.Understanding DBI & DBD: Antennas & Gain

[dervast]

Hi i need information about th dbi and dbd... I know what db is but i am trying to understand dbi and dbd...

Also i want to know what if the antennas or the tvs antennas has some gain.. I know that the gain refers to the antennas surface... that's why the satellite antennas have a big surface for reception... The radio or tv antenna have very small surface so i think they don't have any gain

 

[berkeman]

I believe that dBi is gain over an isotropic antenna. I don't know what dBd is.

A half-wave dipole is an example of an isotropic radiator. When seen from above, the radiation pattern is circular. That is, it doesn't matter at what angle theta the receiving antenna is located around the radiating antenna, it will have the same receive strength at a given radius from the dipole.

Yes, you can get a huge amount of gain with a large parabolic antenna feeding a waveguide, as long as the frequencies are compatible with the waveguide, of course. At lower frequencies, you can still have some modest dBi gain over an isotropic receive antenna like a dipole. One way to do it is to use an array of antennas, which will give you more gain in some theta angles and corresponding less in other angles. Look up the subject of antenna arrays for more info and some illustrations of antenna gain patterns.

 

[dervast]

Thx a lot man do u have anyurl in mind.. I think that if i try google i will lose time tryint to figure out what is valuable and what is not

 

[berkeman]

Here's a homework thread from a couple weeks ago about calculating antenna array patterns. Check out the link that I posted down at the end of the thread.

https://www.physicsforums.com/showthread.php?t=108034

 

[Ouabache]

As I understand it, dBi is the gain over an isotropic antenna. An isotropic radiator is a lossless dimensionless point in free space that radiates equally http://www.hp.com/rnd/images/pdf_html/antennas_figure6.jpg

A reasonable example of isotropic radiator is a vertical 1/4-wave antenna. From above this will radiate iso (same) in all directions.

Whereas dBd is the gain relative to a 1/2-wave dipole antenna. The radiation of a dipole (typically mounted with horizontal polarization), is http://www.phys.ufl.edu/~dorsey/phys6347/worksheets/images/radiation6.gif shaped.

One relationship, comparing the two terms I've seen is
dBi = dBd + 2.15 (http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html)