V/I ratio is forced to Z0
 

On Tue, 27 Sep 2005 02:04:46 GMT, Cecil Moore wrote:


The transmission line length must only be long enough such that
the V/I ratio is forced to the Z0 value. According to some pretty
smart guys I asked, that's about 2% of a wavelength.

Cecil, do you have some quantitative explanation / support for this?

The treatments that I have seen of transmission line tuners where
different Zo lines are directly connected do not suggest corrections /
tolerances of the type you imply.

(IIRC, Terman discusses a fringing capacitance as a means of allowing
for a physical discontinuity.)

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

Owen
--

Owen Duffy wrote:
On Tue, 27 Sep 2005 02:04:46 GMT, Cecil Moore wrote:



The transmission line length must only be long enough such that
the V/I ratio is forced to the Z0 value. According to some pretty
smart guys I asked, that's about 2% of a wavelength.


Cecil, do you have some quantitative explanation / support for this?

The treatments that I have seen of transmission line tuners where
different Zo lines are directly connected do not suggest corrections /
tolerances of the type you imply.

(IIRC, Terman discusses a fringing capacitance as a means of allowing
for a physical discontinuity.)

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

Owen

As I recall it came from someone on sci.physics.electromag.

But think about it. The surge impedance (Zo) is basically just the
ratio of the capacitance per unit length to the inductance per unit
length. Those quantities might vary a little bit from one place to
another, but probably not by much. And there are undoubtedly end
effects which locally pull the capacitance and inductance values away
from the ideal. So the length really need only be long enough for the
variations to average out and for the total values to become large
enough to swamp the end effects.

ac6xg

On Fri, 07 Oct 2005 15:09:25 -0700, Jim Kelley
wrote:


But think about it. The surge impedance (Zo) is basically just the
ratio of the capacitance per unit length to the inductance per unit
length. Those quantities might vary a little bit from one place to
another, but probably not by much. And there are undoubtedly end
effects which locally pull the capacitance and inductance values away
from the ideal. So the length really need only be long enough for the
variations to average out and for the total values to become large
enough to swamp the end effects.

I don't doubt there is a discontinuity that disturbs the fields and
V/I ratio.

What I am asking about is the basis for the 2% of wavelength factor.

If I use RG58C/U on 160m, I read that Cecil is suggesting that the V/I
ratio is significiantly different to Zo for 2% * 160m or 3.2m
(125")from the end of the cable, which seems large when the physical
distance between the inner and outer conductor is 0.001m (0.04").

I am looking for quantitative support for Cecil's 2%.

Owen
--

Definition of what you are all waffling about :

"The input impedance Zo applies only for the duration of time taken
for an echo to be received back from the point where the line
impedance Zo first changes to another value.".

Distance can be measured either in metres or, if you like, fractions
of a wavelength. Wavelength involves frequency which is rather
meaningless because time is already a variable but on a different
arbitrary scale.

Only Cecil could dream up a use for such an effect.
----
Reg, G4FGQ

Owen Duffy wrote:
Cecil, do you have some quantitative explanation / support for this?

Nope, but there were no disagreeing postings.

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

As I remember it came from the spacing between conductors Vs wavelength.
The spacing between conductors is about 0.1 inches for RG-58. How many
times that value would you think it would take for a transmission line
to force its Z0 upon the signals? At 10 MHz, 2% of a wavelength (24 inches)
is about 250 times the spacing between conductors.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
Only Cecil could dream up a use for such an effect.

Sorry Reg, I only dream of six foot tall blonds with big boobs.
The 2% WL value came from sci.physics.electromag.
--
73, Cecil http://www.qsl.net/w5dxp

Only Cecil could dream up a use for such an effect.

Sorry Reg, I only dream of six foot tall blonds with big boobs.
The 2% WL value came from sci.physics.electromag.

========================================

Yes Cec, you've told us before. I read the thread. That newsgroup
has more highly-convincing old-wives than this one has. They are just
a little harder to detect.

2% of wavelength is meaningless unless you also state by how much
input impedance has diverged from Zo after a time T has elapsed.
Wavelength also implies a frequency but what THAT has to do with it is
anybody's guess. It merely adds to the confusion.
----
Reg, G4FGQ

On Sat, 08 Oct 2005 00:25:46 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Cecil, do you have some quantitative explanation / support for this?

Nope, but there were no disagreeing postings.

