Re: Can twisted wire replace shielded wire?

[Cliff]

Characteristic impedance and DC resistance are the same if the line is
infinitely long and unterminated.

How do you arrive at that one?

 

[John Fields]

How do you arrive at that one?

---

+V>---O
|
S1 | <--O--[AMMETER]--+--[INFINITELY LONG COAX CENTER CONDUCTOR]-
|
[VOLTMETER]
|
GND>---------------------+--[INFINITELY LONG COAX SHIELD------------

1. Close S1

2. Read the ammeter and record what you read as 'I'

3. Read the voltmeter and record what you read as 'E'

4. Calculate the resistance of the cable:

E
R = ---
I

5. Since a DC source is driving the cable and the voltage and
current remain constant, there must be no reactive terms which
need to be considered, so:

Z = sqrt R² = R

Thinking about it a little differently, if you consider that when
you close the switch the inductance of the center conductor is
limiting the current that can charge the incremental capacitance of
the cable as the edge propagates down the cable, then that limit and
that capacitance define how much current the voltage source driving
the cable needs to supply while the edge is moving down the cable,
and, since

E
R = ---
I

we wind up back where we started.

 

[DaveB]

How do you arrive at that one?

---

+V>---O
|
S1 | <--O--[AMMETER]--+--[INFINITELY LONG COAX CENTER CONDUCTOR]-
|
[VOLTMETER]
|
GND>---------------------+--[INFINITELY LONG COAX SHIELD------------

1. Close S1

2. Read the ammeter and record what you read as 'I'

3. Read the voltmeter and record what you read as 'E'

4. Calculate the resistance of the cable:

E
R = ---
I

5. Since a DC source is driving the cable and the voltage and
current remain constant, there must be no reactive terms which
need to be considered, so:

Z = sqrt R² = R

Thinking about it a little differently, if you consider that when
you close the switch the inductance of the center conductor is
limiting the current that can charge the incremental capacitance of
the cable as the edge propagates down the cable, then that limit and
that capacitance define how much current the voltage source driving
the cable needs to supply while the edge is moving down the cable,
and, since

E
R = ---
I

we wind up back where we started.

Guess that sums it up

Best
Daveb

 

[David]

David wrote:




Bullshit.

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

 

[Fred Abse]

David wrote:




Bullshit.

If you mean by "DC resistance", the impedance you would see at the input
of an infinitely long line, it's not bullshit, it's perfectly true, even
if the line is made of zero-resistance conductors. You'll just see the
line's characteristic impedance.

 

[Fred Abse]

So the longer conductors are not shorter after all? How long
do you suppose that they are compared to the central conductor ???
Now, about the speed of light in those conductors ... a lead or
a lag between the signal's phases?

What matters is the speed of light in the dielectric *between* the
conductors. That's where the traveling electromagnetic wave is.

 

[Gary]

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

For god 'amighty sake, read your own reference and then read what I
wrote again. Now, read your reference *completely* again.
*Characteristic Impedance*. In your reference, they give exactly the
same formula as I did.


Jeeze.

--
Regards,
Gary

I know not with what weapons World War III will be fought,
but World War IV will be fought with sticks and stones.

Albert Einstein

 

[Gary]

Fred Abse wrote:

If you mean by "DC resistance", the impedance you would see at the input
of an infinitely long line, it's not bullshit, it's perfectly true, even
if the line is made of zero-resistance conductors. You'll just see the
line's characteristic impedance.

*Please* , read my post on calculation of characteristic impedance. I
am not talking about Infinite length transmission lines. I am talking
about the calculation of impedance.
Fer krissake, keep the discussion in context.
What do I give a shit about infinitely long lines. I don't use 'em.
Valid only if you want to discuss voltage and current distribution.
That's not where this started.



--
Regards,
Gary

I know not with what weapons World War III will be fought,
but World War IV will be fought with sticks and stones.

Albert Einstein

 

[VWWall]

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

Quite true if you define DC resistance as the applied voltagr divided by
the, (constant for an infimite line), charging current.

