Re: K Factor & Mutual Inductance

            Re: K Factor & Mutual Inductance
            Wed, 2 Apr 1997 14:08:26 +1200
            "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
            Wellington Polytechnic, NZ

       I'm going to have to throw all my textbooks away :( after 
reading this....

> One equation for the mutual inductance of air core concentric solenoids
> is
> shown in Terman's "Radio Engineers Handbook". Once the mutual inductance
> Lm
> is found the K Factor can be found by the equation
>           K Factor = Lm/(sqrt(LpLs))
> where Lp and Ls are the inductances of the primary and secondary coils.

That equation is good for any coil - air-cored, iron cored or 
whatever? There are no conditions stated by Terman or anyone else to 
my knowledge (except JHC perhaps ;)

> Note that the K Factor or coupling is not only dependent on the physical
> location of the coils but also on several other parameters such as
> resonant
> frequency, Q Factor, log decrement, etc.

Where is the frequency dependence in that equation? Lp? Ls? M? k? I've
not observed any frequency dependence in Lp or Ls from DC to TC 
frequencies in all the measurements I've done. So it must be in k or 
M, right? But what defines either of those? Is the number of flux 
linkages between two coils frequency dependent? Why?


> Also, the critical coupling can
> be
> found when these parameters are coordinated to give Rp = Rs where Rp and
> Rs
> are the primary and secondary effective resistances.

> The JHCTES computer program determines the K Factor and other parameters
> in
> this manner. The results have been found to agree closely with tests of
> actual coils. To my knowledge this is the only computer program
> presently
> available that utilizes this method of finding these parameters. 
>  John Couture