The Q-value of a complete pi-tank circuit can be solved
by ordinary circuit theory by modeling the circuit
as a transducer between the resistive plate impedance
of the final tube and a 50 ohm port.
An expression can then be derived that approximately relates the voltage
reflection coefficient rho as seen looking into the network from the load
end to circuit Q.
This expression is:
Q = (1/2)*(fo/|f1-f2|)*SQR(((1/(1-rho))^2)-1)
where fo is the center frequency of the network, f1 is the higher frequency
for a given rho and f2 is the lower frequency.
SWR is related to rho as rho = SWR-1/SWR+1
This expression only holds for reasonably symmetrical
response curves, which implies that both the circuit Q must be above about
10 and that the frequency excursions should not be toolarge.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Mikael Larsmark" <mike@sm3wmv.com>
To: <amps@contesting.com>
Sent: Thursday, January 05, 2012 4:52 PM
Subject: [Amps] Q-value of a plate tank
> Hi everybody
>
> I have been wondering a bit about how to actually measure the Q of the
> plate tank of an amplifier. Of course I have been looking around a bit
> and I have two different things that kind of contradicts each other, at
> least from what I understand. Or I have missed something.
>
> First, there is the method of using for example an MFJ 259B, which is
> explained here,
> http://web.archiveorange.com/archive/v/0RLC1crR63v6fj2qn5uL
>
> By this method they use the 2:1 VSWR bandwidth to determine the
> bandwidth of the filter and thereby calculate the Q-value. This is OK if
> it is actually the "antenna bandwidth" that does determine the Q-value
> in a Pi-tank.
>
> Another method is to use the same as for example when measuring the
> Q-value of a BPF, where we use the -3dB point as a measure of the
> bandwidth (half power).
>
> But for me 2:1 bandwidth and -3dB point is not the same, or have I
> missed something?
>
> Mike, SM2WMV (SJ2W)
> http://www.sj2w.se/contest/
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>
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