On Sat, 2 Jul 2005 11:15:29 +0000 (UTC), "Reg Edwards"
wrote:

....

Noted.

The most accurate way to estimate D at HF is to measure attenuation
versus frequency over a wide frequency range on several miles of solid
polyethylene coaxial line.

Then separate the constants A and B by plotting on graph paper
Attenuation/Sqrt(F) versus Sqrt(F). and then do a few simple
calculations.

Ok, as I stated earlier, I am working from published specs rather than
my own measurements, and I understand there are issues about the
reliability of published specs.

I didn't plot the values on graph paper to find A and B (or k1 and
k2), but used a least squares regression to find A and B (and r**2,
the correlation coefficient as an indicator of the quality of the
fit).

What I was trying to do was to develop an RLGC model for the line from
published figures (attenuation at spot frequencies, nominal Ro, vf),
and I can do that. The issue about the dielectric came up in trying to
reconcile the G / D / k2 figures with the knowledge that the cable had
PE dielectric and should have had D=2e-4 (I stand corrected on my
misreading of the value 2e-5 from my ITT Handbook by Wes - thanks).

The purpose of the model and validation are for my online transmission
line loss calculator.

The notes to the calculator show the modelled loss vs the data points
in one of the graphs at http://www.vk1od.net/tl/tllc.php .

D can vary noticeably from one length of cable to another manufactured
from a different batch of nominally identical materials.


Ok, noted. I have added a few words to the explanatory notes on the
calculator.

Thanks for the help all.

Owen
--

Owen,

Are you aware of programs -

RJELINE2, RJELINE3, for balanced pair lines,

and COAXPAIR, COAXRATE for coaxial lines ?

These are based on exact, classical transmission line formulae and
will tell you all you could wish to know about behaviour of
transmission lines versus any complex termination from open circuit to
short circuit. No approximation of critical parameters.

However, in the coaxial case, dielectric loss is an input quantity.
;o)
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

On Sun, 3 Jul 2005 06:26:57 +0000 (UTC), "Reg Edwards"
wrote:

Owen,

Are you aware of programs -

Yes, of course, Reg, they and the rest of the suite are interesting...

In validating my calcualator, I have compared it to yours, Dan's
XLZIZL / TLD, Dean's TLA and Kevin's java applet. They all produce
slightly different results, one of the reasons I have tried to
document the assumptions that underly my calculator.


RJELINE2, RJELINE3, for balanced pair lines,

and COAXPAIR, COAXRATE for coaxial lines ?

These are based on exact, classical transmission line formulae and
will tell you all you could wish to know about behaviour of
transmission lines versus any complex termination from open circuit to
short circuit. No approximation of critical parameters.

However, in the coaxial case, dielectric loss is an input quantity.
;o)

Yes, I played with a model of Belden 8262, it is pasted below, and
will probably be line wrapped by a lot of news readers.

I have plugged in the dimensions from the 8262 spec sheet, and it
produces some results that are inconsistent with the specs. The
insulant diameter from the spec sheet is 4.5mm, loss/Km 13dB. (The
value for O was chosen to calibrate the DC loop resistance.) At 1GHZ,
the calculated loss was 35dB against spec of 21dB.

I played with it for a while trying to calibrate against the B8262
specs, but was unsuccessful.

Owen


N. Nominal Zo, ohms . 50.0 TERMINATION/LOAD -------------
ohms ---
D. Inner diameter, mm 0.889 R. Series Resistance
0.0
O. Outer thickness mm 0.080 X. Series Reactance
50.0
V. Velocity Factor Vf 0.6600 EQUIVALENT TERMINATION -------
ohms ---
T. Loss Factor ...... 0.000200 S. Shunt Resistance
2500000000.0
L. Line Length metres 100.000 Y. Shunt Reactance
50.0
F. Freq, kilo-Hertz . 1000.00
----------------------------- LINE CHARACTERISTICS
---------------------------
Insulant diameter 3.14 millimetres. Total Attn 10.5898
dB/kilo-m
Magnitude of Zo 51.79 ohms. Dielectric Loss 0.0275
dB/kilo-m
Angle of Zo -2.112 degrees. RF Resistance 0.1258
ohms/metre
Relative Velocity 0.6373 Inductance 0.2705
uH/metre
Electrical length 0.523 wavelengths. Capacitance 101.1476
pF/metre
DC Loop resistance 49.063 ohms/kilo-m. Conductance 0.1271
uS/metre
----------------------------- INPUT IMPEDANCE Zin
----------------------------
Series Resistance 16.51 ohms. Equivalent Shunt R 271.28
ohms
Series Reactance 64.86 ohms. Equivalent Shunt X 69.07
ohms
Magnitude of Zin 66.93 ohms. Angle of Zin 75.716
degrees
-------------------------- TRANSMISSION PERFORMANCE
--------------------------
Reflection coeff. 1.0375 Line loss when matched 1.0590
dB
Refl.coeff. angle 92.01 degrees. Actual line loss 70.8624
dB
VSWR at termination 54.296 Actual power loss in line 100.000
percent
VSWR at line input 9.696 Skin Depth 0.066 mm Q 13.5
Length = Q(1/4-wave) A(dd 1/4-wave) 3(dB) 6(0dB) =(.2%) -(.2%)
Hit N,D,O,V,T,L,F,R,X,S,Y to change data. B(egin again), E(xit
program)



--

If you want to work from published specs, you should find this
interesting, from the Belden catalog:

It looks like 8214 and 9913F7 are the same except that 9913F7 has a
solid aluminum shield between the insulation and the braided copper
shield. 9913F7 claims lower loss, amounting to 0.9 dB at 400 MHz. One
might conclude that the difference is due to the extra shield,
presumably because it's smooth and not braided.

