[Antennas] Q

Sat, 12 Oct 2002 11:42:57 -0500

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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