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Thread: What does dBi represent for antennas?

  1. #1
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    What does dBi represent for antennas?

    I was given a D-Link wireless router model WBR-1310 which I can probably use somewhere, but there's no antenna. I did some shopping, and some are 2 dBi, others 5 dBi or 7 dBi. Will a higher dBi allow for better reception?
    Give a man a fish and you feed him for a day. Teach him how to use the Internet and he won't bother you for weeks.

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  2. #2
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    dBi is a representation of relative power.

    The unit power is relative to a theoretical antenna, called an Isotrophic antenna (or radiator), which is a conceptual point source in space where all energy is radiated from that point equally in 3-dimensional space (similar to how a star radiates light).

    dB is on a log scale so when comparing the ratings, you have to consider that. Every 3 dB is a doubling, so 2 dBi vs. 5 dBi is twice in both receive and transmit gain (antennas are typically reciprocal).

    In this context, higher dBi is usually better, but it depends. The way an antenna achieves gain is by focusing the energy. An example is how a flashlight reflector focuses the light into a narrow beam (a satellite dish does this).

    Higher gain "rod" antennas typically squish the radiation pattern and make it from a sphere (or 1/2 sphere) into something flatter, like a pancake shape in an extreme case. This might be good if your on one story, but if on two floors directly above/below, then it might work better by orienting the antenna horizontal.

  3. #3
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    I found an antenna that fits on the router from a wireless PCI NIC that I'm not using. Is there any way to determine the dBi of this antenna?
    Give a man a fish and you feed him for a day. Teach him how to use the Internet and he won't bother you for weeks.

    Relztrah

  4. #4
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    Will a higher dBi allow for better reception?
    Define (rhetorical) "better" !?

    Just as with digital television reception the detection of wifi data packets is more-or-less an all-or-none phenomenom. There may well be a particular range at which there is distortion/"pixilation" but in the main reception must either be complete or non-existent. Analog data is more accessible to some sort of reception even if there is interference be that with music or with picture transmission and reception or whatever. If one is getting good digital reception then increasing the gain will not make anything better.

    Is there any way to determine the dBi of this antenna?
    There are basically two approaches.

    Firstly there is equipment that can measure RF outputs but they are mostly for technical use and need knowledge of how to interpret the results. Such Digital Field Meters come in all sorts of degrees of elaboration and expense.

    Secondly is the empirical approach. You move the transmitter and receiver further and further apart (in the correct orientation and in line of sight) until the signal is lost and back-calculate the relevant gain. In the real world this is what most punters want to know and not how much the actual dB have been affected.

    PS This Flash Tutorial is only a bit related but none-the-less quite well put together.
    Last edited by Paul Komski; 05-05-2010 at 04:28 AM.
    Take nice care of yourselves - Paul - ♪ -
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  5. #5
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    Yea, why not see if it works for you? Does the dBi matter in that case? It is probably 1 or 2 dBi.

    If you go to buy an antenna from Radio shack or wherever, you will undoubtedly get a better antenna than that from a cheap PCI nic.

  6. #6
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    You can determine gain by the design of the antenna. For example, a dipole antenna will always have about 2.15 dBi gain. A biquad antenna will have about 10 dBi gain. etc. Antenna modeling programs do a fairly good job at calculating antenna gains.

    An RF field strength meter may give you an indication and a spectrum analyzer will do a better job of it, if you want to actually measure it.


    The further I walk away from my router with my laptop, the slower my throughput goes from 11Mbps down to the kbps range. If I put a higher gain antenna on my router, I would expect better reception (and transmission), or at least faster connection speeds, from farther away.

  7. #7
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    Well, this has indeed been an education and I appreciate the input from all contributors.
    Give a man a fish and you feed him for a day. Teach him how to use the Internet and he won't bother you for weeks.

    Relztrah

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