Richard Clark wrote in
:

On Wed, 28 Feb 2007 01:01:55 GMT, Owen Duffy wrote:

By "line loss" I mean the ratio of power at the load end of the line to
power at the source end of the line, not "forward power" or "reflected
power", but the average rate of flow of energy at those points.

Hi Owen,

What's wrong with conventional terms so that we BOTH know what you
mean? The convention would call this Mismatch Loss. If you dispute
this, then it serves my complaint. Further, convention has no
interest in "forward power" nor "reflected power" except as expressed
as SWR. I thought I was quite terse in this regard.

So, to your challenge:
The problem is 1m of Belden 8262 (RG58C/U type) at 3.5MHz with three
loads, 50+j0, 5+j0, and 500+j0.

Well, as I've pointed out, it is not strictly in the terms of my
challenge, is it?

Richard,

Your challenge was "Give me any normal line attenuation and SWR at the
load, and I will tell you exactly how much additional loss will occur."

I didn't state the VSWR, but it is 10:1 in both cases. The "normal line
attenation" you refer to is I expect the Matched Line Loss" which I have
given you.


The loss for 50+j0 load is 0.025dB (equivalent to the Matched Line
Loss).

Sigh... parentheticals?

What Line Loss to you get for the other two cases?

(I make it 0.24dB for 5+j0, and 0.014 for 500+j0.)

An additional 0.1dB However, this example strains the utility of the
challenge.

Is that your answer, an additional 0.1db due to the 10:1 VSWR? We do not
agree on either answer.

BTW, my figures were not additional loss, but total Line Loss as I
defined it.

You will note that my calculation for the 5+j0 case is less than the
Matched Line Loss, not higher.

In practical transmission lines, most of the loss is in current flowing
in the R component of an RLGC equivalent of the line, the loss in the
copper conductors forming the line. For VSWR1, the net current varies
along the line forming the classic standing wave pattern, and the loss in
incremental lengths of the line varies approximately with the square of
current in that increment.

So in the two cases above, even though the load VSWR is the same, the
loss is quite different due to the different current distribution in both
cases, one is near a current maximum, and the other is near a current
minimum. Any adjustment of Matched Line Loss for VSWR1 using only the
VSWR cannot take the location of the standing wave pattern into account,
and is an inaccurate approximation in some situations.

Many books showing a VSWR based formula for "additional loss due to
VSWR" don't spell out the assumptions underlying the formula. Phillip
Smith does in his book "Electronic Applications of the Smith Chart", he
says "If a waveguide is one or more wavelengths long, the average loss
due to standing waves in a region extending plus or minus a half
wavelength from the point of observation may be expressed as a
coefficient or factor of the one way transmission loss per unit length."
and he gives the ratio as (1+S^2)/(2*S). Though the ARRL shows graphs and
formulas they don't always (if ever) spell out the assmptions.

So, yes I assert that the Line Loss under mismatch conditions may be less
than the Matched Line Loss.

Owen

PS: I calculated my answers using http://www.vk1od.net/tl/tllc.php ,
Dan's TLDETAILS.EXE and ARRL's TLW3.EXE give similar results.