The Telecommunications Industry Association, an international organization which develops standards to which nearly all countries of the world have subscribed, has already taken steps to correct the misleading practice of indiscriminately using dBi where dBd is appropriate.
According to TIA-329-C, published in 2003, base station antenna gain for less than 1 GHz shall expressed in dBd using a dipole antenna as a reference. Antenna gain for 1 GHz and above shall be expressed in dBi using a theoretical isotropic radiator as a reference. There are no exceptions. So, why are some manufacturers still using dBi for their 2m and 70cm antennas? There are probably several answers to that question, such as: 1. Perhaps most antenna buyers don't know the difference between dBi and dBd. 2. Perhaps most antenna buyers believe whatever the ad copy says. 3. Perhaps the company owner is an old-school believer that dBi is the only "true" gain unit. 4. Perhaps the antenna designer knows about TIA-329-C, but chooses to ignore it. It should be obvious that microwaves, which begin around 1 GHz, behave a lot like light and can be focused with a parabolic reflector. Short radio waves are easy to visualize as being generated by a point source, very much like a bulb in a parabolic flashlight reflector. Such point sources can be easily expressed as isotropic radiators, and the leap to dBi is logical. The wavelength of lower-frequency waves in the VHF and UHF spectra are not point sources, and it is illogical to expend any effort "converting" from one reference to the other. As several others have pointed out, there is about 2.14 dB difference between the absolute gain expressed as dBi and that expressed as dBd. Unfortunately, there will always be some "fringe group" that will argue until the end of time that dBi is the Nirvana of antenna gain expression. I doubt that the decision by the TIA to limit dBi as an antenna gain unit to 1 GHz and above will change their beliefs. Getting the antenna manufacturers to properly report the gain of their products is quite another thing. As previous posters have mentioned, some popular antennas are junk that has never been properly tested on an antenna range, resulting in ridiculously inflated and undocumented claims of performance. If clueless buyers believe the hype, nothing is likely to change. That's a shame- but hey, it's the American Way! 73, Eric Lemmon WB6FLY
