Build the Featherweight 6m Yagi.pdf
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Give your signal extra punch on amateur radio's "magic band" with
t h i s c o m p a c t three-element beam f o r
6
meters.
e
Feather-\rxeigl~t
is an ultra-light
VHF Yagi that goes where heavier
anten~ias
can't. Designed especial-
ly for roof-top installations, this compact
three-eleinent beam tips the scales at
a
mere two
I I O L I ~ I ~
and uses inexpensive
S
TV hardware to get high above surround-
iiig obstacles.
I11 additioii to being extremely small
and light, the
F c c ~ t h e i ~ ~ ~
is
g
easy to
i ht
build using comnioii tools and readily-
available hardwa1-e.Best of all, it requires
no tuning to achieve low-VSWR perfor-
mance. And, if you're
a
serious
DX
liuiit-
er, you call add a seco~id
Feather>veight
in a stacked array for extra gain and great-
er capture area.
This particular versioil is a second-
generation relidition of an antenna
I
origiiially presented in our sister publi-
cation,
Corr~rnl~nicritiorzs Qrrai.terly
("Tech Notes," Summer, 1995). The
di-
n~ensionsare virtually the same, with
{nillor hardware changes to reduce
coiistructio11.
weight and si~liplify
Tlie a.sseri~bletl
Feoiher-~~iglit
6-rileter-Yagi at tlie n~~t1zor'~s
station.
or sligl7fl~below.
This places 50.125
side of the anten-
MHz on the opti~nuin
NEC" analysis of predicted
performance
na's perforniance curve, and also pro-
for tlie
Featl~enveighr
antenna (special vides conipeiisatioii for precipitation
thanks to Paul Carr,
N4PC.
for tlie corn- build-up oil rainy days, which lends to
p~~terruii). booin
is
six feet Ioilg, with lower
Fr.
Tlie
a turning radius of just under six feet.
According to EZ-NEC, free-space for-
Frequency of resollance (Fr) is typically ward gain is predicted at 8.1
1
dBi (or 5.97
about 50.250
MHz,
or 125 kHz above the dBd)?, the -3 dB beamwidth is 62', and
S S 3 calliilg fi-equency. The aiiteiina is the front-to-back ratio
(FIB)
is predicted
tuned slightly high because best gain and at 33 dB (real-world
FIB
was measured
front-to-back ratio typically occur at Fr at
-1
8 dB in a roof-top installation). The
use of precise element leiigths and
no-
:'Rick Littlt.fielc1,
KIBQT,
is
a
regulrrr trlrle
hairpin matching eli~ni~iate need
tlie
co~ltr-ib~rtor
CQ VHF
a~irl
to
Coiiiniu~li- for post-construction tuning. The anten-
catioils Quarterly.
na s h o ~ ~ easily handle 200-300 watts
ld
PEP in service.
(tclBi
refers to decibels,
or dB, of gain over an
isotropic
antenna
in free space, ail ideal that exists only as
a
reference;
clBrl
refers to the more real-
of decibels gain over
istic ~ne:~s~ireii~ent
a
dipole
antenna.)
The "Table of Materials and Hard-
ware" provides a checklist of hard~val-e
and materials you'll need prior to assem-
bly. Figure
1
provides an overview of
what the completed antenna will look
like. Refer to both as you work. Begiii
preparatioii by laying out and drilling
24
0
CQ VHF
June
1996
Ham
Radio
Above
50
MHz
3 ELEMENT
6
METER BEAM
10-04-1 995 07114159
Freq
-
50.25
MHz
-3dB:
329, 31
deg
Slobe:
-18.77
dBi
Outer Ring
-
8.11
dBi
Max. Gain
=
8.11
dBi
Elevation Angle
=
0.0
deg
Azimutlz Plot. Featlzerweight antenna modeled
on
"EZ-NEC"
<
3
1
5
-
2
"
Driven Element (1 10-112")
Center Insulator
Reflector (1 17-112")
Figure
I .
Ovet-all view of the Featherweight 6-meter Yagi.
mounting holes in the boom. Note that the
Featherweight
uses equal element spac-
ing, placing the driven-element insulator
at the exact center of the boom. To mark
this location, measure three feet from
either end. The center insulator will be in-
stalled on the
bottom
surface of the boom.
After finding the mid-point, locate and
mark hole locations for the mast-mount
U-bolt. These will be centered on the
side
surface of the boom at 32
'12
inches and
34
114
inches from one end. Drill two 5116-
inch holes through both walls at these
locations (if possible, use a drill press to
ensure the drill axis is perpendicular to
the boom). The reflector and director
mounting holes are positioned '12-inch
back from each end of the boom. Mark
these locations on the
side
surface of the
boom, about 3116-inchup from the bottom.
