RF in shack with vertical and longwire antennas


articles at

Ground Systems

Longwire antennas

J-poles Zepps

and EH antennas

fed dipoles

contains model of

shows how
a sleeve adds
impedance, useful
for VHF and higher

Receiving Common
Mode Noise
how lack of a balun
can contribute to
system noise (it
applies to
antennas as

and Core

selection for
transformers and

testing transmitting

Common Mode and RF in the Shack

Our first common
assumption is that RF
the shack” that
distorts our audio,
causes computer
crashes, or (worse
yet) causes RF burns
is caused by
inadequate station
grounding. RF “in
the shack” often
occurs when we use
makeshift antennas,
such as a
longwire or some
other single-wire
feed system.
Obviously, bringing
a longwire or some
other antenna
directly into the
ham shack or placing
an antenna near the
operating position
will cause high RF
levels and
associated problems.

What may not
quite be so obvious
is we should never
need a station RF
ground when using two-wire
feed lines. This
includes both
shielded coaxial
cables and
unshielded open
wire lines. If
we have
properly operating
coaxial feed lines or
feed lines,  the
operating position
should have minimal
RF even totally absent a
ham shack ground! There
is one exception to
this, direct
radiation from the
antenna into station
wiring can sometimes cause
high levels of RF to
appear on equipment
or wiring.

Contrary to
popular belief that
only balanced dipoles
or balanced antennas need
baluns or
isolating devices or
systems, verticals
and longwires also
require “baluns” (more
correctly called
common-mode chokes
or isolators for this
This article shows
why baluns, or more
properly common-mode
chokes, might be
required when using
unbalanced feed lines
with antennas we
consider to be
At the root of this
problem is

common-mode current

on feed lines and
other wires and

What causes
common-mode current?

Current flows
because there is a
voltage or
electrical potential difference
between two parts of
a system, along with
a path that allows current
to flow between
those points.
This path can be
through empty space,
where special
currents (without actual
electrons flowing)
currents” flow, or
it can be actual
charges moving
through things we know to be conductors.
currents are
sometimes confusing,
or we don’t think of
them very much.
currents flow
between two plates
of a capacitor by
flowing through a
dielectric (even
vacuum or air).
currents also “flow”
between a
vertical or single-wire fed antenna and
“ground return
for that antenna,
such as displacement
currents between a mobile
antenna and a
vehicle’s conductive
body or chassis and the earth around that system.

currents complete the return current path in
antennas. They are
the sole reason
current in
physically large
coils can vary from
end-to-end, and are
problematic in mobile
. They
are the reason an
open-ended antenna
like a dipole,
longwire, or
vertical is able to
have current flow
out to the end of
the antenna, even
though the end just
hangs out there in
the air with nothing
around it!  

When we force
charges up into a
Marconi vertical or
longwire antenna
(making current
flow), we have to
move an equal number
of charges out of
some ground system
or counterpoise at
the feed point. The
ground system can be a
single conductor or
many dozens of
wires, and it can
(and often does)
involve equipment in
the house and/or the
coaxial feed line
shield. The bottom
line is we always
must have the same
current coming into the feed point from some sort of counterpoise as
the current that
moves up into the
antenna at that point! There is no
way around that
rule, and this
requirement for
equal currents
flowing into the
ground and up into
the antenna creates
two problems:

  1. The system
    will not be able
    to compensate
    all the charge
    with a few
    radials without
    voltage driving
    those radials
  2. The
    feed line has to
    connect to the
    antenna, the
    outside of the
    shield may be
    excited by this
    voltage, and
    become part of
    the return path

Another way to
view this is
feed lines or
feed points of our
Marconi (end-fed)
antennas must have
something to push
force current into
the antenna. It is
very much like
physically pushing a
car. If we have very
poor footing, our
feet will move and
slide as we push. We
not only waste
energy that could be
used to move the
car, we have
movement or motion
where we don’t want
movement. The same
is true for a ground
system, as feed line
current up into the
antenna the other
terminal of the
feed line has to be
held steady. We
waste energy that
COULD be in the
antenna, and we have
RF movement where we
don’t want it….on
the outside of the
coax shield and on
anything connected
to that

A dipole, even
though a balanced
antenna, also has
problems with common
mode currents when
fed with  an
unbalanced feed line.
Some feed line
lengths can cause
problems, while
other lengths might
eliminate need for a
balun. There is no
universal magic
length to minimize
common mode currents
because the required
length to minimize
common mode varies
with the routing,
grounding, and
surroundings of the
feed line. If we have
a specific case,
like a vertical
feed line hanging in
free air from a
dipole and running
straight down to
earth and grounded
at earth’s surface,
then the magic
length is 1/4 or any
odd quarter
with no correction
for velocity factor

The feed line

If you are not
familiar with how
coaxial cables work,
you might want to
look at a
simple  explanation
on this site
one of the ARRL

In order for a
conductor like the
outside of the
shield to not have
current flow at
radio frequencies,
it must have the
same electrical
potential and phase
on the shield’s
outer surface

all along the
cable’s length. If
the feed line has a
high series
impedance (common
mode impedance), or
if the potential
difference along the
conductor is low,
very little current
will flow on the
outside of the
shield. Any
coaxial feed line, or
two-wire twinlead or
open wire line, can
have unwanted common
mode currents. 

