Windom off center fed

Windom off center fed

revised some wording
1/11/2010

The modern common
Windom
antenna is not an
original single-wire
fed Windom, but
rather an off-center-fed or
“OCF” two wire feed dipole.
The normal Windom is
described as having
a 1/3 length leg,
and a 2/3 length
leg. In other words
an 80-meter Windom
137 feet long would
have one leg 45.667
feet long, and the
other leg 91.333
feet long.

After spending some
time modeling OCF antennas,
the
closest antenna I
could find to a true
multi-band dipole
with reasonable SWR
on most bands had an
80%/20% leg length
and 200-ohm
feedpoint.
This would be 109.6
feet for one leg,
and only 27.4 feet
for the other leg.
The antenna could be scaled
for other bands.

There is one
caution with this.
The large offset
means the balun must
be particularly good
current balun on the
lowest frequency
bands, and the
terminal best able
to stand the highest
voltage to earth
should be on the
shortest antenna
side. (Nearly all
baluns are not
symmetrical in
voltage to ground
capability for both
terminals.)

Windom Antennas

Loren G. Windom,
ex- W8GZ, was
instrumental in
the development of the
Windom
antenna. Loren lived
in Reynoldsburg,
Ohio. Loren was
originally 8ZO from
Columbus, Ohio in
1922. His call is
now reissued.

Windom’s original
idea was off-center feeding a
horizontal antenna
to present a
feedpoint
impedance of 600
ohms. This is close to
the surge impedance
of a single-wire
feed line, allowing an inexpensive
single conductor to
be used as a
feed line.

In theory, with
suitable selection
of the feedpoint tap
point and antenna
length, a reasonable
match should occur on
multiple
harmonically related
bands.

The Achilles heel
of the original
single wire Windom
is the single-wire
feeder. It has no
“return conductor”
or shield, so the single-wire feeder:

1. radiates and
   couples to
   everything around
   it. This causes
   TVI and RFI and
   couples local
   noise into the
   receiver
2. causes
   considerable
   common mode
   currents along the
   feed line and at
   the matching
   system. The common
   mode currents and
   strong electric
   field extending a
   considerable
   distance from the
   feed wire causes
   
   RF grounding
   problems and common
   mode currents in
   the Hamshack and
   on all the shack
   wiring and
   equipment.

Later
Generations of the
Windom

Later generation
of the Windom are
more correctly
called off-center-fed dipoles (or in
abbreviated form, OCF
dipole antennas).
Some people might
take issue with
using the word
“dipole” because the
antenna has more
than two opposing
polarities along the
length, but in my
opinion it is
perfectly fine.
Physics does have

two-pole “dipoles”
that are rigidly
described as such,
but not in the
context of antennas.
A

dipole antenna
is defined in many
antenna engineering
textbooks and
dictionaries as an
antenna that is
insulated at some
point forming two
halves of no
particular length.
There can be short
dipoles and long
dipoles, sometimes
well over 1/2
wavelength long. If
it is good enough
for people like
Kraus, Jasik, and
others to use the
term dipole to
describe a “two
half” antenna of
varying length, it
is good enough for
me.

The problem with
making a multiband
OCF dipole is
finding a sweet spot
that presents a
reasonable impedance
on all amateur
bands. As frequency
increases the length
of the antenna and
offset of the
feedpoint become
critical. Along with
critical length and
feed line offset
comes increased
sensitivity to
surroundings.

Theory Behind
The Windom

Every
unterminated antenna
has standing waves.
In the context of
standing waves, we
actually are
speaking of voltage
and current
distribution along
the antenna. If we
represent current
levels with a line,
a half-wave 3.5MHz
dipole’s current
distribution looks
like this:

Notice the offset
feedpoint does not
distort the current
distribution along
the wire. Current is maximum
at the center and
minimum at the open
ends. This means the
feedpoint impedance
is lowest at the
center, with current
increasing towards
the ends. The
impedance, if we
break and feed the
antenna with a
two-wire source, is about
50-70 ohms at the
center and very high
(but not infinite)
at the ends. By
offsetting the
feedpoint from the
center we can
increase the
feedpoint impedance
without upsetting
current distribution
or antenna pattern.

The same “dipole”
operated on 14 MHz
(the 4th harmonic)
shows four current
peaks, or four “low
impedance” areas
along the antenna:

When there are
multiple peaks in
current, each
current peak is less
intense. Peak
current at any point
is less than with
the same power
applied to a
half-wave. This of
course means
the impedance at
each low impedance
point (current
maximum) is higher.
The end-impedance is
also lower! Notice
in this case the dot
representing the
feedpoint was offset
but very near the
maximum.

Finally at 28MHz
we have this:

Notice once again
the feedpoint falls
very near a current
maxima. Now there
are 8 maximum
current points. With
a 137-foot long
antenna we have
eight maximums. This
is one maximum every
17 feet. An 8-1/2
foot move in antenna
length or feedpoint
location takes us
from a maximum to a
minimum! This means
we have to be very
careful with antenna
dimensions and
feedpoint location
or we could miss the
maxima.

The Windom or OCF
dipole is a
balancing act. We
must position the
feedpoint and select
the antenna length
to place the
feedpoint at or near
a current maximum at
every desired
operating frequency!
The higher the
frequency the closer
the feedpoint must
be to the center of
the current maxima
on that band. A
large error or
offset can be
tolerated on the
lowest band, but not
on the highest band.
This is because
current changes very
fast with physical
position on the
highest band, and
because maximum
current levels are
lower than on the
lower bands.   

Slight
adjustments in
antenna length and
feedpoint location
affect the highest
bands severely.

SWR Plot 80%
Feedpoint OCF Dipole or
Windom

Like many people,
I’d like to have one
simple cheap antenna
that handles 1500
watts, has a low
SWR, and radiates
efficiently on all
primary HF bands. 
I spent a few hours
of time
modeling OCF dipole
or two-wire feed line
Windom antennas. This is the best
combination I could
find.

Length: 137 feet
of bare #14 to 16
AWG wire

Height: As high
and clear as
possible

Feedpoint:
Located 80% from one
end.  27.4 ft
from one end. 

Here is an SWR
sweep of this
antenna:

As an alternative
this antenna can be
fed at the feedpoint
with a good 4:1
current balun. Be
careful doing this
because many 4:1
current baluns are
very poor designs.
Some baluns
advertised to be
current baluns are
not even current
baluns!

The following
plot shows SWR using
a good 4:1 current
balun.

This provides a
very useable antenna
on
80,40,30,20,15,12,
and 10 meters. The
antenna is not
particularly good
where SWR exceeds
5:1. 60 and
17 meters are
excluded from “good
performance” bands.

66.7% Feedpoint
Windom

Note: This
feedpoint offset
gives up 30 meters.
This is the primary
reason I avoided the
1/3-2/3 feedpoint
position and used the
80% feedpoint
position.

.The following 300
ohm plot shows 300
ohm feed line SWR.
This is the SWR that
would appear on a
300 twinlead feeder:

The 80% offset is
better when using
300-450 ohm lines.

The following 200
ohm SWR plot is for
a Windom which is
1/3 and 2/3 offset,
as shown in the ARRL
Handbook. This is
the SWR that would
appear on a 200-ohm
balun at the
feedpoint:

since June 2006.

c. W8JI 2006

.