Dipoles and the rig's RF ground...
 

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

Dipoles and the rig's RF ground...
 

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).

Dipoles and the rig's RF ground...
 

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/

Dipoles and the rig's RF ground...
 

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).

Dipoles and the rig's RF ground...
 

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

Dipoles and the rig's RF ground...
 

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

Dipoles and the rig's RF ground...
 

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

Dipoles and the rig's RF ground...
 

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.

Ran a Windom in Texas in 1965 (WA5KBO) with only 150W and burned a hole in
my lip (no joke) with the RF on the metal ring around the microphone! The
Windom was a good performer, but I could not effectively ground the rig. I
was in student housing (College Station) and was not allowed antennas but
improvised!

Dipoles and the rig's RF ground...
 

rf 'ground' is a real misunderstood thing. and things like this point out
just how poorly understood it is. there is really no need for a radio to be
'grounded' to prevent rf burns or to have an antenna work properly... the
important thing is to remember that at that point where the rf leaves the
radio on the center conductor of the coax connector the current there must
be exactly balanced by a current going the opposite direction the inside of
the shield of the coax connector. looking at the worst possible case, just
stick a random wire in the coax connector and run the rig off a battery with
a short cable and no other 'ground' wire. current flows out the center
conductor of the connector into the exposed wire and somehow has to get back
to the inside of the connector shell to balance it out... well, the only
place for that current to come from is coupling from the antenna wire back
onto the case of the radio and from there it flows back into the connector.
now, put your hand on a metal part of the radio, or your lip if you are
unlucky, and what happens?? you are much bigger than the case of the radio
and you are fairly conductive, so now you provide a bigger collector for the
current from the antenna so lots of it flows through you to get back to the
radio connector... hence rf burns. how to stop it?? provide a lower
impedance path for the rf to get back to the connector than through you.
that can be a counterpoise wire, a 'ground' wire that collects current from
the soil under the antenna, connect the case of the radio to your car and
the car becomes the collector... OR add an equal sized second wire that
'balances' the current from the wire in the center conductor... the critical
point is that it mut be very nearly identical to the first one so the
current in it is the same... a dipole that is symetric with respect to the
feed point will work, but you have to watch out because the case of the
radio is connected to the coax connector also, which tends to unbalance the
equation since there is no equivalent lump of metal on the center conductor
part of the antenna. to make this job of balancing the currents easier we
normally add a length of coax (to get the antenna farther away from the
radio) then add a balun to help force the currents in the two halves of the
dipoles to be equal so there is no need for current to flow from the radio
back into the inside of the shield. a choke on the outside of the feedline
also can help, but the reason is different... a choke on the coax creates a
very high impedance so that current has a hard time flowing from the case of
the radio or outside of the coax back to the inside of the coax. enough
rambling, just remember, rf ground is a myth, all you need to do is get
those currents equal while preventing the path where they flow from being
through you or something else that could be damaged by them.


"Charles Schuler" wrote in message
. ..



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.

Ran a Windom in Texas in 1965 (WA5KBO) with only 150W and burned a hole in
my lip (no joke) with the RF on the metal ring around the microphone! The
Windom was a good performer, but I could not effectively ground the rig.
I was in student housing (College Station) and was not allowed antennas
but improvised!

Dipoles and the rig's RF ground...
 

Charles Schuler wrote:
Ran a Windom in Texas in 1965 (WA5KBO) with only 150W and burned a hole in
my lip (no joke) with the RF on the metal ring around the microphone! The
Windom was a good performer, but I could not effectively ground the rig. I
was in student housing (College Station) and was not allowed antennas but
improvised!

Dang Charles, I did exactly the same thing in 1957. If you had
asked me, I would have told you to watch out for those metal
microphones when using a Windom.
--
73, Cecil http://www.qsl.net/w5dxp