[Antennas] Q
Dave Shrader
[email protected]
2002年10月12日 13:53:35 -0400
As mentioned below, for resonant circuits the Q is determined from the
-3 dB points. That is a VSWR of 3.65 to 1 for an antenna.
However, common usage in amateur circles defines antenna bandwidth [Q]
not as the -3dB point but at the 2:1 VSWR point.
Deacon Dave, W1MCE
Manager, Textron Antenna Range, retired
--------------------
"Douglas J. Koehler" wrote:
>> Q of an inductor is defined as the inductive reactance at a specific
> frequency divided by the coil's internal resistance.
>> A coil having 200 ohms of reactance and 100 ohms of internal resistance
> would have a Q of 2 (200/100 = 2)
>> If you are given is the inductor's value and the operating frequency, you
> must calculate the reactance by X=2*(pi)*f*L.
>> For a 75 microHenry coil at 3.5 MegaHertz with 60 ohms of internal
> resistance you would calculate it thus:
>> First find the reactance: X = 2*(pi)*3500000*.000075 = 1649.3
>> Then divide by the resistance: Q = 1649.3 / 60 = 27.47
>> For resonant circuits, Q is defined as the resonant frequency divided by
> the bandwidth, bandwidth being defined as the half-power (-3db) points on
> either side of the resonant frequency.
>> Thus a coil resonant at 450 MHz whose half power points are 449 and 451 MHz
> will have a bandwidth of 2 MHz (451-449 = 2) and a Q of 225 (450/2 = 225)
>> At 10:58 AM 10/12/2002, you wrote:
> >Where do I find a good definition and explanation of "Q"?
> >
> >WD8OKN
> >Michigan
> >
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