Drill through both walls with a 5~~6-inch
bit. When drilling, try
to
make
the lower
edge of the drill bit contact the inside sur-
face of the boom as it goes through. This
will provide a flat channel for seating the
element as it passes through the boom.
Now, place a mark on the bottom sur-
face of the boom-directly centered over
the element mounting channel-and drill
through the bottom wall with a '18-inch
bit. This hole will be used when pinning
the element in place later on.
Next, prepare the ~lexiglasm
center in-
sulator and spacer, following the detail
provided in Figure 2. If you have diffi-
culty finding Plexiglas stock, check your
local industrial plastics distributor or
automotive glass replacement shop: most
will have Plexiglas remnants for a rea-
sonable price. Don't substitute a plastic
with unknown insulating properties.
Once you've cut and drilled the Plexi-
glas pieces as shown, you can use the
spacer as a drilling template to mark the
insulator mounting hole locations at the
center of the boom. Use a 7164-in~h to
bit
drill two pilot holes for the #12 sheet-
metal mounting screws.
The hairpin matching inductor is easi-
ly fashioned from
#8
soft-drawn solid-
aluminum ground wire (Radio Shack
#15-035). The exact dimensions are
shown in Figure
3.
When forming the
hairpin, make sure both sides are straight,
symmetrical, and conform to the pattern.
This will be installed later.
The elements are cut from 5/16-inch
thin-wall aluminum antenna tubing. This
normally comes in 12-foot lengths, and
may be purchased from aluminum prod-
ucts supply houses, such as Metal and
Cable Corporation in Twinsburg, Ohio
Ham
Radio Above
50
MHz
June1996
CQ
VHF
25
(also, check your area Yellow Pages for
a local aluminum tubing distributor).
Cut the director exactly 107 inches in
length and cut the reflector to 117
112
inch-
es. Drill a [is-inch hole at the exact cen-
ter of each element (at
53
112
inches for
the director and 58
3w
inches for the re-
flector). These holes will be used later for
pinning the element to the boom.
The driven element is made from two
tubing sections cut to 54
$4
inches each.
To prepare these for mounting, crimp the
last '12-inch of one end flat in a vise, then
drill a 3116-inchhole centered about 114-
inch from the end.
Ctr
I
118"
C;r
7132"
Ctr
31
16"
Sheet
Plexiglas
I
Use Figure 1 as a guide during assem-
bly. Place the Plex~glascenter insulator
and spacer
111
position, sandwiching the
spacer between the insulator and the
boom. Secure both with two #12 x h -
inch sheet-metal mountilig screws, using
a square to confirm that the center insu-
lator is seated exactly
90"
to the boom.
Next, use two #8-32 x l-inch machine
screws to mount the hairpin and driven
element sections. Refer to Figure
4
for
You
Be t h e
Do you know an interesting short
story about a well-known person in the
history of ham radio, radio communi-
cations, or electronics generally?
About the origin of some of the terms
we use every day?
A
significant event
in ham history? Or a person who made
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,
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N.
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e-mail to <CQVHF@aol.com>? "True
facts" only, please.
If
we can't verify
it, we won't print it. If we do print it,
we'll give you a free one-year sub-
scription (or extension) to
CQ
VHF.
mounting details. Make sure the hairpin
center-tap points toward the,fiorzt of the
antenna (away from tlie U-bolt holes).
Once the driven element sections are
bolted in place, secure each to tlie insu-
If
lator with 5116-inchnylon cable clan~ps.
916-inchclamps are not available, use 31s-
inch clamps and build up the driven ele-
ment with a few turns of electrical tape
to provide a snug fit. Confirm that each
side of the driven element sits 90" to the
booni. If either element is off. loosen its
cable clamp and reposition.
To ground the hairpin center tap to the
boom, drill
a
'18-inch pilot hole on-cen-
ter, inserting the drill bit through the hair-
the
pin tap. Sec~tre tap in place using a
#8
sheet-metal screw and a '14-inch
x
5116-
inch diameter spacer, as shown in Figure
5.
The director and reflector are mount-
ed as shown in Figure 6. Slide each ele-
nleilt into place so that the 11s-inch hole
90-degree Angle
/
1
"
x
1
-
112" Plexiglas Spacer
(underneath)
Figure
4.
Ceiltei. block cl.s.serr~hly
clrtciil.
26
0
CQ
VHF
June1996
Ham
Radio Above
50
M H z
#8
flat
washer
Fi,qrci-c
5.
Huirpiri c.critri.-rtr,~~
gi-oifrrtii~z,q
ilrrtiil.
ally correct) way to meet this requirement
is to install ferrite sleeves over the feed-
line at the antenna feedpoint. This creates
a "current balun" that will choke
RF
radi-
ation off the outside of the coax braid anci
prevent pattern distortion.
