Add 6 Meters to Your Triband Trap Yagi
This approach is almost painless, stealthy and gets the job done.
Joel R. Hallas, W1ZR
ack in the 1950s, HF transceivers
were just starting to replace separate
B
receivers and transmitters and our
DX bands were 20, 15 and 10 meters. The
triband trap Yagi became a very popular
antenna for those who wanted to work DX
but couldn’t swing separate monoband Yagis
for each band. Many amateurs also operated
on 6 meters in those days, but the equipment
was usually separate from the HF gear — the
focus of VHF specialists, in many cases.
Fast forward to 2011 a n d alm ost all
current “HF” transceivers also cover MF (160
meters) and VHF (at least to 6 meters), with
similar performance, power and features as on
the HF bands. A look on the towers of many
amateurs will yield a view of the same type
(or even the same) trap tribander from the ’50s.
That Was the Situation at W1ZR
In my case, the triband Yagi was a relic
from the ’80s I obtained for a price too good
to pass up. I didn’t actually have a tower to put
it on, and after getting the neighborhood acclimated to the driven element tied to the top of
my chimney for a few years, I took the plunge
and sunk a pipe mast next to the chimney, put
on a rotator, and — one piece at a time — the
Yagi grew in place of the solo driven element.
During the same period, I retired my old
160-10 meter transceiver to replace it with
a modern unit that covered 160 through
6 meters. Now my radio had outpaced my
antenna farm. I could operate 6 meters using
my 100 foot center fed Zepp, but I had nulls
every few degrees all the way around —
something had to be done.
The Mast Thickens
My challenges were twofold. First, because
my ro tator was mounted on top of a mast,
rather than inside a tower, I had to derate the
rotator’s wind load capability by 50%, and
avoid any bending moments resulting from
loads above the rotator. That put the tribander
right above the rotator. To add 6 meters, my
first thought was to investigate the low wind
load Moxon we reviewed in 2004, secured a
few feet above the tribander.1 Unfortunately,
my modeling indicated that installing them
just a few feet apart would result in significant
degradation of the gain and pattern of both
1
Notes appear at end of article.
From September QST © ARRL
antennas — back to the drawing board.
I had been very pleased with the results
of my 40 and 20 meter skeleton sleeve dipole
described in a recent QST article — could
I use the same technique to add 6 meters to
my Yagi?2 EZNEC modeling indicated that
it could indeed work — and work very well.
This gave me two significant advantages:
I had sidestepped the wind loading and
bending moment concerns. The added elements were right above the rotator and the
thin elements were largely in the shadow of
the tribander’s elements or boom, depending
on relative wind direction.
Perhaps even better — I did not need
an add itional feed line. The HF feed line,
going to the split driven element, would also
feed power to the 6 meter Yagi.
through parasitic coupling, so no connection
to the tribander is required.
This occurs
The Details
Before I proceed, I should give credit
where due. Following publication of my two
band “skeleton sleeve” dipole article, I found
that the parasitic coupling to a single element
was pre sented in an antenna ar ticle in The
ARRL Antenna Compendium, Volume 5
Gary Breed, K9AY, also the developer of the
low frequency receiving loop that bears his
call letters.4 Gary called it a coupled resona-
antenna — perhaps more descriptive a
tor
name. There’s nothing new under the sun — it
would seem.
by
Design Approach
The usual issue with Yagi design is that
there are many variables as well as many
objectives. The primary variables are element length and spacing while the objectives
are generally forward gain, front-to-back
ratio (F/B) and bandwidth. They tend to fight
each other to some extent, and others may
3
find different combinations that are better in
one respect or another.
My goal was to achieve reasonable Yagi
performance with elements comfo rtably
between those of the tribander. Using EZNEC
modeling, I was able to find a set of dimensions that were predicted to work well starting
from the National Bureau of Standards (NBS)
baseline of 0.2 l parasitic element spacing.5
The modeled forward gain was within about
1 dB of a similarly sized three element Yagi
in the same space but without the tribander —
not a bad trade, in my view.
The loss in a mismatched transmission
line is a particular problem at VHF, so it is
important to match to whatever impedance
the Yagi offers. In a traditional VHF Yagi,
the low impedance is generally transformed
to a matched value through an adjustable
matching arrangement. For the coupled resonator with no direct connection this is accomplished, as predicted by Breed’s formula, by
adjusting the spacing between the HF driven
element and our coupled resonator. I found
that adjusting the center-to-center spacing
from about 4 inches (the minimum possible
with the mounting hardware) to the 10 inches
shown, I could increase the impedance of a
single element coupled resonator from 45
to 120 W. The same adjustments resulted in
a reduction of element resonant frequency
from 50.2 to 49.2, so retrimming is required.
