Dipoles and the rig's RF ground...

I realize that dipoles are balanced antennas, but does the rig
itself still need an RF ground too? (I know the radio always needs a DC
ground, of course). How about if the dipole is being used as a
non-loaded "all band" antenna (IE: RIG--TRANSMATCH--LADDER
LINE--DIPOLE) -- would this affect the need for an RF ground on the rig
for operation in the dipole's non-resonant bands? Or is no RF ground
_at all_ required with a dipole; unlike when using random wires or
verticals, and other such un-balanced antennas?

Thanks!

Bill

 

With ladder line it is best to use a balun between the antenna tuner and the
transmission line. An RF ground on the rig is then not required (you won't
have RF voltages on the rig's chassis).

The thing about an RF ground is that due to the length of the ground
circuit, the rig is often not grounded anyway.

 

If your feedline is balanced, that is it has equal and opposite currents
on the two conductors, then there's no current left over to flow to or
from ground and no need for an RF ground connection. All the current
from one conductor goes back on the other. Feedlines can be balanced
even if they're coax and/or the antenna is unsymmetrical; they can be
unbalanced even if the antenna and feedline are symmetrical.

If the feedline isn't balanced, the difference current (that is, the
difference between the currents on the two feedline conductors) will
find its way to ground however it can. This often creates undesirable
effects. But if you can't avoid it, it's better to provide a low
impedance path for the ground current if possible. And that can
sometimes be difficult to do.

(I know the radio always needs a DC

No, it doesn't. It needs an AC safety ground if connected to the mains,
and a lightning ground if that's a possible hazard. But DC isn't important.

How about if the dipole is being used as a

The trick is to get the feedline balanced on all bands. That requires
either a truly balanced tuner, or a combination of a good balun and
impedances on all bands at the balun which the balun can handle.

Roy Lewallen, W7EL

 

ORIGINAL MESSAGE:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Any antenna which requires a connection to ground should be shown the
trash can immediately. Ground (earth) is a lousy conductor and does
nothing to help your signal. RF belongs up in the air, not down in the
dirt.

If you find that connecting a ground wire actually improves your
signal, you have a SERIOUS problem in your antenna.

73, Bill W6WRT

 

Most modern shacks have to long distance between the rig and ground. It's
not equal to an ungrounded rig but you might experience hf in your shack.

This is best solved by a short ground cable to a proper ground rod.

The next best thing is to buy or build an artificial ground. It's very
simple and can be made to cover all ham bands easily and will always give
you a perfect length of the earth cable... By cheating of course - but it
works!


Cheers

M0DFI

 

Get a virtual earth! They are easy to build for all ham bands!

It's basically a phasing unit for the earth connection which can null the
voltage on the earth at the RF Rig!


Cheers

M0DFI

 

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's the ideal case and in that ideal case you need no RF ground at the
radio. The ideal case, however, rarely ever exists in practice.

And end-fed wires can be a whole different ball game. I had to use a 16
foot counterpoise once to "ground" a rig in a 2nd story location when I
end-fed a very long wire with it.

 

Thanks Guys -- I really appreciate the clarifications on grounding! It
sometimes gets a bit confusing for me.

Best Regards,

Bill

 

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).

 

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

 

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.

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.

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

 

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.).

 

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.

 

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.

 

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

 

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.

 

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