Question on SWR
 

I make here reference to the well-known chart, shown in almost all the ARRL
Antenna Handbooks, setting the relationship among the real SWR (i.e. that
measured at antenna), the SWR measured at transmitter and the "line loss".

In the text they explain that the "line loss" to be considered when using the
chart is the line loss under matched conditions (i.e. that given by the line
manufacturer), that is without taking into account the extra loss caused by SWR.

They make the example of a line having a (matched) loss of 1.0dB with an SWR of
4.5 at antenna. The graph shows that the corresponding SWR at transmitter is
3.0.

Unless I am wrong, a simple calculation shows that, in the considered example,
the SWR at transmitter is about 2.3 rather than 3.0.

Here it goes (please note that, for the assumed SWR at antenna of 4.5, the extra
loss caused by SWR is just 1.0dB):

- actual loss on the forward wave: 1.0dB (matched) + 1.0dB (extra by SWR), for a
total of 2.0dB
- return loss corresponding to an SWR at antenna of 4.5: 4.0dB
- actual loss on the reflected wave: 1.0dB (matched) + 1.0dB (extra by SWR), for
a total of 2.0dB
- return loss at transmitter: 8.0dB
- SWR measured at transmitter (corresponding to a return loss of 8.0dB): about
2.3

It is interesting to note that the chart would give an identical result if by
"line loss" they would mean the total line loss (that is also including the
extra loss due to SWR) rather than just the matched line loss (as they state in
the text).

Any comment?

Thanks and 73

Tony I0JX

Question on SWR
 

I realized my error!

In my budget I counted the extra line loss caused by SWR twice:

- first time when I have added 1 dB to the loss of the forward and the return
wave

- second time when I took some power out of the antenna (to account for an SWR
of 4.5) and delivered it back to the transmitter

ARRL is always correct!

Sorry for the useless noise

Tony I0JX

Question on SWR
 

"Antonio Vernucci" wrote in message
...
I realized my error!

In my budget I counted the extra line loss caused by SWR twice:

- first time when I have added 1 dB to the loss of the forward and the
return wave

- second time when I took some power out of the antenna (to account for an
SWR of 4.5) and delivered it back to the transmitter

ARRL is always correct!

All Hail the ARRL!

Question on SWR
 

"Antonio Vernucci" wrote in
:

I realized my error!
....
ARRL is always correct!

The ARRL information on "extra loss due to VSWR" is may be incomplete in
that it may not the assumptions that underly the formula used for the
graphs.

The very concept that SWR necessarily increases loss from the matched line
loss figure is flawed.

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

Question on SWR
 

Owen Duffy wrote:
The ARRL information on "extra loss due to VSWR" is may be incomplete in
that it may not the assumptions that underly the formula used for the
graphs.

It is possible for a feedline with a high SWR to have
lower loss than the matched-line loss. For instance,
if we have 1/8WL of feedline with a current miminum
in the middle of the line, the losses at HF will be
lower than matched line loss because I^2*R losses tend
to dominate at HF.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Question on SWR
 

"Owen Duffy" wrote in message
...
"Antonio Vernucci" wrote in
:

I realized my error!
...
ARRL is always correct!

The ARRL information on "extra loss due to VSWR" is may be incomplete in
that it may not the assumptions that underly the formula used for the
graphs.

The very concept that SWR necessarily increases loss from the matched line
loss figure is flawed.

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.

Question on SWR
 

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

Question on SWR
 

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

Question on SWR
 

"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.)

Question on SWR
 

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