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

As I remember it came from the spacing between conductors Vs wavelength.
The spacing between conductors is about 0.1 inches for RG-58. How many
times that value would you think it would take for a transmission line
to force its Z0 upon the signals? At 10 MHz, 2% of a wavelength (24 inches)
is about 250 times the spacing between conductors.

It seems different people have this conceptual model of "a
transmission line forcing its Z0 upon the signals" in a gradual way,
though differing propositions for the length of line that does not
behave as predicted by Zo.

An extension of that thinking is in the proposition that I have seen
that a Bird 43 cannot give valid readings unless there is at least a
quarter wave of 50 ohm line on each side of itself. In this case, the
magnitude of significantly affected line seems to be 25%, someone
else's is 2%, can they both be correct?

It seems to me that apart from the region of the significant
distortion of the fields local to some kind of discontinuity, that the
fields further along the line at a distance from the discontinuity
large compared to the dimension of the discontinuity (which will often
be the conductor spacing) should be as constrained by the physical
parameters of the line (V/I=Zo for each travelling wave).

In the case of the Bird 43, I suggest that if had, say, at 1MHz, 75
ohm line and a 75 ohm load on the load side, that the V/I raio for the
travelling waves in the region of the sampling element would be so
close to 50 ohms as to not materially affect the accuracy of
measurements on the 50 ohms coupler section, irrespective of the fact
that the sampling element has only 0.02% of a wavelength of 50 ohm
line on its load side.

(For avoidance of doubt, nothing in the foregoing is to imply the Bird
43 would be directly measuring or indicating the conditions on the 75
ohm line.)

Owen
--

Cecil Moore wrote:
Owen Duffy wrote:
Cecil, do you have some quantitative explanation / support for this?

Nope, but there were no disagreeing postings.

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

As I remember it came from the spacing between conductors Vs wavelength.
The spacing between conductors is about 0.1 inches for RG-58. How many
times that value would you think it would take for a transmission line
to force its Z0 upon the signals? At 10 MHz, 2% of a wavelength (24 inches)
is about 250 times the spacing between conductors.

Maybe the electromagnetics people have a useful way to visualize it...

Deep inside the coax, the electric field lines between the inner and
outer of the coax are exactly at right-angles to the main axis. Where
that is exactly true, you have a pure TE10 mode so it's also valid to
assume that V/I is exactly equal to Zo.

Very close to the end of the coax, the electric field lines from the
center conductor start to reach out and connect with whatever is out
there beyond the end of the shield. Then you no longer have pure TE10
and can no longer assume that V/I=Zo.

Coming at it from the other direction, the question would be: how far
into the coax must you go before the field lines become accurately at
right-angles?

We can be sure that the field lines won't suddenly snap from being
divergent to being accurately at right-angles, so what we're really
asking is: how far before the field lines are near-enough at right
angles to make V/I=Zo a good engineering approximation?

Intuitively, the diverging field lines only seem likely to occur within
a few diameters of the end of the shield. Field lines always connect
with highly conducting surfaces at right-angles, and they won't like to
be sharply bent to run along the axis of the coax.

In other words, the effect would seem to be mainly a function of shield
diameter D. Again intuitively, I can't see where wavelength would come
into it, unless D itself is a significant fraction of the wavelength
(which is normally never true, and even microwave engineers try to avoid
it).

Following this picture of diverging field lines, there should also be a
secondary effect depending on how the inner and shield of the coax are
connected to the circuit outside.

All of this suggests that it's impossible to give a single answer that
would be valid for all cases (unless you choose a number that's so big,
it can't fail to be correct... like "120 radials" :-)

However, none of this speculation is of any practical consequence. All
practical experience indicates that if a line is so short that V/I is
not quite equal to Zo, the impedance transformation along that line will
be so small that the effect of any Zo error is lost in the noise.



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

"Cecil Moore" wrote in message
. com...
Reg Edwards wrote:
Only Cecil could dream up a use for such an effect.

Sorry Reg, I only dream of six foot tall blonds with big boobs.
The 2% WL value came from sci.physics.electromag.
--
73, Cecil http://www.qsl.net/w5dxp

then please take it back there. it makes no sense as it would force the
effect to get longer and longer at lower frequencies. the more logical
effect is fringing effects from edges of the shield if it isn't properly
connected to a proper termination... this is something that i can measure
with my tdr, and it is definately a very short range effect. just think, if
i pulse a line with a 1khz square wave and the effect got longer with lower
frequency what would the return pulse look like?