By using the 10 inch spacing for the three
element case, the low impedance of the Yagi
configuration was transformed to close to the
desired 50 W (see Table 1).
A Few Caveats
This project was initiated on a trap tribander with a split dipole feed. Although this
is the arrangement of many such Yagis, other
configurations will be encountered. Some
may include a shunt transmission line section
across the feed or other matching arrange-
Table 1
Measured SWR at Antenna
Frequency SWR
50.0 1.2
50.1 1.1
50.2 1.1
50.3 1.2
50.4 1.6
Figure 1 —
I used the DX
Engineering (www.
dxengineering.
com) stainless
steel saddle
clamps, as well as
their telescoping
aluminum tubing
and stainless
element clamps.
Lower cost nonstainless hardware
could be used if
the budget is tight.
ments. They may also work but I haven’t
tried them. If you’re not sure, try it with just
the driven element before you commit to the
whole project.
Elements
For elements, I selected aluminum tubing
in diameters of 1⁄2 and 3⁄8 inches and a wall
thickness of 0.058 inches. These telescope
nicely. For the center of each element, I used
a 3 foot section of 1⁄2 inch tubing with the ends
slit and compressed on the smaller section on
each side using stainless hose type clamps
of the appropriate size. I obtained my tubing
and clamps from DX Engineering, which
offers the tubing in 3 foot and 6 foot lengths.
The 3 foot, 1⁄2 inch tubing is available with
one end pre-slit for a slight additional charge,
and they may offer it with both ends slit by
the time you read this. I had reasonable luck
slitting the other end using either a band saw
or a hacksaw with the tubing in a vise. The
outer 3⁄8 inch sections were made from 6 foot
lengths, two required per element — each cut
to 4 feet 4 inches long.
Element Mounting
I chose to mount the elements insulated
from the boom to avoid having to make the
required correction for all metal construction.
I used a 3 × 6 inch piece of 1⁄4 inch polycar-
bonate for each insulator. I would guess other
materials could be used, but polycarbonate
comes highly recommended and was readily
available.
6,7
To mount the insulator to the boom, and to
the elements, I choose stainless steel saddle
clamps also fr om DX Engineering. These
clamps are very nicely constructed (see
Figure 1).
Although they are more expensive
than the hardware store variety, I thought they
were worth it. For those on a tight budget, less
expensive clamps may work fine for many
years, but will make for tougher disassembly.
Once you have the clamps selected, carefully lay out the insulator for drilling. Figure 2
indicate s the construction lines that I laid
on the handy paper that came applied to the
McMaster-Carr polycarbonate. It is important
that the holes be lined up quite closely. If the
clamps for one side or the other aren’t paral-
lel, the tubing may bind. If misaligned, the
6 meter elements will not be parallel to the
HF elements.
If your shop gear and skills are up there
with Barry Shackleford, W6YE, you could
make the holes just a bit larger than the U
bolts. For me, using a hand drill and a vise, I
found I had to open them up just a bit with a
rotary grinding tool.
Mounting the Elements
Figure 3 shows the dimensions of the
6 meter elements on the Yagi. Note that I have
referenced them all to the center of the HF
driven element. This should allow for some
differences between tribanders. For the record,
my tribander is a Wilson Electronics SY33.
This looks a lot like the very popular Mosely
TA-33, but has slightly wider element spacing.
Table 2 provides a summary of dimensions
for the NBS 0.2 l spacing case that I used.
In case your tribander has a shorter boom, I
have also included dimensions for a version
with 0.2 l reflector spacing and 0.15 l direc-
tor spacing. EZNEC modeling predicts that
this version has about 0.5 dB less gain than
the larger version. I haven’t actually tried the
shorter version.
Figure 2 — Dimensions of the construction lines and hole locations for the 3 × 6 inch
pieces made from the McMaster-Carr polycarbonate sample pack. The dimensions
assume a 2 inch boom and
obtain correctly sized saddle clamps and lay out accordingly.
1
⁄2 inch inner element section. For a different size boom,
Performance
Modeling is fine, but on-air measurements
remove any guesswork. I made measurements of W1AW (50 air miles away) code
prac tice on 20, 15 and 10 meters before
and after adding the 6 meter elements and
found no difference. I asked W1AW Station
Manager Joe Carcia to put a signal on a clear
6 meter frequency. I was very pleased; gain
was at least as good as EZNEC predicted (see
Figure 4). My F/B was not as good, perhaps
due to reflections from multiple antennas.
From September QST © ARRL
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