Half power point
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half-power point
(′haf ¦pau̇·ər ′pöint)
(electronics) A point on the graph of some quantity in an antenna, network, or control system, versus frequency, distance, or some other variable at which the power is half that of a nearby point at which power is a maximum. half-power frequency
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Half power point
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half power point
A frequency at which the power is 50% of maximum. This corresponds to 70.7% of maximum current or voltage.
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Half power point
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The half power point of an electronic amplifier stage is that frequency at which the output power has dropped to half of its mid-band level. Other names include the cutoff frequency.
This occurs when the output voltage has dropped by 1/√2 or 0.707 (approximately 1.5 dB) and the power has dropped by half (approximately 3 dB). A bandpass amplifier will have 2 half power points, whilst a low pass amplifier will have only one. A high pass amplifier stage will have only the lower half power point.
The bandwidth of an amplifier is usually defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth.
3 dB
The half power point is approximately 3 dB because 
so 
the decibel measure of a ratio r is defined as 
This mathematical coincidence also means that 
(precisely, 
taking the logarithm of this equation yields 
). This approximation is notably used in loose usage of the prefix kilo- (and associated mega-, giga-, tera-, etc.) in computing to refer either to 
or 
– see kilobyte.
Using 3 dB rather than the correct value of 3.010... yields a power factor of 
which differs from a factor of 2 by about 0.24%. As logarithmic errors add, using 6 dB to approximate a factor of 4 difference yields an error of about 0.48%, and so forth.
Antennas
The half power point or 3 dB point of an antenna beam is the angle of boresight at which the antenna gain has fallen 3 dB below the peak.
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