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