Antonio Vernucci wrote:

Hi Walt,

I have always used a good balun on all my antennas, and therefore I am
not too convinced that, in my case, the SWR change I observe when adding
(or removing) a piece of coax in my station could be due to RF presence
on the coax braid.

Anyway, I have not yet read a clear and convincing explanation of why
the presence of RF on the coax braid would cause the SWR meter to give a
different reading when moving it along the line.

I appreciate that, with a hot braid, the coax cable becomes part of the
antenna and then radiates, but I cannot clearly focus why this can cause
the SWR meter to see different impedances at different points of the
line. Impedance is the ratio between RF voltage (between center
conductor and braid) and (differential mode) RF current. So, I do not
well visualize how the presence of a common mode RF current can
influence the meter reading.

73

Tony I0JX

If you keep the same total line length and move your meter to different
points along the line, current on the outside of the line won't change
the indicated SWR except to the extent that the SWR meter is poorly
shielded and affected by the current. However, I believe the problem was
stated in such a way that the SWR was seen to change as line was added
or removed, that is, as the line length changed. And that could change
the actual line SWR for the reason you described.

Roy Lewallen, W7EL

"Antonio Vernucci" wrote in
:

....
I have always used a good balun on all my antennas, and therefore I am
not too convinced that, in my case, the SWR change I observe when
adding (or removing) a piece of coax in my station could be due to RF
presence on the coax braid.

Perhaps you should set aside your confidence that there is no common mode
current, and do some experiments with shielded loads and coax lines.

For example, you could parallel two 50 ohm loads at a T piece, and
measure the VSWR into the branch line at different lengths and locations.
Try it also with some sections of 75 ohm line if you have some available.

Come back and describe the experiments and results if they do not behave
as expected.

If you get info fine detail about the effect of a VSWR meter calibrated
for 50+j0 on a line that is not exactly 50+j0 (eg practical lines), my
notes at http://www.vk1od.net/transmissionlin...splacement.htm
might help you to understand what may be happening.

When you have convinced yourself about the soundness of the broad
statement that for most practical cases, VSWR decreases smoothly from the
load to the source, and the decrease is accounted for exactly by the line
loss.


Anyway, I have not yet read a clear and convincing explanation of why
the presence of RF on the coax braid would cause the SWR meter to give
a different reading when moving it along the line.

If you maintain the common mode current path exactly, and move the SWR
meter to a different location, the VSWR should be seen to decrease
smoothly from the load to the source, and the decrease is accounted for
exactly by the line loss.

If you change the common mode current path in any way, you may cause a
different load impedance at the far end of the coax, and that will affect
the VSWR.


I appreciate that, with a hot braid, the coax cable becomes part of
the antenna and then radiates, but I cannot clearly focus why this can
cause the SWR meter to see different impedances at different points of
the line. Impedance is the ratio between RF voltage (between center

If the VSWR1, then the impedance varies all along the line.

The VSWR meter doesn't directly read the impedance, but it shows the
VSWR, and although the impedance may vary radically, the VSWR decreases
smoothly from the load to the source, and the decrease is accounted for
exactly by the line loss. (I think I have said that before.)

conductor and braid) and (differential mode) RF current. So, I do not
well visualize how the presence of a common mode RF current can
influence the meter reading.

See above and my notes.

But, while you are fixated on the belief that you have zero common mode
current, what you report cannot be explained by conventional transmission
line theory.

Owen

PS: "most practical cases" means the VSWR as indicated on an instrument
calibrated for 50+j0 and used on low loss nominal 50 ohm line at HF or
above.

On 9 jun, 22:37, "Antonio Vernucci" wrote:
The possibility of common-mode current on the outside of the braid has
been mentioned, but nothing has been mentioned concerning whether a
balun is used if the feedline-antenna connections is unbal to bal. If
there is no balun where should be one, seems to me it's a no-brainer
that the problem is common-mode current causing the different SWR
readings with different lengths of feedline.

Walt, W2DU

Hi Walt,

I have always used a good balun on all my antennas, and therefore I am not too
convinced that, in my case, the SWR change I observe when adding (or removing) a
piece of coax in my station could be due to RF presence on the coax braid.

Anyway, I have not yet read a clear and convincing explanation of why the
presence of RF on the coax braid would cause the SWR meter to give a different
reading when moving it along the line.

I appreciate that, with a hot braid, the coax cable becomes part of the antenna
and then radiates, but I cannot clearly focus why this can cause the SWR meter
to see different impedances at different points of the line. Impedance is the
ratio between RF voltage (between center conductor and braid) and (differential
mode) RF current. So, I do not well visualize how the presence of a common mode
RF current can influence the meter reading.

73

Tony I0JX

Hello Antonio,

Do you have access to a 2m or 70cm set? If so, take a piece of 50 Ohms
coaxial cable (of about 1m). Plug it onto your set (or a separate VSWR
meter) and add a quarter wave to the other end. Do nothing with the
braid at the other end.

Now select low power setting. Measure the VSWR while you move you hand
along the cable. You will notice change in VSWR for sure.

The reason for this is that the QW antenna requires a good image. The
image impedance is in series with the radiator impedance (simple point
of view). This image is the cable, VSWR meter and set. When you change
this, you will notice change in VSWR. The easiest way to change the
image/ground/counterpoise is capacitively with your hand.

Of course you may assume that you don't have common mode current
(because of a balun), but best is to turn such an assumption into a
fact.

Best regards,

Wim
PA3DJS
www.tetech.nl
please remove the obvious letters in the PM