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

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!

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!

On Sat, 07 Jan 2006 23:51:47 +0000, Dave wrote:

rf 'ground' is a real misunderstood thing. and things like this point out
just how poorly understood it is.
[snip]

Broken shift key, huh?
--
Flap!
The Pig Bladder from Uranus, still waiting for that
hot babe to ask what my favorite planet is. ;-j

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

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.

I only made that mistake once! That burn was very slow to heal, by the way.
I clearly understood several principles after that. Close-talking the mic
and over-modulation was the least of them.

"Charles Schuler" wrote in
:



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.

I only made that mistake once! That burn was very slow to heal, by
the way. I clearly understood several principles after that.
Close-talking the mic and over-modulation was the least of them.

This is where those artificial ground things come in handy. But the end of
the counterpoise needs to be where it can do no harm, as that's where the
artificial ground sticks the voltage.


--
Dave Oldridge+
ICQ 1800667

"Charles Schuler" wrote in
:




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!

A true windom with a single wire feed, or one of the latter-day kind with
twinlead?


--
Dave Oldridge+
ICQ 1800667

A true windom with a single wire feed, or one of the latter-day kind with
twinlead?

Single wire feed. I was a student (living off of my wife) and used what I
could get my hands on.