On Tue, 08 Nov 2005 23:09:51 GMT, Cecil Moore wrote:

Owen Duffy wrote:
In my 100m of W551 with a 16+j0 load at 30MHz, the loss in one metre
of line nearest the load is over 4%, the good news is that since 75%
of the transmitter power is already lost, the weighted effect of that
4.3% is nearer 1% of tx output.

What the heck is one "metre"? Netscape says that is misspelled and
probably should be corrected to "metro". Why aren't you guys on
the English system?

Met The fundamental base of the metre is the quarter of the
terrestrial meridian, or the distance from the pole to equator, which
has been divided into ten millions of equal parts, one of which is of
the length of the metre.

I think we saw the light before the English, but I think they have a
partial metrication now.

If the loss in each meter is 4%, wouldn't the loss in 100 meters
be 400%? What am I missing?

I did not say "the loss in each meter is 4%", I said "the loss in one
metre of line nearest the load is over 4%".

Firstly, percentage losses on cascaded sections are not additive...
you know that. Losses multiply, dB losses add because adding exponents
is multiply the fundamental quantity.

As I have said before, you seem to be under the misconception that the
overall loss (ie Pin/Pout) per unit length of a transmission line
operating with VSWR1 is constant,

It is not necessarily a constant. It is for a lossless cable, and I
think it probably is for a distortionless cable... but I would have to
check that.

(It is true that the loss per unit length of a transmission line
operating with VSWR=1 is constant.)

We were discussing an example based on Wireman 551 ladder-line. The
dominant factor affecting loss at 30MHz is the series resistance
element. Does it make sense that since in that example, the magnitude
of the current varies by nearly 25:1 along the line, that the I**2*R
loss per unit length along the line is not constant, and will vary by
a factor approaching 625:1?

Owen
--

Owen Duffy wrote:
We were discussing an example based on Wireman 551 ladder-line. The
dominant factor affecting loss at 30MHz is the series resistance
element. Does it make sense that since in that example, the magnitude
of the current varies by nearly 25:1 along the line, that the I**2*R
loss per unit length along the line is not constant, and will vary by
a factor approaching 625:1?

25% of the power is delivered to the load. There are eleven current
maximum points in 100m on 10m. Does that 11% of the feedline really
contribute 59% of the losses? Does the remaining 89% of the feedline
really only contribute 41% of the losses?
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 09 Nov 2005 03:25:47 GMT, Owen Duffy wrote:



We were discussing an example based on Wireman 551 ladder-line. The
dominant factor affecting loss at 30MHz is the series resistance
element. Does it make sense that since in that example, the magnitude
of the current varies by nearly 25:1 along the line, that the I**2*R
loss per unit length along the line is not constant, and will vary by
a factor approaching 625:1?

I knocked up a quick graph of the attenuation from point x to the load
for this scenario. (I think / hope it is correct!)

http://www.vk1od.net/lost/W551Example.htm

Is the shape of the curve (the cyclic variation over each electrical
half wave diminishing away from the load, and the general shape of the
curve a surprise? The effects plotted here might not be explained by
the ARRL charts.

Owen
--

On Wed, 09 Nov 2005 05:46:00 GMT, Cecil Moore wrote:

Owen Duffy wrote:
We were discussing an example based on Wireman 551 ladder-line. The
dominant factor affecting loss at 30MHz is the series resistance
element. Does it make sense that since in that example, the magnitude
of the current varies by nearly 25:1 along the line, that the I**2*R
loss per unit length along the line is not constant, and will vary by
a factor approaching 625:1?

25% of the power is delivered to the load. There are eleven current
maximum points in 100m on 10m. Does that 11% of the feedline really
contribute 59% of the losses? Does the remaining 89% of the feedline
really only contribute 41% of the losses?

When I have written about loss per unit length, I have implied "loss
at the rate of y per unit length". If you have tried to apply the 4+%
figure to one meter at each maximum, then you are unlikely to get any
meaningful results for a number of reasons. See the graph I just
posted (our posts crossed in the mail so to speak).

I haven't stated it it the post, but it should be obvious that the
rate of attenuation is the slope of the line in the plot referenced in
the post.

Owen
--

On Tue, 8 Nov 2005 13:23:46 -0500, "Fred W4JLE"
wrote:

I choose a ladderline length to always feed the antenna at a current node.


I don't know what this really means, and how it is relevant to the
discussion that the ROT in question doesn't hold true for longer feed
line lengths.

You have previously dismissed 100m of feed line with the eloquent
"sucks" comment, whatever that means.

What has feeding an antenna at the current node got to do with feed
line length? Is it intended to imply that feed line length or the ROT
are not relevant when you feed the antenna at a current node?

Is it just a trite comment to consume bandwidth?

Owen

"Owen Duffy" wrote in message
.. .
On Mon, 7 Nov 2005 22:21:35 -0500, "Fred W4JLE"
wrote:

How many hams find a feedline length of 100 Meters acceptable? Lets look
at
a more realistic length of 100 feet and the loss is less than 2 dB at 30
Mhz. The loss is much less on the lower bands. With all due respect, your
example sucks!

BTW, I make your example 2.7dB, not less than 2dB. (Perhaps you labour
under the misaprehension that loss per unit length is a constant in
this situation).

Fred, what you have highlighted is the unstated assumption of some
limit on length. You know enough to choose a length so that the ROT is
true, but does a learner soaking this up know as much?

