# Re: I've lost my k. Can someone help me find it?

```Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
>
> Original poster: "Daniel Barrett by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dbarrett1-at-austin.rr-dot-com>

>     Can someone (Antonio???)  set me straight on how to calculate K given a
> pri/sec assembly's lower (F1)  and upper (F2) resonant frequency?

k=((F2/F1)^2-1)/((F2/F1)^2+1), assuming correct tuning.

>     I'm a 'digital guy' so please don't flame me too bad for not knowing my
> network theory too well ;)
>
>     I got the impression *somewhere* that k = sqrt( F2/F1 - 1 ). However, I
> suspect this may not be correct, and I can't figure out where I found this
> tidbit.

With F2=2*F1 this gives k=1, wrong. The correct value is k=0.6. And the
expression can produce k>1, impossible.

>     And I keep finding what appears to be some contradictory information as
> I seach some the archives.
>
>     I think I understand the 'count the number of half cycles per notch'
> method, but my lack of appropriate HV/HA probe is preventing me from getting
> a clean enough look at the ringdown on the scope to do this reliably...

I prefer this: Look at the primary voltage, count the number of full
cycles until the first notch, and multiply by 2, obtaining "b". Let
a=b-1. k=(b^2-a^2)/(b^2+a^2), and the circuit is operating in "mode"
a-b,
meaning that F2/F1=b/a. The test can be done at low power, with a low-
impedance square wave generator connected across the spark gap.

Antonio Carlos M. de Queiroz

```