As a practical matter, to match a generator to a given line, you can use
a long length of co-ax cable to simulate a load at the cable's
characteristic impedance. The line needs to be long enough to have
~10-20 db loss at the frequency in use.

 

[Gary]

Fred Abse wrote:

If you mean by "DC resistance", the impedance you would see at the input
of an infinitely long line, it's not bullshit, it's perfectly true, even
if the line is made of zero-resistance conductors. You'll just see the
line's characteristic impedance.

You can simulate the same "infinite length transmission line" by
terminating a transmission line into a *purely* resistive load whose
resistance is exactly the same as the characteristic impedance of the
transmission line.

That's the purpose of overall system matching. Maximum transfer of
energy from generator to load. The load may be either a piece of
equipment, a dummy load or an antenna. In either case we attempt to
remove all reactive components and make them appear truly resistive. So
what???

That doesn't change the characteristic impedance of the transmission one
"tic". The object of the exercise here was to show that a 75 Ohm line
mated to a 50 Ohm line does not produce an "insertion" loss but instead
results in a "reflection" loss due to mismatch.

I have no idea how the infinite line blurb got in here at all. True
though it may be, it's of no consequence other than to show voltage and
current distribution and that was not part of the original problem.


--
Regards,
Gary

I know not with what weapons World War III will be fought,
but World War IV will be fought with sticks and stones.

Albert Einstein

 

[David]

For god 'amighty sake, read your own reference and then read what I
wrote again. Now, read your reference *completely* again.
*Characteristic Impedance*. In your reference, they give exactly the
same formula as I did.


Jeeze.

So why are you all freaked out?

''REVIEW:
A transmission line is a pair of parallel conductors exhibiting
certain characteristics due to distributed capacitance and inductance
along its length.
When a voltage is suddenly applied to one end of a transmission line,
both a voltage "wave" and a current "wave" propagate along the line at
nearly light speed.
If a DC voltage is applied to one end of an infinitely long
transmission line, the line will draw current from the DC source as
though it were a constant resistance.
The characteristic impedance (Z0) of a transmission line is the
resistance it would exhibit if it were infinite in length. This is
entirely different from leakage resistance of the dielectric
separating the two conductors, and the metallic resistance of the
wires themselves. Characteristic impedance is purely a function of the
capacitance and inductance distributed along the line's length, and
would exist even if the dielectric were perfect (infinite parallel
resistance) and the wires superconducting (zero series resistance).
Velocity factor is a fractional value relating a transmission line's
propagation speed to the speed of light in a vacuum. Values range
between 0.66 and 0.80 for typical two-wire lines and coaxial cables.
For any cable type, it is equal to the reciprocal (1/x) of the square
root of the relative permittivity of the cable's insulation.''

 

[David]

Quite true if you define DC resistance as the applied voltagr divided by
the, (constant for an infimite line), charging current.

As a practical matter, to match a generator to a given line, you can use
a long length of co-ax cable to simulate a load at the cable's
characteristic impedance. The line needs to be long enough to have
~10-20 db loss at the frequency in use.

Can it be on a spool?

 

[John Fields]

Fred Abse wrote:



*Please* , read my post on calculation of characteristic impedance. I
am not talking about Infinite length transmission lines. I am talking
about the calculation of impedance.
Fer krissake, keep the discussion in context.
What do I give a shit about infinitely long lines. I don't use 'em.
Valid only if you want to discuss voltage and current distribution.
That's not where this started.

---
Regardless, if you want to talk about the charachteristic impedance
of a peculiar transmission line, you _have_ to start with an
infinitely long unterminated line.

If you want to talk about something shorter, then you need to
terminate the line with a resistance equal to the generator's
resistance.

Can you understand why that's so?

 

[Paul Hovnanian P.E.]

["Followup-To:" header set to sci.electronics.basics.]

