Dave wrote:
... unless you want to argue it out with cecil.

Sorry, I have no argument with Owen. I do have
an argument with people who say replacing the
RG-213 on a G5RV with 300 ohm twinlead all the
way to the tuner will reduce losses on 80m -
tain't so.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

On Fri, 17 Oct 2008 13:36:04 GMT, "Dave" wrote:

Try the line loss calculator at http://www.vk1od.net/tl/tllc.php to
calculate the loss in 1m of RG58 at say 2MHz with loads of 5 and 500 ohms
(both VSWR=10).

Now refer to the ARRL... does it explain the difference?

Owen

yeah, when you use the full complex Z0 and probably the full transmission
line equations it gets a bit more complex. but for amateur use that graph
is close enough. the difference between 5 and 500 ohm loads of .07db or so
for 100m just ain't worth quibbling about for normal amateur hf use. unless
you want to argue it out with cecil.

To catch Owen's drift, you really need to perform his suggested
exercise.

73's
Richard Clark, KB7QHC

"Dave" wrote in
:

....
yeah, when you use the full complex Z0 and probably the full
transmission line equations it gets a bit more complex. but for
amateur use that graph is close enough. the difference between 5 and
500 ohm loads of .07db or so for 100m just ain't worth quibbling about
for normal amateur hf use. unless you want to argue it out with
cecil.

You either misread my example (it was 1m not 100m) or you labour under
the misapprehension that loss per unit length under mismatched conditions
is constant at all displacements along the cable.

When approximations that hold under some conditions replace the
underlying principles, we dumb down. The formula and graphs for
"additional loss due to VSWR" without statement of the assumptions under
which it is valid are an example... where now, so many people accept the
concept that VSWR necessarily increases loss.

The OP was trying to reconcile calculated losses in a particular
scenario, and one of the contributions to error was the "additional loss
due to VSWR". It is fine with me that understanding doesn't appeal to you
Dave, but you don't need to press that approach on the rest of us.

Owen

(PS: if we take a length of 50 ohm coax sufficiently short that current
distribution is approximately uniform, and consider the losses under
matched conditions and under a 500 ohm load with same load power where
voltage is three times and current is one third, it is intuitive that
since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G), that loss will be LESS (than Matched Line Loss)...
approximately one tenth in that case... so why swallow the ROT that high
VSWR necessarily increases loss.)

since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G)

A quick question. If most of the the cable loss is due to I^2R, how can one
explain that the foam versions of common coaxial cables show a much lower loss
than versions having solid PE insulation?

For instance RG-213 is rated at 8.5dB loss for 100 meters at 144 MHz, while
RG-213 foam at only 4.5 dB. If G is relatively unimportant with regard to loss,
how can one explain that a change of insulation material yields such a
tremendous change in loss?

Thanks and 73

Tiony I0JX

"Owen Duffy" wrote in message
...
"Dave" wrote in
:

...
yeah, when you use the full complex Z0 and probably the full
transmission line equations it gets a bit more complex. but for
amateur use that graph is close enough. the difference between 5 and
500 ohm loads of .07db or so for 100m just ain't worth quibbling about
for normal amateur hf use. unless you want to argue it out with
cecil.

You either misread my example (it was 1m not 100m) or you labour under
the misapprehension that loss per unit length under mismatched conditions
is constant at all displacements along the cable.

When approximations that hold under some conditions replace the
underlying principles, we dumb down. The formula and graphs for
"additional loss due to VSWR" without statement of the assumptions under
which it is valid are an example... where now, so many people accept the
concept that VSWR necessarily increases loss.

The OP was trying to reconcile calculated losses in a particular
scenario, and one of the contributions to error was the "additional loss
due to VSWR". It is fine with me that understanding doesn't appeal to you
Dave, but you don't need to press that approach on the rest of us.

Owen

(PS: if we take a length of 50 ohm coax sufficiently short that current
distribution is approximately uniform, and consider the losses under
matched conditions and under a 500 ohm load with same load power where
voltage is three times and current is one third, it is intuitive that
since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G), that loss will be LESS (than Matched Line Loss)...
approximately one tenth in that case... so why swallow the ROT that high
VSWR necessarily increases loss.)

why should i swallow your rot that shows when you push the limits of the
equations you get results that 'seem' to defy logic. I understand perfectly
well what you are saying, and i do understand the full complex forms of the
transmission line equations. what i am saying is that for most 'normal'
amateur use the graph presented in the arrl book is adequate. if you insist
on pushing computerized calculations to the absurd limits i'll lump you in
with art and his over optimized ball of wire antenna.