Does a vertical
or longwire present
significant common mode
voltages to the
feeder, voltages that can
cause common mode
currents? You bet it
does! The only
vertical (or
longwire) that would
not cause such
problems is one with
a very good or
nearly perfect
ground system, and
that means something
at the feed point
that looks like a
large infinite
groundplane. Even
then, the feed line must
exit below that
groundplane to be
“shield current
free”. Of course current fed antennas are more of a problem, voltage fed
antennas have much less return current at the feed point. Keep in mind though
that voltage fed antennas, like an end-fed halfwave, do not have infinite feed
impedance. End-fed halfwaves still require a counterpoise, always! People
sometimes fail to recognize the counterpoise, but one always must be there or it
would be impossible to feed the antenna.

Aren’t Four
Elevated Radials

Here is a model
of a groundplane
with four radials:

Groundplane with feedline model

EZNEC ver. 3.0
Balun 80 vertical
1/3/04 7:19:05 PM
Frequency = 3.6 MHz.
Wire No.
Wire No.
1.359 (This is your
feed line or mast)
Wire No.
1.985 (These are the
Wire No.
Wire No.
Wire No.

We can see
significant current
flows over wire 2,
which would be the
coax shield, a mast,
or both. 



There is a trick
with Eznec. By
inserting an
additional source in
the mast or feed line
and setting current
of that second
source to zero, we can
see the voltage
required across a
feed line isolator to force
current to zero. In
this case the
voltage across a
perfect isolator would be:

Source 2 Voltage
= 145.5 V. at 67.97
Current = 0 A. at
0.0 deg.
Impedance is

At 1500 watts the
radial system common point
needs to have 145.5
volts to earth

prevent current flow
along the outside of
the shield! If we
elevate the common
point to 145.5 volts
at 68 degrees phase
angle, we now have
the following
currents at 1500

Wire No. 1: 6.4 A
Wire No. 2: 0 A
(coax shield or
Radials: 1.58 A each

How many times
have we been told
four carefully tuned
resonant radials make a
zero-resistance ground? Obviously
any claims that four
resonant radials
create a
near-perfect ground
reference are
not true. With four radials
the antenna might
have high
efficiency, but the
antenna is

Since the antenna is
not perfectly
unbalanced, we
cannot connect a
feed line without
some feed line
lengths or feed line grounding
causing appreciable
unwanted current to flow over
coax shield. 

You can see an
example of this in
link to groundplane

Other Systems

With fewer
radials the
situation becomes
much worse! (As a
matter of fact, this
is a good reason to
use as many radials
as we can even if
the radials are

Claims that four
elevated radials
form a “perfect
ground better than
120 radials”
are obvious
nonsense! If it was
a perfect ground,
there would be no
potential difference
to earth and no
common mode current
flowing to
ground” ! Four
radials may be a
reasonably good
ground when elevated
a considerable
distance above lossy
media, but it is not
zero impedance.

If the antenna
has a high base
impedance, it will
have less current at
the feed connection.
(Sorry, but we
cannot do this by
using a folded

Longwire and
Windom antennas
really aren’t much
different than
verticals. They are
a form of Marconi
antennas, and
require a
counterpoise or
ground of some sort.
As with verticals,
common mode current
flowing into the
antenna must be
balanced by current
flowing into a
ground system. You
can see how that
works at this link

end-fed antennas.

Instead of
bringing the
longwire directly to
an antenna tuner, a
better solution is
using an RF ground
system independent
of the station
safety ground, and
keeping that ground
isolated from the
station safety
ground. That can be
accomplished by
adding a good
heavy-duty 1:1 choke
or current balun a
few feet from the
tuner, and
connecting the RF
ground to one output
terminal and the
antenna to the other
terminal. With a
two-wire fed Windom
(really an OCF
dipole), the two
wire feeder should connect
to the choke balun
or isolator and then
to the antenna.

The balun MUST be
a current balun,
rather than a
voltage balun. The
balun or isolator should be at
or near the base of
the antenna. In
tough situations a
second choke 1/4
wavelength away will


The cure for
common mode problems
caused by
or smaller ground systems is inserting
a good 1:1 choke balun in
the system at the
antenna feed point.
The coax should also
be kept away from
the radials as it
exits the area of
the radials and the
antenna. An antenna
with a poor ground
using few radials
cannot have a
support mast
grounded to the
radial common point
(at least it
shouldn’t if
designed properly).
There is no
exception to this! A radial system of 25 or more full-size radials would likely
have minimal common mode on an unbalanced feeder, but current rapidly increases
as the ground system becomes smaller.


Feeding end-fed antennas