its where the gain is max and stable
3dB of gain rolloff is a linear reduction of gain by a factor of 2. That makes it a good reference point.
In a 3dB coupler, the "dB" stands for decibels, which is a unit to measure the attenuation or gain of a signal. The 3dB value indicates that the signal power is divided equally among the output ports, resulting in a 3dB loss compared to the input signal power.
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Bandwidth is typically measured from the two -3dB points on each end of the response curve. You find the two points where the response is -3dB (half power) and measure the distance between them. That is your bandwidth.
-3db equates to half of the original power. if you had 1 watt input. and the output is 1/2 watt. The power gain is calculated by 10 log (Pin/Pout) ( in bels its log(Pin/Pout) in decibels its 10 log (Pin/Pout) ) That is 10 log (1/ 1/2) = 10 log 2 = -3 that's -3db for you It communications and in so many other fields. the 3db mark is taken as kind of datum for useful power. that is to say beyond this the magnitude will reduce below half.
3 db is double the power, so an input of 1 KW would yield an output of 2 KW for a 3 db gain.
DB (decibels) gain is the log based 2 times 3 relative power change measured from input to output of a circuit. For instance, +3db is twice the power, +6db is four times, +9db is eight times. Similarly, -3db is half the power, -6db is quarter the power, and -9 db is eighth the power.Keep in mind that DB is relative power, not voltage, so if you are measuring voltage, then +3db is square root of 2 (1.414) times the voltage, etc.
If u see the frequency response of an amplifier, initially the gain increases, then remains constant for some time and then decreases. If we draw a line 3dB below the contant frequency region we get two point , one at the increasing gain region and the other at the decreasing gain region. The frequency range between these two points respectively called as fl and fh is called midband frequency range.
bandwidth refers to the amount of the frequency spectrum that a signal resides in.example, the spacing between channels on radio are determined by their bandwidth. beamwidth is a physical characteristic of an antenna. they generally mark the point bore sight of a directional antenna that are at -3dB or half power
50% Coupler :D
3Db