[HBR] HBR-11 IF xformer critcal coupling?

[Hopperdhh at aol.com]

Hi Tom,

Determining the coupling coefficient of IF transformers is  not a trivial 
task.  Two publications I would recommend are the RCA  "Radiotron Designer's 
Handbook" and the Meissner "How to Build" Instruction  Manual.  The latter 
especially goes into detail about the practical  problems of mixed capacitive and 
inductive coupling, as well as how to connect  the transformer so that the 
capacitive and inductive coupling are aiding each  other.  At 100 Kc this is not as 
big of a problem as it is at 455 Kc, of  course.

Yes, the circuit loading affects the coil's loaded Q which is a  factor in 
how much coupling is required for critical coupling.  High  circuit Q needs 
lower coefficient of coupling for critical coupling than low  Q.  The tube manuals 
specify about 1 megohm for Rin and Rout of most  pentode amplifiers.  But in 
high impedance tube circuits even a very small  amount of stray capacitive 
coupling can have a big effect.  So, to know the  percent of critical coupling is 
a difficult problem in the real world.
 
A Q-Meter is the best piece of test equipment for testing coefficient of  
coupling in low frequency IF circuits.  If you pursue this path you  will learn a 
lot about IF amplifiers, but the time could probably be better  spent 
elsewhere!

So, having said all that, I agree with Darrell's  advice!  Build it first and 
go from there.

Dan K9WEK
 
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