.Understanding DBI & DBD: Antennas & Gain

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Discussion Overview

The discussion centers on understanding the concepts of dBi and dBd in relation to antennas and their gain. Participants explore the definitions, implications, and applications of these terms, as well as the relationship between antenna size and gain.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant seeks clarification on the definitions of dBi and dBd, noting their understanding of decibels (dB) but not the specific terms.
  • Another participant proposes that dBi represents gain over an isotropic antenna, while expressing uncertainty about dBd.
  • A half-wave dipole is mentioned as an example of an isotropic radiator, with a circular radiation pattern that provides consistent receive strength at a given radius.
  • Participants discuss the potential for significant gain with large parabolic antennas and the use of antenna arrays to achieve varying gain across different angles.
  • One participant provides a relationship between dBi and dBd, suggesting that dBi equals dBd plus 2.15.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of dBi and dBd as they relate to isotropic and dipole antennas, but there is some uncertainty regarding the specifics of dBd. The discussion includes multiple viewpoints on how gain is achieved and the implications of antenna size.

Contextual Notes

Some participants reference external resources for further information, indicating a desire for additional context and examples. There is also mention of a previous homework thread related to antenna array patterns, suggesting a continuity of discussion on this topic.

dervast
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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
 
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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.
 
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
 
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)
 
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