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Thread: What's the difference between "dB", "dBi", and "dBm"?

  1. #1
    FPV-The RC modeler dream! HobbyWireless's Avatar
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    What's the difference between "dB", "dBi", and "dBm"?

    We get this question a lot, and many people gets confused and even some of our staff have use it wrongly.
    Here is a very simple and basic explanation about some of the differences.

    dB is just a ratio for gain, and alone it is used to compare two different levels of power "gain" ex. amplifiers outputs "X"dB of the input. It can also be relative to voltage levels.

    dBm is typically used for transmitters output power, but can also be use for amplifiers but it's not same as the ratio for the gain.

    dBi is referenced ISOTROPIC source typically used for antennas. If I write 10dB antenna, I mean nothing specifically, because I have not mentioned what is in reference to.

    These are the most typical ones. There are others such as dBW, dBm, dBd well-know by HAM users, but I don't want to get into.
    We don't know everything on the subject, but if someone else wants to add any extra information without confusing others, feel free to do so.

    Hobby Wireless
    Last edited by HobbyWireless; 19th February 2016 at 02:18 PM.
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  2. #2
    FPV Crew Chief kalikraven's Avatar
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    I think you left out one of the most important ones that I see in the FPV community. Decibels over Isotropic Too often I see antenna's advertised as 2.15 dBi when in reality it has no effective gain at all.

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    FPV-The RC modeler dream! HobbyWireless's Avatar
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    Quote Originally Posted by kalikraven View Post
    I think you left out one of the most important ones that I see in the FPV community. Decibels over Isotropic Too often I see antenna's advertised as 2.15 dBi when in reality it has no effective gain at all.
    There are lot to be talked about it, and there are plenty of good and deep information on the internet on the subject. We just listed some of them in response to the most typical questions we receive about it.
    But anyone is welcome to post additional information on the subject.
    So feel free to explain your point.

    Note: Here is a very informative page, in case of anyone wants to know more about antennas and their definitions.
    http://www.antenna-theory.com/definitions/main.php
    Last edited by HobbyWireless; 22nd February 2016 at 06:35 PM.
    FPV is not just our hobby, it's our passion!

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