Good article Jim. Beware of some things that you read on the web though. For example an article on antennas in wikipida says that by using a folded dipole rather than a regular dipole "increases the radiation resistance" by a factor of 4. While the folded dipole does provide for a higher feed point resistance it does NOT increase the radiation resistance.
73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Jim Cicirello > Sent: Tuesday, February 20, 2007 7:57 AM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Antenna dBd Vs dBd > > I found this article on Antenna dBd Vs dBi that explains why some > antennas show more gain than others. This is because the gain used in > measuring some antennas is 2.15 dB more when expressed in dBi. > > http://www.maxstream.net/support/knowledgebase/article.php?kb=146 > > Antenna Gain: dBi vs. dBd Decibel Detail > Antenna gain is measured in either dBi or dBd. > > It is important to note that antenna gain is different than amplifier > gain. Antennas do not have a power source that allows the antenna to > create additional energy to boost the signal. An antenna is similar to > a reflective lens in principle - it takes the energy available from > the source and focuses it over a wider or narrower area. > > Antenna gain is then a measure of the amount of focus that an antenna > can apply to the incoming signal relative to one of two reference > dispersion patterns. MaxStream specifies all antenna gains in dBi. > > dBi is the amount of focus applied by an antenna with respect to an > "Isotropic Radiator" (a dispersion pattern that radiates the energy > equally in all directions onto an imaginary sphere surrounding a point > source). Thus an antenna with 2.1 dBi of gain focuses the energy so > that some areas on an imaginary sphere surrounding the antenna will > have 2.1 dB more signal strength than the strength of the strongest > spot on the sphere around an Isotropic Radiator. > > dBd refers to the antenna gain with respect to a reference dipole > antenna. A reference dipole antenna is defined to have 2.15 dBi of > gain. So converting between dBi and dBd is as simple as adding or > subtracting 2.15 according to these formulas: > > * dBi = dBd + 2.15 > * dBd = dBi - 2.15 > > Specifying antenna gain in dBd means that the antenna in question has > the ability to focus the energy x dB more than a dipole. > > Beam Width > Because higher gain antennas achieve the extra power by focusing in on > a smaller area it is important to remember that the greater the gain, > the smaller the area covered as measured in degrees of beam width > (think of an adjustable beam flashlight). In many cases a high gain > antenna is a detriment to the system performance because the system > needs to have reception over a large area. > > Hope this helps, it helped me understand better what some Mfg may be > using. > > 73 JIM KA2AJH > > > > > > > Yahoo! Groups Links > > >
