Roy Lewallen wrote in
:

Dave Oldridge wrote:

If the antenna is TRULY balanced and the feedline dressed well away
from it at right angles you should have no common-mode currents on
the feedline.
. . .

That only prevents one of the two ways common mode current can be
created, by coupling. It can also be created by conduction. A common
example is a coax-fed dipole, where the current in the outer feedline
conductor splits between the antenna conductor and the outside of the
coax. An equivalent problem can occur when a dipole is fed with
symmetrical line such as ladder line, and one conductor of the line is
connected to the rig's chassis at the rig end. The current on the
inside of the chassis is equal to the current from the "hot"
conductor, and this splits between the transmission line conductor and
the outside of the chassis. A detailed explanation of conducted common
mode current can be found at
http://eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL

Still, if the antenna is TRULY balanced (a situation that only rarely
actually happens), you won't get common-mode currents. I've never had a
problem with them with well-grounded (from an RF standpoint) ground-
mounted verticals either.

Essentially this is why I recommend using open wire or twinlead and
feeding it through a proper balanced-line tuner. Years ago, I built an
amplifier that literally had a balanced line output and fed a 600-ohm
feeder direct off two taps on its output coil. That feedline was only
ten feet long and I worked a TON of 80m DX an the inverted vee that it
connected to. And I could always tap the coil so as to have ZERO RF in
the shack (though my landlady's little 7.5 watt light bulbs used to light
on some frequencies when the house wiring picked up direct from the
antenna).


--
Dave Oldridge+
ICQ 1800667

Is it not true that if the currents on the transmission line are unbalanced
(i.e., unequal on the two conductors) then the transmitter must already be
connected to ground? If not, what is the path of the differential current?

Chuck

Dave Oldridge wrote:

Roy Lewallen wrote in
:

Dave Oldridge wrote:

If the antenna is TRULY balanced and the feedline dressed well away
from it at right angles you should have no common-mode currents on
the feedline.
. . .

That only prevents one of the two ways common mode current can be
created, by coupling. It can also be created by conduction. A common
example is a coax-fed dipole, where the current in the outer feedline
conductor splits between the antenna conductor and the outside of the
coax. An equivalent problem can occur when a dipole is fed with
symmetrical line such as ladder line, and one conductor of the line is
connected to the rig's chassis at the rig end. The current on the
inside of the chassis is equal to the current from the "hot"
conductor, and this splits between the transmission line conductor and
the outside of the chassis. A detailed explanation of conducted common
mode current can be found at
http://eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL

Still, if the antenna is TRULY balanced (a situation that only rarely
actually happens), you won't get common-mode currents. I've never had a
problem with them with well-grounded (from an RF standpoint) ground-
mounted verticals either.

Essentially this is why I recommend using open wire or twinlead and
feeding it through a proper balanced-line tuner. Years ago, I built an
amplifier that literally had a balanced line output and fed a 600-ohm
feeder direct off two taps on its output coil. That feedline was only
ten feet long and I worked a TON of 80m DX an the inverted vee that it
connected to. And I could always tap the coil so as to have ZERO RF in
the shack (though my landlady's little 7.5 watt light bulbs used to light
on some frequencies when the house wiring picked up direct from the
antenna).

On Tue, 03 Jan 2006 16:24:01 GMT, chuck wrote:

Is it not true that if the currents on the transmission line are unbalanced
(i.e., unequal on the two conductors) then the transmitter must already be
connected to ground? If not, what is the path of the differential current?

Chuck

Think about it. If there is no path there would be no current!

Danny, K6MHE




email: k6mheatarrldotnet
http://www.k6mhe.com/

chuck wrote:
Is it not true that if the currents on the transmission line are unbalanced
(i.e., unequal on the two conductors) then the transmitter must already be
connected to ground? If not, what is the path of the differential current?

Your thinking is correct. If you had a small, battery operated rig, you
could theoretically force current balance by completely isolating it
from ground. You might burn yourself when you touch it, though.