It looks like 9914 is the same as 9913F7 except that 9914 has a solid
rather than stranded center conductor. The spec for 9914 shows less
loss, amounting to 0.4 dB less at 400 MHz. One might conclude that the
difference is due to stranded vs. solid center conductor.

I think your analysis isn't valid for two reasons. The first is that
you're relying on published specifications. (For example, I just
measured the Z0 of ten "50 ohm" cables of different brands and types I
have on hand -- it ranged from 44.6 to 56.8 ohms. I already reported a
loss measurement that was quite different from the spec.) The second is
that your model is overly simplified. It looks like you're trying to
account for all the loss mechanisms in a coax cable by considering only
the dielectric and conductor losses in idealized materials, or at best
two factors which are proportional to F and proportional to the square
root of F. In order to get a decent fit, I think you'll have to include
additional factors for such things as the effect of stranding the center
conductor and the extra loss associated with a braided shield, which
might well be different functions of frequency. Of course, tinned
conductors, which are very common, will alter the loss characteristics
depending on the composition of the plating, and change the shape of the
loss vs. frequency curve at a frequency depending on the depth of the
plating. Once you start really digging, I think you'll find other
mechanisms, such as perhaps how tight a woven shield is -- it wouldn't
take much of an air gap between the shield and the insulation to have
some pretty noticeable effects.

I went through this excercise of attempting to predict coax cable loss
many years ago (my notes say 1991) and abandoned the effort as being too
time consuming. I hope you'll be willing to devote the time and effort
necessary to come up with a reasonably accurate model.

Roy Lewallen, W7EL

Roy said -
I think your analysis isn't valid for two reasons. The first is that
you're relying on published specifications. (For example, I just
measured the Z0 of ten "50 ohm" cables of different brands and types
I
have on hand -- it ranged from 44.6 to 56.8 ohms. I already reported
a
loss measurement that was quite different from the spec.)

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

The greatest source of divergence between manufacturers'
specifications and what you think you've actually got is due to
manufacturing variation in cable dimensions.

Such as :

Inner coaxial wire diameter as it is drawn through diamond dies.
Ovality of wire drawn through worn dies.
Tightness of stranded inner conductors affecting diameter.
Diameter over the extruded polyethylene insulant. Affected by
temperature.
Off-centre eccentricity of inner conductor within the insulant.
Ovality of the polyethylene extrusion.
Diameter of braiding wires.
Tightness of braid over polyethylene.
Longitudinal tension in braid.
Tightness of copper or aluminium tapes over polyethylene.
Tightness of PVC jacket or other protection over braid or tapes.

- and a dozen other dimensional factors which I have long forgotten.

There's also variation in the conductivity of annealed copper wire and
contaminants in polyethylene due to lack of cleanliness in storage.

During manufacture, as the product is drawn through machinery,
electrical characteristics change. They can become cyclic. When
measuring long lengths small reflections can accumulate causing
attenuation versus frequency curves to exhibit a slow ripple about the
average slope of A*Sqrt(F)+B*F dB.

( The most unreliable manufacturer's specification I have seen,
associated with attenuation and power rating, allowed cable to be used
at a temperature of the melting point of polyethylene. Would burn the
skin off your hands. Testing equipment???? )

Important factors are the inevitable errors in all measuring
instruments, especially so-called SWR meters, and the delusions of
accuracy usually suffered by everybody involved. It's so easy to draw
the wrong conclusions!
----
Reg, G4FGQ

On Sun, 3 Jul 2005 11:20:01 +0000 (UTC), "Reg Edwards"
wrote:

Roy said -
I think your analysis isn't valid for two reasons. The first is that
you're relying on published specifications. (For example, I just
measured the Z0 of ten "50 ohm" cables of different brands and types
I
have on hand -- it ranged from 44.6 to 56.8 ohms. I already reported
a
loss measurement that was quite different from the spec.)

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

The greatest source of divergence between manufacturers'
specifications and what you think you've actually got is due to
manufacturing variation in cable dimensions.


Thanks Roy and Reg,

I understand your point that the calculation engine is more precise
than the model accuracy (against specs), and the product manufacturing
tolerances introduce an even larger error in the predictions. Not
mentioned, but installation and through life degradation introduces
yet another uncertain and possibly greater issue.

Appreciate all the input.

I am sure you are right Roy, this could go on indefinitely. I think I
have gone far enough for the purpose, probably too far, but that is
the cost of confidence that I have gone far enough!

Thanks again.

Owen
--