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

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

"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?

Dave wrote:
The 2% WL value came from sci.physics.electromag.

then please take it back there. it makes no sense as it would force the
effect to get longer and longer at lower frequencies.

So how long does the coax have to be to force V/I to Z0
when the applied signal is DC?
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
t...
Dave wrote:
The 2% WL value came from sci.physics.electromag.

then please take it back there. it makes no sense as it would force the
effect to get longer and longer at lower frequencies.

So how long does the coax have to be to force V/I to Z0
when the applied signal is DC?
--
73, Cecil http://www.qsl.net/w5dxp

just enough for any fringe effects to become negligible... no more than a
couple diameters if the coax as a rough guess.

Dave wrote:
just enough for any fringe effects to become negligible... no more than a
couple diameters of the coax as a rough guess.

Apparently, the 2% of a wavelength that I was remembering was
at 10 MHz. 1'/(98.4*0.66) rounded to 2%. All I was interested
in at the time was proving to Reg that one foot of coax forces
Vfor/Ifor = Vref/Iref = Z0, the boundary conditions assumed
by Bird Wattmeter designers. Since Kevin was not familiar with
PL-239's, I erred on the side of caution with the 2% estimate.

****Quote****
Newsgroups: sci.physics.electromag
From: "Kevin G. Rhoads"
Date: Tue, 07 Oct 2003 12:49:14 -0400
Subject: Transmission Line Question

Cecil wrote:
It addresses it adequately but doesn't answer any particulars.
Given PL-239 connectors and RG-213 coax, I wonder what the
answer would be for 10 MHz?

I'm not familiar with the connectors in question. Assuming
they are properly attached, they should not introduce much
mode diversion. For 10 MHz I would expect that all other modes
would be non-propagating (i.e., evanescent) even though RG-213
is a large coax (improved RG-8 apparently). The speed of propagation
is listed as 66%, so the nominal wavelength is 3/2 times the free
space wavelength for the TEM mode. 3/2 x 30m = 45m, which implies
the decay rate in space for non-TEM modes is going to be large
as the cable diameter is .405" (jacket) which implies the
spacing from inner to outer conductors will be less than .203".
For order of magnitude estimate, assuming the lowest non-TEM mode
can be approximated using a characteristic equation that really
is only applicable in Cartesian geometries:
(1/45m)**2 = (1/.203")**2 + kz**2
Clearly, kz must be imaginary to make this work. thus an
evanescant, non-propagating wave:
kz**2 = (1/45m)**2 - (1/.203")**2
To the accuracy used to date, the first term on the right
is negligible, so the decay rate, alpha, can be estimated:
alpha**2 = - (kz)**2 = (1/2.03")**2
Or, the lowest order undesired mode should reduce intensity
by a factor of 1/e (0.37) in about 2.03"; power will reduce
by that factor squared in the same distance (.135). In
about four inches, undesired mode power is down to about
0.018, in six inches, .00248, and after a foot, 6.14x10-6

You should double check my algebra, but I think the estimate
is reasonable. To put it into other terms, since the wavelength
in the coax dielectric is 45m and the conductor to conductor
spacing is about 2", any non-TEM mode will suffer attenuation
in E-field intensity with a space-rate constant rounghly
equal to the conductor to conductor spacing. INtensity
drops by 1/e = 1/2.71828 every 2 inches. Power availalbe
drops faster, being square of intensity.

So unless almost all the power diverts into an undesireable
mode (by a factor of more than a million to one), one foot
of cable should see pure TEM at the end.

HTH
Kevin
****End Quote****
--
73, Cecil http://www.qsl.net/w5dxp