A small riddle, just for fun
 

Yesterday, while repairing my antenna, something came to my mind I had never
focused on before.

Let us consider a bipole, that is a "black box" having TWO terminals and
including plain passive elements only (like capacitors, inductors, ... , no
diodes or other special devices), arranged the way you prefer, it does not
matter.

In my mind it was quite clear that, when fitting such a bipole into a circuit,
the sense makes no difference, i.e. one can reverse the two terminals with no
consequence. As a matter of fact, the bipole has an equivalent impedance that
remains the same independently of the way it is put in the circuit.

Yesterday a case occurred to me in which this is not actually true.

Instead of directly telling which it is, just for fun I wonder whether anyone
can figure out a case in which a bipole may not be reversed without
consequences. Not difficult, but it anyway requires some thinking.

Although probably unnecessary, let me recall that a filter is typically a
THREE-terminal device (IN, OUT, GROUND), not a TWO-terminal one.

73

Tony I0JX
Rome, Italy

A small riddle, just for fun
 

On 31 ene, 21:20, "Antonio Vernucci" wrote:
Yesterday, while repairing my antenna, something came to my mind I had never
focused on before.

Let us consider a bipole, that is a "black box" having TWO terminals and
including plain passive elements only (like capacitors, inductors, ... , no
diodes or other special devices), arranged the way you prefer, it does not
matter.

In my mind it was quite clear that, when fitting such a bipole into a circuit,
the sense makes no difference, i.e. one can reverse the two terminals with no
consequence. As a matter of fact, the bipole has an equivalent impedance that
remains the same independently of the way it is put in the circuit.

Yesterday a case occurred to me in which this is not actually true.

Instead of directly telling which it is, just for fun I wonder whether anyone
can figure out a case in which a bipole may not be reversed without
consequences. Not difficult, but it anyway requires some thinking.

Although probably unnecessary, let me recall that a filter is typically a
THREE-terminal device (IN, OUT, GROUND), not a TWO-terminal one.

73

Tony I0JX
Rome, Italy

Hello Tony,

The answer may be in your own text (the ground issue). There may be a
third path via ground (capacitive coupling).

You can add a very good common mode choke at the input terminal of you
bipole. In that case the path via ground is blocked.

Try a very simple bipole: metallic case connected to terminal 1,
terminal 2 connected to nothing. When the center conductor of your
source is connected to terminal 1, you have the ground path.

The effect of reverse connection will be less when you use a very
small battery powered source that is completely floating. Measuring
data you can get out of it via optical link...

Best regards,

Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me.

A small riddle, just for fun
 

The answer may be in your own text (the ground issue). There may be a
third path via ground (capacitive coupling).

Hi Wim,

I acknowledege that your example leads to an asymmetric bipole, that one cannot
reverse it without consequences.

However, in the case which occurred to me, there is no need to assume the
existence of a third path via ground for justifying the asymmetry. Actually the
asimmetry remains even if the bipole would be in free space!

Forgive me if I do not immediately reveal my case, but I would like to see if
there are some more guesses before closing the issue.

73

Tony I0JX

A small riddle, just for fun
 

K1TTT wrote:


as long as it is ONLY 2 ports, has ONLY passive linear components, AND
is small enough to be considered a lumped element, then you can always
reverse the terminals and not know the difference. The most
frequently violated condition is the last one, put in a piece of coax
with that is long enough to be measured at the highest frequency you
will use and all bets are off.

An electrolytic capacitor acts differently if reverse biased. The results
are usually bad. It does qualify as two leaded, passive and linear when used
properly.

A small riddle, just for fun
 

On Jan 31, 9:38*pm, "Antonio Vernucci" wrote:
The answer may be in your own text (the ground issue). There may be a
third path via ground (capacitive coupling).

Hi Wim,

I acknowledege that your example leads to an asymmetric bipole, that one cannot
reverse it without consequences.

However, in the case which occurred to me, there is no need to assume the
existence of a third path via ground for justifying the asymmetry. Actually the
asimmetry remains even if the bipole would be in free space!

Forgive me if I do not immediately reveal my case, but I would like to see if
there are some more guesses before closing the issue.

73

Tony I0JX

as long as it is ONLY 2 ports, has ONLY passive linear components, AND
is small enough to be considered a lumped element, then you can always
reverse the terminals and not know the difference. The most
frequently violated condition is the last one, put in a piece of coax
with that is long enough to be measured at the highest frequency you
will use and all bets are off.

