ual-Band Wire Beam for 17 and 12 Meters
From August 2005 QST © ARRL
he enthusiasm generated by the
Moxon
1
has left many amateurs
wondering if a multi-band ver-
sion is possible. Interaction be-
tween elements, however, has denied the
builder an easy answer. Nevertheless, a
dual-band version is possible for non-
consecutive bands.
The Design
In one of his articles, W4RNL de-
scribes three methods of nesting Moxon
rectangles that allows the antenna to re-
tain its desirable characteristics.
2
(For a
detailed explanation, I encourage the
A Dual-Band Wire Beam
1
Notes appear on page 32.
Figure 1The dual-band Moxon antenna layout, with basic element dimensions.
for 17
and 12
Meters
T
This wire beam Moxon antenna offers
double duty on 17 and 12 meters
with one 50
feed line.
reader to study this enlightening piece.)
The method presented here is the first
and simplest of the three versions to
build. The antennas 50
feed line at-
taches to the 12 meter driven element,
and from there a 75
transmission line
feeds the 17 meter driver.
The original antenna designs were
based on the use of
3
/
4
and
1
/
2
inch alumi-
num tubing for construction, but I wanted
to use wire elements, in order to allow
for portable operation. Reducing the ele-
ment diameter will reduce the SWR band-
width, but this does not become an issue
Allen Baker, KG4JJH From August 2005 QST © ARRL
Table 2
Cutting Schedule
Description
Quantity
Total Length
Materials
(Inches)
Spreader reinforcement
6
12
1
/
2
" OD fiberglass tubing*
Main spreader extension
4
48
1
/
2
" OD fiberglass tubing*
Main spreader
4
96
3
/
4
" OD fiberglass tubing*
Feed line spreader
1
48
3
/
4
" OD fiberglass tubing*
Balance spreader
1
48
3
/
4
" OD fiberglass tubing*
17 meter driver
2
153
14 gauge Flexweave wire and ring
terminals
17 meter reflector
1
325
1
/
4
14 gauge Flexweave wire and ring
terminals
17 meter insulator
2
10
5
/
8
Screen base (
1
/
2
" wide)
12 meter driver
2
115
14 gauge Flexweave wire and ring
terminals
12 meter reflector
1
235
5
/
8
14 gauge Flexweave wire and ring
terminals
12 meter insulator
2
6
1
/
8
Screen base (
1
/
2
" wide)
*All fiberglass tubing is
1
/
8
" wall.
Figure 2A top view of the antenna
center hub during construction, together
with the guy anchor post.
Figure 3A bottom view
of the antenna center hub.
Table 1
Modeled Antenna Data
Elevation
Free-Space
30 Feet
Center Frequency (MHz)
18.118
24.94
18.118
24.94
Gain (dBi)
5.97
5.81
10.94
10.84
Front-to-Back Ratio (dB)
27.99
30.64
21.35
25.46
Complex Impedance (
)
45.1
j5.3
44.4 +
j12.9
48.3
j8.9
41.6 +
j12.8
SWR
1.16:1
1.35:1
1.2:1
1.4:1
for the narrow 12 and 17 meter bands.
Numerous EZNEC modeling sessions
yielded a dual-band design that appeared
to equal Moxon wire monobander param-
eters.
3
Figure 1 shows the basic antenna
layout and Table 1 shows the modeled
data for both free-space and 30 foot el-
evations. Detailed construction drawings
are available at the ARRL Web site.
4
The Hub
To ensure a rugged and weatherproof
design, fiberglass spreaders are mounted
on an aluminum plate using stainless steel
U-bolts. A view of the center hub, to-
gether with the top guy anchoring post
can be seen in Figure 2. Two die-cast alu-
minum flanges secure the mast to the hub
on the bottom, and the guy line post on
the top. The hub is made from a one-foot
square of aluminum plate of 0.190 inch
thickness. Using a paper template taped
to the aluminum plate, center punch and
drill all holes. Next, cut the plate into a
circle to reduce the weight. On the top of
the plate, draw a 4 inch diameter circle,
in the center, with a permanent marker.
This circle will be used later to align the
spreaders. Four
3
/
8
-16
× 1
1
/
2
inch stain-
less steel hex bolts and stop nuts are used
to attach the flanges to the top and bot-
tom sides of the plate. Figure 3 shows the
hub bottom.
The Spreaders
The four main spreaders are made of
3
/
4
and
1
/
2
inch OD
×
1
/
8
inch wall fiber-
glass tubing. A feed line spreader sup-
ports the coax and transmission line on
one side, while a balance spreader pro-
vides balance and element support on the
opposite side. A 12 inch piece of
1
/
2
inch
OD tubing is inserted into one end of each
3
/
4
inch tubing at the hub for reinforce-
ment. A cutting schedule is shown in
Table 2.
To keep from drilling holes and weak-
ening the spreaders, the wire elements
and guy lines are attached using nylon
collars.
