On Sat, 27 Nov 2004 21:43:14 GMT, (Robert Lay
W9DMK) wrote:

On Fri, 26 Nov 2004 10:57:25 -0700, Wes Stewart
wrote:


Yes. But the ITT Reference Data For Radio Engineers uses this paper
as a reference.

If you have Mathcad, a sheet that implements some of the equations was
included as a reference in my Balanced Transmission line paper.

http://users.triconet.org/wesandlinda/LineCalc.mcd


Dear Wes,

I was happy to find that the MacAlpine paper is the first part of
Chapter 22 of the ITT Handbook, as the latter is much more readable.

I did not pick up on the MathCad files, because I do not have MathCd -
however, the material from MacAlpine and Ricardi have answered most of
my concerns.


|I hope that MacAlpine agrees with what Dave and Richard are telling
|me, because their responses seem to be correct and are exactly what I
|was afraid of - that I've been sucked into another example of the
|strange terminology used to describe "losses".
|
|I have always thought of "loss" as a conversion to another form of
|energy (typically heat energy) which is lost from the system.
|Apparently, the kind of "loss" being described in the example that I
|gave is not a loss at all.

I was premature in those two paragraphs, above. I can see now that the
Additional Losses Due to SWR really are dissipative and are unrelated
to the "Mismatch Losses" and "Transducer Losses" defined on page 22-12
of the ITT Handbook, 5th Ed.


Yes it is. A simple-minded way of looking at it is if the SWR is
greater than unity then increased current is flowing in the line. The
line has resistive loss, so the I^2*R loss increases. The current
isn't constant (there is a current standing ratio, ISWR, just like a
VSWR) so there are peaks and valleys in the current and as you have
figured out, the longer the line and the higher its nominal loss, the
lower the ISWR is at the line input.

My interpretation of your "Yes it is." is that you mean that the
Additional Losses Due to SWR are truly heat losses and are due to the
ohmic losses in the hot spots of the line. Then we agree on that
point. Your paragraph above is much more succinct than the papers by
MacAlpine and Ricardi, but it certainly tells the story.

So the loss per unit length is non-linear and varies with distance
from the mismatch, but it is a real dissipative loss.

I don't know that I would have used the term "non-linear", but I would
certainly agree that it varies along the line in accordance with the
current loops.

For those interested in the loss in the shorted or open stub case,
maybe this will be of interest:
http://users.triconet.org/wesandlind...ching_Loss.pdf

I took that pdf and added it to the collection. There were several
things about that paper that filled-in gaps of detail in MacAlpine.
However, neither paper gives us much hope for a simple model of these
losses. Nonetheless, it makes hash out of the material in The ARRL
Antenna Book. In all fairness, the Antenna Book cannot cover all
aspects of these topics in detail. Unfortunately, the material in the
Antenna Book is, in my opinion, very misleading in several specific
areas, as follows:
- The Antenna Book gives only one expression for Total Line
Loss (combining ML loss and the Additional Loss Due to SWR). If we
accept Macalpine's model, there are different relationships for the
range of SWR from 0 to 6 and for the range from 6 upwards.
- Antenna Book does not explain that the hot spots are very
localized and that the additional losses can be quite dependant upon
the length of the line in wavelengths. For example, the losses in a
segment of line less than 1/3 wavelength might be insignificant in
comparison with a segment of line greater than 1/3 wavelength simply
because the shorter segment may not contain a hot spot. In other
words, one cannot apply the Antenna Book equations, blindly, because
of several factors that are not even mentioned, and for short line
segments it is quite possible that there would be no signicant losses
due to SWR.
- The most misleading information in The Antenna Book is on
pages 24-11 and 24-12 where it is shown that a 100 foot RG-213
feedline will suffer 25 dB of Additional Loss Due to SWR at 1.83 MHz
because of the very short antenna. I believe that when the equations
from the ITT Handbook are used instead, that the actual losses will be
far, far less.

Just today, I made a careful measurement on an RG-8/U line of 5.33
meters length at 30 MHz and terminated with a 4700 + j 0 load. The
Matched Line Loss of that line at 30 MHz is 0.9 dB per 100 feet, and
its Velocity Factor is between 0.75 and 0.80 The input impedance was
actually measured at 2.45 -j15 ohms for an SWR at the input of 22.25.
The SWR at the load end was 94. Those two SWR's establish a total loss
on the line of 0.15 dB. If one were to blindly apply the formula in
The Antenna Book on page 24-9, the result obtained would be 4.323 dB.


I have finally resolved this problem. The last paragraph, immediately
above, represents the problem that I have been unable to reconcile
until now.

Surprisingly, it was not until today that I finally made a computation
of the input power to the line for the configuration above.
Specifically, I was able to compute the voltage and current at the
input to the line that would produce a 100 volt reference voltage
across the 4700 ohm line. That is the obvious thing that had to be
done in order to establish a reference power for purposes of computing
losses. That calculation resulted in an applied voltage at the line
input of 29.2 volts at angle -171.5 degrees and a current of 1.917
amps at -90.78 degrees. Computing power into the line as E*Icos(theta)
= 9.024 watts. The power delivered to the load is 100 volts squared
divided by 4700 ohms, which is 2.127 watts.

Therefore the efficiency is 23.6% and the losses in the line are 6.275
dB. In all fairness, I did have to change one assumption in the data
above. I had to revise my attenuation value of 0.9 dB per 100 ft.
upwards to a value of 1.72 dB per 100 ft. in order to get my measured
impedance at the line input to be consistent with that line impedance,
length, load value and velocity factor.

Up until today, I could not see the losses being that high. In fact,
everything that I used to compute losses based on the measurements
above told me that the losses were on the order of 0.3 dB or less,
depending on which foolish method I was using. The method that really
sucked me in was the method based on two SWR readings - one at the
load and one at the input. That method, gives either 0.15 dB or 0.3
dB, depending upon whether you believe the scale at the bottom of the
Smith Chart or whether you believe the nomograms on pages 22-7 or 22-8
of the ITT Reference Data for Radio Engineers, 5th Edition.

So, once I tackled it head on and just did the brute force, obvious
calculation, I got a loss figure that exactly corresponds to the
losses predicted in The ARRL Antenna Book, 17th Edition, page 24-9.

So, I hope everyone had fun and learned something in the process. I
know I did.

73,

Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk