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1, dB
dB relative value is a characterization of the value of the ratio of pure, only that the relative size of the relationship between two quantities, no units, when considering the power compared to a B or a small number of large power dB, according to the following formula: 10log (A power / B power), if calculated using both the voltage ratio, use 20log (A voltage / B voltage).
[Example] A B power than double the power, then 10lg (A power / B power) = 10lg2 = 3dB. In other words, a power larger than the B power 3 dB. Conversely, if a power is the power of half of the B, B is a power than the power of a small 3 dB.
2, dBi and dBd
dBi and dBd is the amount of the antenna power gain, both of which are a relative value, but not the same reference point. a reference to dBi omni-directional antenna, dBd reference point for the dipole, so the two are slightly different. Generally believed that the same gain, represented by dBi dBd than that out to a large 2.15.
[Example] For the side of the antenna gain of 16dBd its converted into units of dBi gain when compared 18.15dBi (generally ignore decimal places, as 18dBi).
[Example] 0dBd = 2.15dBi.
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3, dBc
dBc is a relative value of said power unit, and dB is calculated exactly the same. In general, dBc relative to the carrier (Carrier) in terms of power, in many cases, used to measure the relative value of the carrier power, such as used to measure interference (co-channel interference, intermodulation interference, crosstalk, with outside interference, etc.) as well as coupling, stray and so the relative value. Place in the use of dBc, in principle, can also use dB instead.
4, dBm
dBm is an absolute value of said power (1mW power can also be considered based on a ratio basis), calculated as: 10log (power value / 1mw).
[Example] If the power P is 1mw, converted into dBm after 0dBm.
[Example] For 40W of power, according to the dBm value of units after conversion should be:
10log (40W/1mw) = 10log (40000) = 10log4 +10 log10000 = 46dBm.
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5, dBw
And dBm as, dBw is a unit of the absolute value of said power (1W power can also be considered based on a ratio basis), calculated as: 10log (power value / 1w). dBw and dBm the conversion relationships between: 0 dBw = 10log1 W = 10log1000 mw = 30 dBm.
[Example] If the power P is 1w, converted into dBw after 0dBw.
In short, dB, dBi, dBd, dBc is the ratio between two quantities, indicating the relative size between the two quantities, and dBm, dBw is that the absolute size of the power value. In dB, dBm, dBw calculation, pay attention to the basic concepts, with a dBm (or dBw) by another dBm (dBw), the result is dB, such as: 30dBm - 0dBm = 30dB.
Generally speaking, in engineering, dBm (or dBw) and dBm (or dBw) only between addition and subtraction, not multiplication and division. The most used is the subtraction: dBm dBm is actually two power by dividing the signal power and noise power is divided signal to noise ratio (SNR). dBm dBm is actually added two power multiplied.
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