[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
RE: [Repeater-Builder] Antenna dBd Vs dBd
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
Re: [Repeater-Builder] Antenna dBd Vs dBd
On 2/20/07, Jim Cicirello [EMAIL PROTECTED] wrote: 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 finally clicked for me one day when someone used the example of one of the original non-LED small MagLite flashlights. Stand about 20 feet from a wall in a semi-dark room... Take the lens off... the MagLite looks like a little candle... light radiating in all directions. Note the amount of light on the wall at a particular spot. This is your isotropic radiator... well kinda... close enough for this demo, anyway. A point of light. Screw the lens back on and aim the flashlight at the wall. Turn the focus (more gain) to make the spot on the wall smaller and smaller... note how the spot gets brighter. (Well the reflected light off the wall back to your eyes is what you're really seeing... but anyway, ignore that part...) The flashlight is transmitting the same amount of light throughout this entire exercise, but as you focus the lens on the spot you want the light to illuminate you send more light that direction. Same thing works the other direction (careful not to hurt eyes, here if you actually try any of this stuff!... of course). You can have someone look at our little tiny MagLite across a mile or more with a telescope -- a REALLY high gain light antenna (lens)... so to speak. If you can make the mental leap here and pretend these are all your favorite high and low gain antennas (for whatever band...) you start to get the idea... In your imagination, it helps show why the guys with big beams can always still hear the pip-squeaks with bad antennas but they can't hear off to the side -- 'cause they're looking with a TELESCOPE for them! -- and how the gain of a particular antenna squeezes its pattern out to the horizon (we hope)... and not the sky... Just imagine your antenna radiates light (not RF... but hey, it's still the all part of the spectrum) and where all that light is going to go, and what little lights you'll be able to see out that direction if you're using highly directional antennas... makes for a decent (but not 100% perfect) mental model... I always thought it'd be neat (if I were a good graphics design coder, which I most definitely am NOT) to build a path prediction software package that would show each transmitter as a 3-dimensional light source. 3-dimensional instead of 2-dimensional like most path prediction software is now. But talk about a nightmare to code. Sure would look nifty, though. Especially if you could simulate real system activity in a linked system and fly around the radio system in 3D, like Google Earth... see where all that RF is really going... add topographic data... etc etc etc. I suppose the big boys (commercial or government) who can afford to pay someone to write such code, probably have toys like this, somewhere... Nate WY0X