A small riddle, just for fun
 

On Mon, 31 Jan 2011 21:20:57 +0100, "Antonio Vernucci"
wrote:

Instead of directly telling which it is, just for fun I wonder whether anyone
can figure out a case in which a bipole may not be reversed without
consequences. Not difficult, but it anyway requires some thinking.

Hi Antonio,

This would fall into the area of a hidden variable (a ghost terminal,
so-to-speak) and is called Common Mode. This occurs in the situation
that includes the proximity of ground which is often neglected as an
influence.

The dipole could be unbalanced forcing currents to be out of balance.
This is notorious with cabled down leads which preserves the sense of
two terminals, but the imbalance with ground forces a third terminal
into the reality of implementation.

If we were to divorce ground from this (free space) and simply looked
at the two lead impedance at the drive point, then adding a cable will
force the same imbalance (albeit to a different degree, as found with
the proximity of earth). If the dipole is off-center fed, this will
be more profound (even though when it is measured at the feed point,
the measurement is immune to pole switching).

Reversing the "polarity" of the cable connection will create a new
topology (although the Common Mode will persist with new
characteristics).

Although probably unnecessary, let me recall that a filter is typically a
THREE-terminal device (IN, OUT, GROUND), not a TWO-terminal one.

Well, even though I have anticipated ground, a filter can easily be a
two terminal device without need for ground - being a series
component. This two terminal description is quite typical too.

73's
Richard Clark, KB7QHC

A small riddle, just for fun
 

On Mon, 31 Jan 2011 17:23:33 -0600, joe wrote:

An electrolytic capacitor acts differently if reverse biased. The results
are usually bad. It does qualify as two leaded, passive and linear when used
properly.

Hi Joe,

The same can be said of a diode. Hence the electrolytic capacitor is
non-linear, which violates the premise.

73's
Richard Clark, KB7QHC

A small riddle, just for fun
 

On Jan 31, 11:23*pm, joe wrote:
K1TTT wrote:

as long as it is ONLY 2 ports, has ONLY passive linear components, AND
is small enough to be considered a lumped element, then you can always
reverse the terminals and not know the difference. *The most
frequently violated condition is the last one, put in a piece of coax
with that is long enough to be measured at the highest frequency you
will use and all bets are off.

An electrolytic capacitor acts differently if reverse biased. The results
are usually bad. It does qualify as two leaded, passive and linear when used
properly.

an electrolytic that acts that way is not a linear component.

A small riddle, just for fun
 

"Antonio Vernucci" ha scritto nel messaggio
...
Forgive me if I do not immediately reveal my case, but I would like to see
if there are some more guesses before closing the issue.

From my very poor knowledge of trasmission lines...
... reversing inner and outer conductor of a coaxial cable ?? :)


-.-. --.-

A small riddle, just for fun
 

On Jan 31, 12:20*pm, "Antonio Vernucci" wrote:
Yesterday, while repairing my antenna, something came to my mind I had never
focused on before.

Let us consider a bipole, that is a "black box" having TWO terminals and
including plain passive elements only (like capacitors, inductors, ... , no
diodes or other special devices), arranged the way you prefer, it does not
matter.

In my mind it was quite clear that, when fitting such a bipole into a circuit,
the sense makes no difference, i.e. one can reverse the two terminals with no
consequence. As a matter of fact, the bipole has an equivalent impedance that
remains the same independently of the way it is put in the circuit.

Yesterday a case occurred to me in which this is not actually true.

Instead of directly telling which it is, just for fun I wonder whether anyone
can figure out a case in which a bipole may not be reversed without
consequences. Not difficult, but it anyway requires some thinking.

Although probably unnecessary, let me recall that a filter is typically a
THREE-terminal device (IN, OUT, GROUND), not a TWO-terminal one.

73

Tony I0JX
Rome, Italy

I have a circuit I've been working on lately which has a simple series
LC in it, no other connection to the node between the inductor and
capacitor. It turns out that the order of the inductor and capacitor
makes a big difference in the circuit performance. I anticipated that
it would, and put them in the intuitively obvious order, only to find
out that it was the wrong order! A proper model cleared things up
quite nicely. However, in no way would I call that particular part of
the circuit a "two terminal" network. The effect is the same as Wim
mentioned.

Cheers,
Tom