Why Signal Strength is a Negative Number

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Home Forums Android App Known Bugs Why Signal Strength is a Negative Number

This topic contains 2 replies, has 2 voices, and was last updated by  GregTuve 1 year, 11 months ago.

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  • #6104

    GregTuve
    Participant

    Dbm stands for DeciBels per Milliwatt. For a cell phone user, it will always be a negative figure (unless he climbs a local tower). However signal strength can theoretically become positive when the radiated energy amount being collected at any moment exceeds one milliwatt. A milliwatt in visible light is very bright. For example, a typical laser pointer emits about half a milliwatt all in the same direction. You wouldn’t want to point the beam through your pupil very long. Db anything is a relative scale, so it’s like doubling brightness every time you halve the distance: You can always get a little closer or stronger. In such situations, you have to choose an arbitrary “1”, a 2 raised to the N where n=1. Since an instantaneous radiation intensity of one full milliwatt is hopelessly unrealistic/bright, you can simply look at the absolute value of the signal strength (which may include the strength of local interference sources as well). Some bar meters calculate “signal strength” in terms of both radiated energy received and the signal-to-noise ratio also. Everyone knows that five bars is betters than four but one of those bars could be coming from a reflection that is garbling your data hopelessly. Regardless of these details, the smaller the absolute value of the strength number, the better off you are. Each time a signal strength number goes “down” 3 decibels (e.g. from -87 to -83), the amount of radiated energy being collected has doubled. My working estimate of what a minimum signal strength is… is currently -96. Someone else found that -85 was about their minimum, but these figures probably vary depending on encoding schemes. You can have good voice and good TV at a given signal strength with poor or zero data streams. That’s because data modulations fluctuate between 1′s and 0′s like mad so those are the most sensitive to interference and lost information. Because the amount of energy radiated by a local cell tower is not equal in every direction, walking towards the base of a tower will drop your signal strength to zero. At the base, all of the energy is going over your head. I guesstimate that you can’t get closer to a cell tower than about half a mile: That is, if you’re walking across an empty plain, signal strength will begin to decline somewhere around the half-mile mark unless you can find a ridge that brings you up to the antenna height and points towards the tower. Maybe someone in a plains state can tell us.

  • #6753

    Brendan
    Keymaster

    Thanks for the info

  • #6754

    GregTuve
    Participant

    I wanted to edit this to correct some minor errors but the edit button has gone missing. Oh well.

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