Figure
7
shows how to prepare the
feeclline balun for installation. Fil-st, slip
three FB43-562
1
coax sleeves over the
antenna-end of the
RG-8X
coax line.
Then, strip back the plastic outer insula-
tion
1
inch and prepare pigtails as shown.
on
Install
a
spade l ~ l g each pigtail (crimp-
type lugs should be both criinped
crncl
sol-
dered to ensure a good electrical connec-
tion). Pigtail length should not exceed
1
inch including the lug, since longer leads
may lower the antenna's Fr. Position the
three FB43-5621 ferrite sleeves to with-
in 114-inchof the end of the outer cover-
A
lightweight antenna deserves a light- ing and wrap three to four turns of elec-
weight feedline!
RG-8X
(or mini-8) foam trical tape around the coax-behind the
coax provides an ideal feed for tlie
F e ~ i t h -
third sleeve-to form
a
stop.
er~jeiglzt
antenna. This cable is readily
Finally, apply two to three thin coats of
available, easy to install, and will perform "Seal-All" (or similar waterprooi'ing
well for rooftop runs of up to 60 or
70
feet sealant) to the braid area of the pigtail to
at 50
MHz.
For longer runs, use an ultra- retard water migration into the cable.
low loss cable, such as Belden 9913, to When the sealant is fully dry, connect
reach the antenna site, then use
RG-8X
each coax pig-tail to a driven element
up the TV mast and around the rotor.
~nountingstud usiilg
#8
nuts (stainless
The antenna's driven ele~nentis bal- steel wing nuts inay also be used).
anced and requires a
1
:
1
balun at the feed-
When ~nouiitingthe
Featl7ervveight.
point. Theeasiest (if not the most technic- note that the center insulator is positioned
in the rniddle of the element lines up with
the 11s-inc11hole in the bottom surface of
the boom. Pin the element in position with
#6 hardware. Tighten firmly to prevent
movement, but avoid crushing tlie alu-
inin~trn
tubing.
This co~npletesassembly of tlie basic
antenna. Make sure all elements are per-
pendicular to tlie boorn. Also. confilm
that the ele~nents in line and that spac-
are
ing between element tips is approxiniate-
ly 35 inches. If necessary, beid the ele-
ments slightly to correct any misalign-
ment. To prevent wind vibration ill the el-
ements, use
a
pair of pliers to crimp each
element tip closed (TV-antenna style).
on the
hoito117
side of the boom. if possi-
ble, use
3
TV-antenna mounting liit that
includes boom reinlhrcernent plates for
a
I
-inch square boom
( I
-inch x
I
-inch is the
stanclrlrd boom size for TV antennas).This
will protect the boom from being crushed
or distorted by the mounting hardware.
If
LI
reinforcement plate is not available, con-
sider malcing one-or at least using fend-
er washers-to
spread out the force
applied by the
U-bolt mounting
nuts. Coax
should exit the driven element at a 90°
angle to tlie center insulator and run down
the mast. Secure feedline tightly to the
mast with electrical tape-just below the
b"l~ln-toi'rovide
aiidlJrevent
brealtage
at
'he pigtails.
If you want to use your
Ferrtl7er~~eigllt
for mo~intailitopping "roving" in a con-
or
test, this optional method for ~nounting
tlie director and reflector will enable you
to I-emoveeleiiieiits quickly and pack the
antenna for easy transportation.
First. fabricate two 6-inch element
sleeves from 3s-inch dia~neter
thin-wall
alun~inumtubing, as shown in F i g ~ ~
8.
re
To prepare the ele~nent
sleeves, slot each
end at
90'
with a handsaw, making Sour
of
slots of about
3 ~ 4
an inch long each.
Then, drill
a
11s-inch pinning hole at the
exact center 6f each tube. If your anteii-
na
is already assembled, relnove the cli-
rector and reflector from the boon1 and
element ~nounting
ream the 5~~c,-incli
holes out to 3/~-inch accommodate the
to
larger-diameter ele~nent
sleeves. Cut the
reflector and director elements in half at
their exact center. To re-install the ele-
ments, insert each half illto the appropri-
ate sleeve and clamp in place using small
stainless-steel hose clamps (available at
most hardware stores).
To collapse the driven element sections
for transport, either unbolt and relnove
them or unscrew the cable-clamps and
swing each ele~nent
back parallel with the
boom. With
a
little practice, assembly and
disassembly taltes only a minute. This
modif-kation has little or no effect on
antenna Fr or perfor~nance.
Boom
end
Figrii-r
6.
E l f ~ ~ 111oi~7zti1ig r ( / i / .
t ~ ~ i t d~
The
F e n t h o - b ~ ~ ~ i
provides about the
ght
same mast-loading as a small "economy-
sized" TV antenna. Because of this, you
can mount it significantly higher than a
comparab!e co~i~mel-cially-built
6-meter
Yagi. Standard TV-antenna mounting
28
0
CQ
YHF
e
June1996
Ham Radio Above
50
MHz
hardware and virtually any light-duty
rotor will do the job.
