Hi Allan,
Here is a way to look at it that may help: A 6 db increase in voltage doubles the voltage. A 3 db increase in power doubles the power. A 6 db increase in power quadruples power (4x). (An easy way to do that is.. 2x power = 3 db. 2x power again = 6 db). Now if we do it by ohms law, let's say we have 10 volts to start with across 50 ohms. E squared /R = P. So 10x10=100 volts divided by 50 ohms = 2 watts. If we increase the 10 VOLTS by 6 db that gives us 20 volts. To find the power now across our 50 ohms we have: 20x20=400 volts divided by 50 ohms = 8 watts. If we go back and increase our POWER by 6 db from our 2 watts that gives us 8 watts. We end up with the same power increase either way. So it doesn't matter if we are using power or voltage as a reference, the amount of energy (signal strength) works out the same way. The only thing we have to watch out for is to not mix the two. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of allan crites Sent: Tuesday, February 20, 2007 11:40 AM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Antenna Gain Specification - dBi versus dBd Now that dBi Vs dBd seems to have been settled, perhaps there is someone who cares to explain if antenna manufacturers express their antenna gain with voltage dB's or in power dB's. There is a difference. Allan Crites WA9ZZU Gary Schafer <[EMAIL PROTECTED]> wrote: > -----Original Message----- > From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] <mailto:Builder%40yahoogroups.com> .com] On Behalf Of Eric Lemmon > Sent: Tuesday, February 20, 2007 1:21 AM > To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com > Subject: [Repeater-Builder] Antenna Gain Specification - dBi versus dBd > > 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? Unfortunately dBd is not a cure all. Dbi is the true way to compare antennas however not always the most convenient. One problem with dBi is that it assumes free space. The dBd also assumes free space but is impossible to test in true free space. When you get into the lower frequencies it becomes more of a problem. >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. There is nothing unique about 1GHz over the lower frequencies being like light waves. > 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. Neither are sources above 1GHz "point sources" any more than a 1 MHz signal is. It is all a matter of scale. >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. This is only in free space. > > 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. Antenna theory sometimes becomes like religion for many. >Getting the antenna > manufacturers > to properly report the gain of their products is quite another thing. It is a good thing to have some standard that figures are reported by to lower the confusion factor. > 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. Some of the prominent manufacturers do not have a range to do testing but that doesn't necessarily make them junk. Some very good performing antennas come out of those factories and a few duds also. 73 Gary K4FMX 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 >
