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Thread: Graph height

  1. #1

    Default Graph height

    I'm using inSSIDer to help figute out which wifi channel to use. The 2.4ghz graph appears to show the signal strength of each other AP superimposed rather than added. In other words, if a channel has routers at -40, -50 and -60, the highest point on the graph is -40 rather than something higher showing the total signal on the channel.

    I would have thought showing the total signal strength on a channel would be more useful for figuring out which is the best channel for me to use. Or is the best technique to ignore the graph and try channels 1, 6 and 11 and see which results in the highest signal strength for me? Or am I misunderstanding?

    BTW, I have a lot of APs in the neighborhood

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  2. #2


    You should bear in mind that dBm is logarithmic, so doubling power adds almost exactly 3 dB, quadrupling power adds 6dB and so on. So, for example, even if two channels reached you at -40 dBm, the sum of those two channels would be only 3 dB higher, at -37 dBm.

    In your example, channel 1 shows -50 dBm and -63 dBm as the two biggest signals:
    Ch1 power ~= 10^(-50/10) + 10^(-63/10) = 0.0000105 mW
    Converting back to dBm:
    Ch1 total dBm ~= 10 x log10(0.0000105) = -49.8 dBm
    (all rounded to 3 significant figures)

    That difference is barely bigger than the largest signal at -50 dBm, because it's 20 times more powerful than the next largest.

    So, for simplicity, looking at the top signal in any channel is pretty accurate, in fact.


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