On 1/29/2014 4:12 PM, Fred Abse wrote:
On Tue, 28 Jan 2014 13:03:35 -0600, amdx wrote:

I have beads* on a coax and want to know the R and the L.
I have measured the R at 3.85MHz, It is 3,350 ohms. I have also measured
the phase shift, voltage leading by 17ns. The period of 3.85Mhz is 260ns.

I'm not sure I understand what you're trying to measure. I assume that
it's the effect of the "beads" on the inductance of the outer conductor,

That is correct.

in which case, 3350 ohms looks way OTT. No pot core material I know of is
*that* lossy.


That's what everyone seems to think. I'm the odd man out.
If you haven't looked at the picture, here it is. The ferrite is 18
inches long. That is a quarter in front.
http://s395.photobucket.com/user/Qma...40099.jpg.html



How long is the coax sample? 17ns is the delay of approx. 5 meters of wire
at 3.85MHz, without any inductive loading.

It is 11 ft long.

You did connect to the outer conductor at both ends, didn't you.

Yes, outer conductor connected at both ends. To different inputs of the
measuring device.

Preferably short inner to outer, at both ends.

That I have not done. I'll try that.
........

.......

OK, I'm back. Shorting the center pin to the shield, (at both ends)
made absolutely no difference in the magnitude or phase of the measurement.



I want to calculate the impedance of the reactance.

LOL, I had to go back and see if I said that.

Now I'll say what I meant.
I want to know the magnitude in ohms of the reactance.
John S already solved that for me, thanks.
He knows how to solve for what I want, not what I ask for. :-)


There's no such thing. Reactance is merely the imaginary part of a
(complex) impedance.

Already restated.


Can anyone solve this for me?
I would like to see the math, because I want to measure again at 7.5MHz.

My first step was to find the phase angle, 23.5*.
Do we agree there?

Lets look at what you have:

You have voltage/current=3350 ohms. That is the *magnitude* of the
impedance, at an angle of 23.5 degrees, current lagging.

That's 3350 angle 23.5 ohms.

We now do a polar to rectangular conversion on that, giving:

3072 +j1335 ohms.

Good, agreement with John S.

3072 seems way too high for the loss component, 1335 ohms is 55
microhenries,at 3.85MHz.

Everyone agrees the loss component is to high. Oh, except me. :-)
I think I have stated, I thought the L would be higher than the R.
That's not what I'm measuring.
I have no experience in ferrite losses, and no education regarding
losses in ferrite. But I think my measurement are in the ballpark.


One thing I suggest is that you do the whole thing again, without, and
then with, the "beads". That way, you can eliminate propagation delays.

I'll try another piece of RG-58/U, I can't get the ferrite of the cable
without cutting off a PL259.

This evening I'll wind a 55uH coil and find a 3,072 resistor. I'll put
these in series and see how it measures compared to my lossy ferrites
beads on a cable.

I already know this measures about 6% high, probably because of the
sense resistor.
Thanks, Mikek