Twisting wires helps reduce noise induced from ambient H fields. If you
have a high ambient E field, you can still get common mode noise coupled
to a bare twisted pair. Particularly on a high impedance circuit. This
becomes a problem at low signal levels, where a CMRR sufficient to
reject the noise may be impractical. Low signal level circuits are where
one finds high impedances as well. In this case, carefully designed
shielding can help.

transformers typically have very a high CMRR. this may be whay they are used
in network cards.

Yes. But some circuits must be DC coupled.

 

[Cliff]

What matters is the speed of light in the dielectric *between* the
conductors. That's where the traveling electromagnetic wave is.

So if all we have is a bare wire no current can flow?

 

[Cliff]

Characteristic impedance is purely a function of the
capacitance and inductance distributed along the line's length, and
would exist even if the dielectric were perfect (infinite parallel
resistance) and the wires superconducting (zero series resistance).
Velocity factor is a fractional value relating a transmission line's
propagation speed to the speed of light in a vacuum. Values range
between 0.66 and 0.80 for typical two-wire lines and coaxial cables.
For any cable type, it is equal to the reciprocal (1/x) of the square
root of the relative permittivity of the cable's insulation.''

How odd then that the speed of propagation in conductors
varies.

 

[Cliff]

How do you arrive at that one?

---

+V>---O
|
S1 | <--O--[AMMETER]--+--[INFINITELY LONG COAX CENTER CONDUCTOR]-
|
[VOLTMETER]
|
GND>---------------------+--[INFINITELY LONG COAX SHIELD------------

1. Close S1

2. Read the ammeter and record what you read as 'I'

3. Read the voltmeter and record what you read as 'E'

4. Calculate the resistance of the cable:

E
R = ---
I

5. Since a DC source is driving the cable and the voltage and
current remain constant, there must be no reactive terms which
need to be considered, so:

Z = sqrt R² = R

Thinking about it a little differently, if you consider that when
you close the switch the inductance of the center conductor is
limiting the current that can charge the incremental capacitance of
the cable as the edge propagates down the cable, then that limit and
that capacitance define how much current the voltage source driving
the cable needs to supply while the edge is moving down the cable,
and, since

E
R = ---
I

we wind up back where we started.

In the DC case the current would drop off with time
as the capacitance near the source becomes charged
and resistive effects predominate.

 

[David]

So if all we have is a bare wire no current can flow?

Correct. There must be a return path to the generator or battery.

 

[David]

I think most of you people are somewhat confused when dealing with the
characteristics of transmission lines of all types. For example, the
mixing of impedances as indicated above results in a "return loss" or
reflection loss. There also ohmic losses in transmission lines as well
as dielectric losses. Impedance is a constant characteristic of a
coaxial transmission, not a variable. It is a fixed quantity and is
calculated in the following manner.

Characteristic impedance and DC resistance are the same if the line is
infinitely long and unterminated.

How do you arrive at that one?

---

+V>---O
|
S1 | <--O--[AMMETER]--+--[INFINITELY LONG COAX CENTER CONDUCTOR]-
|
[VOLTMETER]
|
GND>---------------------+--[INFINITELY LONG COAX SHIELD------------

1. Close S1

2. Read the ammeter and record what you read as 'I'

3. Read the voltmeter and record what you read as 'E'

4. Calculate the resistance of the cable:

E
R = ---
I

5. Since a DC source is driving the cable and the voltage and
current remain constant, there must be no reactive terms which
need to be considered, so:

Z = sqrt R² = R

Thinking about it a little differently, if you consider that when
you close the switch the inductance of the center conductor is
limiting the current that can charge the incremental capacitance of
the cable as the edge propagates down the cable, then that limit and
that capacitance define how much current the voltage source driving
the cable needs to supply while the edge is moving down the cable,
and, since

E
R = ---
I

we wind up back where we started.

In the DC case the current would drop off with time
as the capacitance near the source becomes charged
and resistive effects predominate.

The transmission line is infinite.

 

[Cliff]

Correct. There must be a return path to the generator or battery.

That bare wire could be in a loop back to the source ...

Now, about "earthen grounds" ... <G>.

Where's's Roger N. when he's needed?