In practice, differential current can flow to ground via capacitive
coupling from the rig to ground, or by flowing onto any connecting wires
such as power supply, speaker, or key leads, which will radiate.

Common mode feedline current is generally sinusoidally distributed. So
you can have a relatively small common mode current at the rig but a
much larger one at places on the feedline, if the rig happens to fall at
a common mode current node. When the current is due to coupling, a
single balun will be ineffective if placed at a natural current node, or
in some other cases will only change the distribution by moving the
node, rather than reducing the overall current. In those cases, two
baluns placed about a quarter wavelength apart are effective.

Roy Lewallen, W7EL

chuck wrote in
nk.net:

Is it not true that if the currents on the transmission line are
unbalanced (i.e., unequal on the two conductors) then the transmitter
must already be connected to ground? If not, what is the path of the
differential current?

There's the rub, you see. It's RF and, on some frequencies, standing waves
don't NEED to feed current to a ground for there to be common mode currents
on the line. But yes, whatever current is required at the end of the
transmission line will seek to ground itself through the radio.

My amp had the output coil center-tapped to RF ground and the feedlines
were tapped off it an equal amount on either side.


--
Dave Oldridge+
ICQ 1800667

Dave Oldridge wrote:

Still, if the antenna is TRULY balanced (a situation that only rarely
actually happens), you won't get common-mode currents.

That's true only if by "balanced" you mean that the two feedline
conductors carry equal and opposite currents. In that case, common mode
current is zero by definition. But if you really mean symmetrical, as
most amateurs do when they say "balanced", you certainly can have common
mode current.

A detailed explanation of how that happens is in the article at
http://eznec.com/Amateur/Articles/Baluns.pdf, and the article by Walt
Maxwell, W2DU at http://www.w2du/r2ch21.pdf which is referenced at the
end of the first article? Note particularly figures 3 and 4 of the
Baluns.pdf article.

I've never had a
problem with them with well-grounded (from an RF standpoint) ground-
mounted verticals either.

The reason this provides balanced feedline currents is that the
impedance to ground at the base of the antenna is much less than the
impedance looking back from the feedpoint down along the outside of the
feedline. Consequently, the large majority of the current from the
inside of the coax shield flows to ground rather than down the outside
of the coax. And laying the coax on the ground keeps coupled common mode
current down.


Essentially this is why I recommend using open wire or twinlead and
feeding it through a proper balanced-line tuner.

That combination will produce a truly balanced system with no common
mode current. But it's not the only way.

. . .

Years ago, I built an
amplifier that literally had a balanced line output and fed a 600-ohm
feeder direct off two taps on its output coil. That feedline was only
ten feet long and I worked a TON of 80m DX an the inverted vee that it
connected to. And I could always tap the coil so as to have ZERO RF in
the shack (though my landlady's little 7.5 watt light bulbs used to light
on some frequencies when the house wiring picked up direct from the
antenna).

Roy Lewallen wrote in
:

Dave Oldridge wrote:

Still, if the antenna is TRULY balanced (a situation that only rarely
actually happens), you won't get common-mode currents.

That's true only if by "balanced" you mean that the two feedline
conductors carry equal and opposite currents. In that case, common
mode current is zero by definition. But if you really mean
symmetrical, as most amateurs do when they say "balanced", you
certainly can have common mode current.

No, I mean ELECTRICALLY balanced. And with the feedline at right angles
to the antenna so that it doesn't pick up anything by induction. It's a
tricky thing to do, yet back in the old days hams used to feed dipoles or
extended double zepp antennas with open wire line and not get much RF in
the shack. I know mine didn't. I was putting nearly 700 watts into the
antenna and you could touch the amp chassis without any RF burns. Didn't
have the fancy tools I have now for testing things, but still managed a
good, clean and loud CW signal from an angled dipole. Worked a lot of DX
on 75 with that antenna, including a nightly sked with Midway Is. for
traffic (from Vancouver, BC.).


--
Dave Oldridge+
ICQ 1800667