| Author |
Message |
    
DeadlyEyes
| | Posted on Saturday, June 08, 2002 - 10:31 pm: | 
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If you are considering buying an antenna and the specs state the gain in Dbd or perhaps in Dbi what does that mean?
Well here is the difference between the two references.
Dbd is the gain in signal strength compared to an actual comparative dipole antenna. They compare the signal from the beam to the signal transmitted by an actual dipole antenna. In other worlds a real world comparison using real antennas.
Dbi is the gain in signal strength compared to a theoretical model of a reference dipole. In other words a comparison to a mathametical model.
The Dbi antenna while it is not an actual real antenna is very close to being a real thoucable reference standard. Sometimes you have to be practical about certain things. If you see a Dbi reference for antenna gain do not panic. The numnbers are probably close enough to where the normal user could not tell the difference.
Just thought you might want to know. |
Znut
| | Posted on Tuesday, June 11, 2002 - 6:32 pm: |
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Hey folks,
Yeah, that theoretical antenna is called an isotropic antenna. It would have the least relative gain of any RESONANT antenna because it radiates in all directions in a perfectly spherical pattern. A vertical dipole radiates in a large "doughnut" shaped pattern. A horizontal dipole radiates in a "bipolar" pattern. In other words the signal is strongest broadside to the antenna and weakest at the ends. So as you see, with the signal being more concentrated where the dipole's pattern lies, it has some gain over an isotropic antenna.
Antenna gain compared to a dipole is much more useful for us like Deadly Eyes said. We really have no idea of the practical performance of an isotropic antenna because it is almost impossible to construct. Where do you feed the coax? Where do you provide the signal? Is it worth setting it up miles and miles from the ground (antennas are affected by the earth)? Etc.
Good subject! Thanks.
Znut |
bruce
| | Posted on Wednesday, June 12, 2002 - 9:11 am: |
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DBI is about 2 DB higher than DBD Sadly there also is a ground enhansement effect good for 3 db more and of course ALL antennas have that built into the gain so now that 1/2 wave has 5 db gain then we have to add the FUDGE FACTOR that good for another 3 db well what you know now we have a 8 db gain antenna! REAL WORLD
IF DBD = 0 db
then DBI =2db
and remember dbd is gain over a 1/2 wave dypole |
Marconi
| | Posted on Wednesday, June 12, 2002 - 7:49 pm: |
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Bruce, not that it really matters but I think you have it backwards. |
bruce
| | Posted on Wednesday, June 12, 2002 - 10:54 pm: |
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My mistake Marconi i did not state that well it should have been dbi is 2 db number wise over dbd since dbi is a referance but the real world is dbd can be measured not DBI SO a DBD measurement LOOKS 2 db smaller than dbi then add all the bull
I just came from HgA screening and my insulin hadnt kicked in at 9am. |
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