical, sometimes called just dB). The truest reference is dBd since it can be physically measured with an actual ½ wave dipole antenna.
The differences in reference values are well known:
dBi = -2.15 dBd
dBd = +2.15 dBi
dBq = -0.7 dBd
dBd = +0.7 dBq
Many antenna manufactures like to use the dBi reference to gain an extra 2 dB: Makes the antenna look better when compared to a true dBd measured antenna.
ex Motorola Systems Engineer
VHF - 1.25db
UHF - 2.15db
Unless I operate from the moon of course. This is fine as I live within the primary coverage area of all the repeaters I use.
An omnidirectional antenna attempts to flatten the isotropic theoretical (as in the "ball" example.) The energy that would have gone up and down with an isotropic antenna is directed to goe out "fairly" uniformly in all horizontal directions. So the gain of an antenna is the degree to which it can redirect the energy from being wasted going "up" or "down" and focusing it outward horizontally (if that is the desired direction of the signal.)
A directional or beam antenna focuses the signal even more compactly in a specific direction thereby achieving a higher db in the focused directions than what an omni could achieve.
I need to recall some of Mr. Wizard's experiments.