wrote in message
...
On Dec 20, 4:32 am, Ian White GM3SEK wrote:
Cecil Moore wrote:
But the rules for black boxes do not allow measurements
on the inside. This is how they help clarify the thinking.

So instead of sweeping technical facts under the rug,
you hide them in a black box. In both cases, the only
apparent purpose is to maintain ignorance.

It seems that whatever part of the system you don't
understand, you draw a black box around it so you
don't have to understand it.

No, it is a perfectly normal technique to test a theory or model. The
black box reveals just enough information to solve the problem, and
nothing more.

In this particular case, the impedance at the terminals of the black box
is the only *necessary* information to solve the transmission-line
problem (in the steady state, at one frequency). It is not necessary to
know how that impedance was created.

But Ian,

Suppose the box is labeled -j567 ohms.

Then I ask, "at what frequency is this impedance -j567?".

I find that the impedance for -j567 ohms is 4 Mhz.

Now I take a length of 600 ohm VF = 1 transmission line and vary the
length until I am at resonance with whatever is in the black box at 4
MHz. Resonance would imply 90 degrees total phase shift.

My measurement shows that the length of 600 ohm line to cause this
effect is 43 degrees.

Assuming my measurement is correct, doesn't that tell us a little more
about what is inside the box? It isn't just "any" -j567 ohm impedance
that can cause resonance with a 43 degree 600 ohm line. It is probably
not a discreet capacitor, it would likely be some sort of transmission
line or something that that has 10 deg length, correct?

With a few more measurements, we can determine the Zo of the
transmission line that "appears' to be in the black box, correct and
essentially verify that it a transmission line. We should be able to
both measure and calculate Zo.

If we choose our independent measurements carefully enough, we should
be able to define exactly what is in the black box with only 2
terminals.

I agree you need more than a smith chart (which was where I made my
mistake before).

AI4QJ



No, you can't. if the frequency is fixed, is sinusoidal, and steady state,
then every box that measures -j567 ohms is perfectly equal. that is the
whole idea of a 'black box' not only can't you tell what is inside, it
doesn't matter what you do on the outside, it will always looks the same.
that is the whole purpose of it, you reduce a part of the circuit to a
single component that has well known performance so you remove that part
from the problem.

Dave wrote:
if the frequency is fixed, is sinusoidal, and steady state,
then every box that measures -j567 ohms is perfectly equal.

If the goal is to measure the phase shift at an
impedance discontinuity in a transmission line, why
would someone deliberately put the impedance discontinuity
inside a black box? That defeats the goal of measuring
the phase shift.
--
73, Cecil http://www.w5dxp.com