5
These collars slide over the
fiberglass tubing and tighten with a set-
screw. Four
1
/
2
inch ID collars are drilled
and tapped (opposite the setscrew) to ac-
cept
1
/
4
-20 nylon screws which are used
as the 17 meter element corner anchors.
Four
3
/
4
inch ID collars are drilled and
tapped (opposite the setscrew) to accept
1
/
4
-20 nylon screws, which are used as the
12 meter element corner anchors. The
nylon screws and collars provide a weath-
erproof method of securing the elements
without adding metal that might affect
resonance. The collars also provide an
easy way to adjust the elements. A par-
tial view of the inner element (12 meter)
collars can be seen in Figure 4.
The Feed Line/Balance Spreaders
Two type-C,
1
/
2
inch PVC conduit bod-
ies are mounted on the feed line spreader From August 2005 QST © ARRL
and provide access to the feed line, trans-
mission line, and driven element terminal
posts. The conduit bodies slide over the
3
/
4
inch tubing and are secured with stainless
steel setscrews. After you have determined
the location of each conduit body, cut slots
(inside the conduit bodies) on the top of
the
3
/
4
inch fiberglass feed line spreader to
allow coax cable entry and exit. Two
3
/
4
inch
ID collars are mounted on the balance
spreader for element support. Once again,
drill and tap both collars (opposite the set-
screw) to accept a
1
/
4
-20 nylon screw. Also,
drill and tap two
1
/
4
-20 holes in the bottom
of both conduit bodies for the setscrews.
Figure 5 shows a close-up of one of the
conduit bodies.
The Elements
The elements are cut from 14 gauge
Flexweave wire, according to the cutting
schedule of Table 2. One hundred feet of
wire will provide enough length for both
bands. The insulators are made from
1
/
2
inch
wide strips, cut from screen base (used for
screened-in porches). The elements are at-
tached to the insulators and feedpoints by
crimping and soldering ring terminals to
the wire endssecuring them to the insu-
lators with #6 thread-cutting screws. You
should be careful to include the lengths of
the ring terminals when cutting the wire to
get the correct lengths (Figure 6).
The Guy Lines
Nylon collars are drilled and tapped
to accept
3
/
16
inch stainless steel eyebolts
that are used for the guy line tie points.
Four
1
/
2
inch ID collars are used for the
four main spreaders and two
3
/
4
inch ID
collars for the feed line and balance
spreaders. Using a combination of forces,
that are provided by the guy lines and el-
ements, the main spreaders are stressed
into an S curve to supply the tension
needed to stretch the elements taut.
Prepare the guy line post by cutting a
14 inch piece of 1
1
/
4
inch schedule 40 PVC
pipe. Measure 2 inches down from the end,
Figure 4Element and guy anchor collars. The inner (12 meter)
element collars are shown.
and drill a
1
/
4
inch OD hole all the way
through the center of the pipe. Next, mea-
sure 1
5
/
8
inches down from the same end
and 90
° away from the first hole, and drill
a
3
/
16
inch hole all the way through the cen-
ter of the pipe. Install two
1
/
4
inch and two
3
/
16
inch stainless steel eyebolts with hex
nuts. With the eyebolts on top, secure the
pipe into the top hub-mounted flange by
tightening the setscrew.
Prepare six guy lines using Kevlar
twine, fishing swivels, and clips. The
Kevlar is stronger than fishing line and it
will not stretch as much as nylon or poly-
ester. The total length for the four main
spreader assemblies (Kevlar length plus
swivel/clips on each end) should be 96
inches. Make the feed line and balance
spreader guy line assemblies 46 inches
long. I used standard fishing line knots
on each end and, with a little practice, I
was able to get the lengths correct.
Each guy line is attached to an alumi-
num/stainless steel turnbuckle at the guy
Figure 5One of the conduit bodies. These house the
transmission and phasing line junctions to the driven elements.
Figure 7The guy wire center post, turnbuckles, and hub. Note the conduit boxes at
the lower right. These serve as feed line junctions to the driven elements. The feed line
and phasing line pass through the fiberglass support arm.
Figure 6Be sure to include the ring terminals when measuring element length. From August 2005 QST © ARRL
line post to make adjustments easier, as
shown in Figure 7. The other ends of the
long guy lines attach to the four
1
/
2
inch
OD collars with eyebolts mounted on the
main spreaders, and the short guy lines
attach to the two feed line/balance
spreader
3
/
4
inch OD collars. Figure 8
shows a closeup of a typical guy line
anchoring collar, together with an ele-
ment support collar.
Assembly
Since I wanted to be able to disas-
semble the antenna, I fastened the spread-
ers together with screws and nuts. Insert
a 48 inch long piece of
1
/
2
inch OD fiber-
glass tubing into one end of the
3
/
4
inch
OD
× 96 inch long main spreader with a
12 inch overlap. Drill two
9
/
64
inch OD
holes through both tubes3 inches from
each overl