[AMPS] Tuned Input Circuit Q

[Tom Rauch]

> Is there a formula for calculating tuned input circuit Q. I see a
> chart in Bill Orr,s Handbook for various input impedances with Q,s of
> 2-3. Im probably missing something here. How would you calculate for a
> tuned input Q of 5. Im trying to learn so if its a dumb question I
> apologize for my lack of experience. Is it roughly Rin/xc1? Im curious
> as how to arrive at the values for a tuned input pi net for a given Q.
> 73,s Jeff

Hi Jeff,

Most Handbooks use the Q of one end of the network as the Q, but 
that is actually incorrect. The actual Q involves all parts of the 
system.

Eimac almost certainly is using the entire network's Q, rather than 
the Q at one end of the network. 

The nice thing is Q has very little effect on the system, it is fairly 
non-critical as long as the Q is more than 1+ the square root of the 
ratio of the impedances. Less than that and the network won't act 
like a pi. To behave like a pi, the network has to look like a step 
down and step up L network connected back to back. In other 
words, phase shift has to be somewhat more than 90 degrees and 
the impedance someplace in the network to ground has to be less 
than the lowest of the source and load impedances.

In many cases I have looked at, the "simple Q" of two is nearly an 
overall Q of five. Orr and Eimac actually somewhat agree. 

Other than matching, which sets the minimum usable Q, the only 
other reason for a high input system Q is to provide a reasonably 
low shunt impedance for harmonics. That prevents the cathode of 
the tube from moving around at harmonics caused by the shorter 
than 360 degree conduction angle of the tube. Another thing is if 
any harmonics from the shorter than 360 degree conduction angle 
of the tube reach a solid state exciter, they can "fool" the SWR 
shutdown circuit into thinking the RF is being delivered to a 
mismatched load. 

Because of this, all input circuits should be a low pass C-L-C pi 
network or a parallel tuned network, and not any form of  "T" or an 
L-C-L high-pass pi network.

Fortunately Q is very non-critical in most applications, that is why 
so many approximations and rules-of-thumb work.
73, Tom W8JI
W8JI@contesting.com 

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