AI4QJ wrote:
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.

That isn't quite the concept. The claim that we're testing is that, at
the one frequency where all of the mystery boxes have the same
impedance, you will not be able to distinguish between them using
steady-state measurement techniques. Therefore we have to tell you what
that frequency is (if we let you find it out for yourself, using
variable-frequency test equipment, we'd all be stepping outside the
agreed rules).

So the labels on all the mystery boxes already say "-j567 at 4MHz", and
that will be the only frequency at which you can make measurements.

My point (and Roy's, and Dave's, and that of any textbook author) would
be that there is categorically no way to tell these boxes apart by means
of a steady-state impedance measurement at 4MHz.

To tell them apart, you would have to do something *else* - for example,
make more measurements at other frequencies, use time-delayed
reflectometry, or pry the boxes open.


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.

"At resonance with" is not the right concept; see below.

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?

No. You have indeed confirmed that this particular length of this
particular type of line is one of the myriad possibilities for what
*could* be inside the mystery box. But you haven't added anything to
your information about what actually *is* in there.


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?

Not correct - you have jumped to a conclusion about things being "at
resonance" with one another. All we're saying is that the impedance has
the same *value* at a given frequency. That doesn't imply resonance in
any way.

Would it help if I confirmed that one of the boxes genuinely does
contain a discrete capacitor of... what would it be, 70.17pF? No, it
wouldn't help at all, because there is still no way you could tell which
one it is, without stepping outside of the agreed rules.

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.

At one single frequency, there aren't any "independent" steady-state
measurements that could help you (independent of what?). As Roy says,
whatever methods you propose will not hold up to detailed examination,
because it fundamentally cannot be done.

The fundamental point is that impedances of the same value can always be
substituted for one another, and at a given frequency there is no
steady-state measurement that distinguish between them. That
substitution principle is what allows us to use a dummy load to make
tests on a transmitter, and it forms the basis for impedance bridges and
most other forms of impedance measurement.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
My point (and Roy's, and Dave's, and that of any textbook author) would
be that there is categorically no way to tell these boxes apart by means
of a steady-state impedance measurement at 4MHz.

Better yet, you can increase the ignorance level even farther
by prohibiting anyone from applying a signal. Then even empty
black boxes cannot be distinguished.

The fundamental point is that impedances of the same value can always be
substituted for one another, and at a given frequency there is no
steady-state measurement that distinguish between them. That
substitution principle is what allows us to use a dummy load to make
tests on a transmitter, and it forms the basis for impedance bridges and
most other forms of impedance measurement.

Methinks a field strength meter will distinguish between
a dummy load and an antenna. :-)
--
73, Cecil http://www.w5dxp.com