--

Owen Duffy wrote:
http://www.vk1od.net/lost/W551Example.htm

Is the shape of the curve (the cyclic variation over each electrical
half wave diminishing away from the load, and the general shape of the
curve a surprise? The effects plotted here might not be explained by
the ARRL charts.

The graph is unclear. What does it mean that 6% loss occurs at
100 neters? Is that 6% loss per meter at the source? There's 4%
loss at 50 meters. Does that mean the average loss per meter is
4%? Where is the 4% loss in the meter closest to the load plotted?
--
73, Cecil http://www.qsl.net/w5dxp

Owen Duffy wrote:
What has feeding an antenna at the current node got to do with feed
line length?

Feedline length for an average system may be about 75 ft.,
the distance from the antenna to the transceiver. If a
current maximum point occurs at 87 ft., make the feedline
87 ft. long with (usually) no tuner required.

I assume what Fred thinks sucks is your implication that the
average ham feeds his antenna with 100 meters of feedline.
What percentage of hams do you think actually use 100 meters
of transmission line?
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 09 Nov 2005 14:46:07 GMT, Cecil Moore wrote:

Owen Duffy wrote:
http://www.vk1od.net/lost/W551Example.htm

Is the shape of the curve (the cyclic variation over each electrical
half wave diminishing away from the load, and the general shape of the
curve a surprise? The effects plotted here might not be explained by
the ARRL charts.

The graph is unclear. What does it mean that 6% loss occurs at
100 neters? Is that 6% loss per meter at the source? There's 4%
loss at 50 meters. Does that mean the average loss per meter is
4%? Where is the 4% loss in the meter closest to the load plotted?

The loss scale is in dB, it is the loss in dB at position x metres
from the load.

If you examine the graph, you will find that the slope of the loss vs
position line is as high as about -22dB/100m at the load, it has a
minimum slope of close to 0dB/100m, and you can see that at large x,
the slope approaches the matched line loss of -1dB/100m.

(You find the -22dB/100m by using a ruler to scale off the slope.
-22dB/100m is -0.22dB/m, or 10**-0.022 which is 0.9506, which
corresponds to a loss of almost 5% in that one metre of line nearest
the load. These aren't mental gymnastics!)

You could calculate an average loss per meter figure, but I don't know
what you could you use it for? The fact that this line is not straight
(as some people seem to assume) means that working with average
numbers is inherently inaccurate.

Owen
--

On Wed, 09 Nov 2005 15:02:35 GMT, Cecil Moore wrote:

Owen Duffy wrote:
What has feeding an antenna at the current node got to do with feed
line length?

Feedline length for an average system may be about 75 ft.,
the distance from the antenna to the transceiver. If a
current maximum point occurs at 87 ft., make the feedline
87 ft. long with (usually) no tuner required.

Well, I guess you are guessing at what Fred meant. But such a current
maximum on the feed point repeats every electrical half wave, and such
an approach doesn't preclude using feed lines much longer than 75
feet.


I assume what Fred thinks sucks is your implication that the
average ham feeds his antenna with 100 meters of feedline.

That is his misinterpretation if that is the case. I did not say an
"average ham", but I assert that it is not all that uncommon to have a
ham antenna located at 100m or more, and the ROT falls down.

Thing is, about averages, is that the detail you throw away to
calculate the average may have been relevant. Further, there is little
consolation to the guy who has 10dB worse than average performance
because he has used longer feed line under your ROT, to know that a
whole lot of guys using shorter feed line are enjoying better than
average performance and on average, it all balances out.

It is quite feasible to place an antenna at longer distances if you
want, but ladder line should be operated at lower VSWR for acceptable
losses, or better feed line used... and the ROT doesn't say that.

It is the unstated length assumption (of apparently 75') of your ROT
that makes limits its validity to the people who are most likely to
lap it up.

What is an "average ham" now days? Is it one that doesn't have a real
interest in the technical side of the hobby, the "I just wanna talk on
the radio" set... they like ROTs, gives them something to parrot on
air.

Owen
--

Hardly trite, it limits the line length to a maximum of 164 feet.

"Owen Duffy" wrote in message
...
On Tue, 8 Nov 2005 13:23:46 -0500, "Fred W4JLE"
wrote:

I choose a ladderline length to always feed the antenna at a current
node.


I don't know what this really means, and how it is relevant to the
discussion that the ROT in question doesn't hold true for longer feed
line lengths.

You have previously dismissed 100m of feed line with the eloquent
"sucks" comment, whatever that means.

What has feeding an antenna at the current node got to do with feed
line length? Is it intended to imply that feed line length or the ROT
are not relevant when you feed the antenna at a current node?

Is it just a trite comment to consume bandwidth?

Owen

"Owen Duffy" wrote in message
.. .
On Mon, 7 Nov 2005 22:21:35 -0500, "Fred W4JLE"
wrote:

How many hams find a feedline length of 100 Meters acceptable? Lets
look
at
a more realistic length of 100 feet and the loss is less than 2 dB at
30
Mhz. The loss is much less on the lower bands. With all due respect,
your
example sucks!

BTW, I make your example 2.7dB, not less than 2dB. (Perhaps you labour
under the misaprehension that loss per unit length is a constant in
this situation).

Fred, what you have highlighted is the unstated assumption of some
limit on length. You know enough to choose a length so that the ROT is
true, but does a learner soaking this up know as much?

--