In most locations, the antenna may be
mounted up to 10 feet above the rotor
without over-stressing the bearings (if
possible, use aluminum mast above the
rotor). To avoid property damage and
personal injury, plan your installation
carefully in advance and arrange to have
at least one extra pair of hands available
during the raising. If you plan to mount
the antenna on an existing tower, allow
at least 5 feet of vertical spacing from
VHF Yagis, and more from larger HF
Yagis (the more, the better).
Although the
Featherweight
may seem
somewhat delicate and flexible by com-
munication-antenna standards, it is sur-
prisingly resilient. My own stacked array
has survived a number of New Hamp-
shire ice storms and still remains aloft. If
standard TV antennas can survive in your
climate, the
Featherweight
should cer-
tainly survive as well.
Figure
7.
Balun
nizd
feedline pigtail cletnil.
this,
be
sure to transpose the pigtail
con-
nections on the second antenaa:
if this is
not done, the two Yagis will be fed out-
of-phase and the signals will cancel each
other instead of adding together for im-
proved gain.
Mount the first antenna as close as pos-
sible to the top of the rotor, and mount
the second at the very top of the 12-foot
stacking mast (if you use thin-wall mast,
it may be helpful to install a short length
of wooden dowel in the top and bottom
to prevent U-bolts from crushing the tub-
ing wall). This arrangement will provide
about 11 feet, 6 inches of spacing, which
approximates 518-wavelength at 50 MHz.
Electrically, this spacing provides a good
compromise between optimum gain and
a clean pattern that's free of side-lobes.
If
you cut the elements exactly to
length and formed the hairpin accurate-
For improved weak-signal perfor-
mance, you may stack two
Feather-
weights
with 518-wave spacing (see
photo). Overall gain should increase to
between 8 and 9 dBd, and the
apparent
gain
on some signals may be higher due
to the larger capture area provided by us-
ing two antennas.
To keep my stacked array light and
easy to manage,
I
used a single length of
12-foot x 1 114-inch thin-wall aluminum
as a stacking mast. This configuration
was still easy to support using TV hard-
ware and a small TV-type rotor.
If you decide to stack, you'll need a
stacking harness to match impedances
and distribute in-phase power to both
antennas. To make a harness, use RG-59
75-ohmcoax (solidor foam). Start by cut-
ting one
electrical-wavelength
of cable,
using a grid-dip meter or VSWR analyz-
er to determine the exact electrical length.
Then install the specified FB43-5621 fer-
rite baluns and pigtails at each end, as
detailed in Figure 7 (RG-59 is the same
diameter as Mini-8, and the preparation
procedure is the same).
Next, measure exactly one quarter way
down the length of the harness, and cut
the cable in two. Install PL259s on these
ends and splice back together using a
coaxial
"T"
adapter. This becomes the
array's 50-ohm feedpoint.
Finally, connect the phasing harness to
the antennas (see Figure 9). When you do
Table
of
Materials and Hardware
ITEM
QTY
1
1
2
1
1
1
2
2
Aluminum
117 112"x
5~16"
dia. thin-wall tubing (reflector element)
107" x 5 ~ ~dia. thin-wall tubing (director element)
6"
54
3/4"
x
5/16"
dia. thin-wall tubing (driven element)
72" x 1" x 1" square .047" thin-wall aluminum boom
Hairpin matching stub (from #8 solid ground wire) 1 112" x 5 112"
lt4"
x
5/i6"
aluminum spacer (made from element scrap)
Stainless Steel Hardware
#12 x 3/4''pan-head sheet metal screws
#8-32 x 1" pan-head machine screws
#8 flat washers
#8-32 hex nuts
#8 x 3/4" sheet metal screw
#6-32 x 3/4"pan-head machine screws
#6-32 x 112"pan-head machine screws
#6 star lock washers
#6-32 nuts
Other Materials
1
112" x 10" x 31s" Plexiglas driven element insulator
1
'12" x 1" x
318"
Plexiglas insulator spacer
5/16"
(or 318") plastic cable clamp
FB43-5621 ferrite balun sleeve
#8 spade lugs (crimp or solder type)
Antenna mounting U-bolt kit with nuts, lock washers, and mast clamp
Reinforcement plate for 1
"
x 1" square tubing
Tube, Seal-AllTM other waterproof sealant
or
Rover Option
6" x
31x"
thin-wall aluminum tubing sections
Small stainless-steel hose clamps
8
6
1
2
2
4
4
1
1
2
3
2
1
1
1
2
4
Ham Radio Above
50
MHz
June
1996
CQ
VHF
29
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