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My
Fan Dipole Project
By WA2HIP - John
In 2008 I built 2 fan dipoles that are shown on the home page of
my web site (http://wa2hip.com )
. One fan dipole was for 75/40/20 and the
other was for 20/15. The purpose of
building 2 was to have an East-West broadside and a North-South broadside on
the dipoles.
These 2 dipoles are at right angles to each
other and the 20/15 dipole is located about 6 feet below the 75/40/20 fan
dipole. The 75/40/20 fan dipole is at a
height of approximately 50 feet.
Here I
will discuss my plans, thinking, details, and construction tips.
1.
I
used a 2-inch diameter PVC pipe about 10 inches long.
2.
I
Installed on each side about 1/3 of the way down, standard eyebolts.
3.
About
2/3 of the way down another eye bolt (will speak of this later)
4.
Drilled
2 holes about 1/2 of way down and ran a SHORT (10 inch) length of #14 stranded
and insulated wire.
5.
Just
about 1 inch above these holes I put 1 machine screw on each side with the
threads on the inside of the pipe and then used a crimp lug to connect 1 end
each of the #14 wire.
6.
Connect
the SO-239 to these 2 screws. Keep the
leads short. I used #14 solid wire.
7.
Got
2 clean-outs with plugs. On top one I drilled and installed another
eyebolt for supporting the top
8.
On
bottom plug I installed a standard SO-239 connector.
9.
Installed
the bottom (PVC cement and ran wires from center and shield to each of the
machine screws.
10. Installed top. Note
that if any repairs are needed internally the top cleanout plug can be removed.
Notes:
·
The
purpose of the extra eyebolt near the lower end is for a support line for the
coax (tape the coax to the line about every 24 inches and leave about 3 inches
of slack in the coax between tape points.
·
Connect
the legs of the antenna to the eyebolts by twisting and then taping (avoids
slippage),
·
Skin
back the insulation of the legs near the twist and connect the ends of the
pigtails to these points
·
The
pigtail leads allow for some flexing and also provide a GOOD connection between
the feed line and the antenna legs
·
Shape
the pigtails so there is a "drip loop" similar in shape to that on
the electric service entrance to your house. It will keep water out of
the center PVC pipe
·
Note
that the top eyebolt can be used for hoisting up to the trees and the support
rope, aside from supporting the coax can also act as a retrieval line for the
center sport.
Photos
of Center Insulator:
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Top View |
Bottom View |
Supports:
For support I used nylon rope. For the East-West (broadside) dipole the
center would be located between 2 trees.
Thus I ran a “suspension” rope between the trees and then installed a
pulley in the center.
For the North-South (broadside) I used the
same 2 trees but about 5 or 6 feet lower.
To facilitate this I hung the wire below the rope in a “catenary”
fashion. This is similar to that seen
on the power supply for an overhead-electrified railway or light-rail system.
Then ends
of the East-West dipole are in trees. Again
I used nylon rope for these.
For the 3-band dipole I used 12-inch
lengths of ¾ inch PVC. I drilled a hole
(completely thru both sides) at 1 inch from each end and in the center. The antenna wire will pass thru these holes.
These should be spaced about every 6 feet (or so). But the first one should be about 1-½ feet
from the center insulator.
Note: I
found that it was beneficial to hang a small weight from the insulators at the
ends of the 15 & 20 meter dipoles.
This keeps the wind from twisting the dipoles.
For my dipole I used #12 insulated and
stranded wire. I was given a good-sized
quantity of this when my place of work moved from one building to another
I used the standard calculation of “234 /
Frequency (MHz)” to calculate the length of the dipole’s legs. IMPORTANT NOTE –
Research showed that with insulated wire the velocity factor is reduced by
about 5%. Thus the dipole’s length needs
to be reduced accordingly.
Yet, another technique I applied is to
calculate the length for the highest frequency (including the 5% reduction) and
then to further reduce the length by about another 3% to 4%. This further reduction I will discuss in the
“Tuning” below.
Now, take the wires, run them thru the
insulators. Next twist the wire around
the upper eyebolts (see photos) of the center insulator. You should have 7 or 8 turns around the
wire. Tape these so they will not slip.
At the ends twist the wire around the PVC
insulator and back on itself. Again
about 7 or 8 turns and tape it.
Skin the wires insulation near the center
insulator; attach a short pigtail (about 6 to 8 inches) to each wire. Solder these and tape.
Skin the wires insulation near ALL of the
ends and attach a length of wire that is about equal to the 3% to 4% reduction
(see above). In fact, ADD another
2%. This will be a “Tuning Stub”.
Solder these and tape.
Getting The Ropes Up In The Trees
There are a multitude of techniques and I
will describe what worked best for me.
I began with a 30 pound bow and some simple
and cheap target arrows. I drilled a
small hole just in front of the “nook”.
Thru this hole I passed 20-pound test nylon fishing line and tied it.
Place the spool of fish line on its
side. DO NOT plan on letting it “spool:
off because this will be too much drag.
Just let the line “spin” off the side of the spool.
For me, to achieve the 45 to 50 foot reach,
I found that standing near the tree and aiming up at about a 60 degree angle
worked best.
Once the arrow gets to the other side of the
tree you may need to “play” it out until the arrow reaches near enough to the
ground to grab it.
Attach a length of mason line to the fishing
line and pull it back thru the tree.
Then I used some small diameter rope and pulled it back with the mason
line.
Now you are ready for the final and larger
diameter rope. I used ½ inch nylon
rope.
When attaching this to the small rope. DO
NOT knot it. I will get stuck in the
tree. Rather, place the 2 ropers
overlapping for about 12 to 14 inches and wrap tightly with the mason line and
finally electrical tape. This will slip
thru and over the tree fairly easily.
Connect all
and you can pull your creation into the sky!!!!! Check clearances and location and note where you want make
adjustments for clearance, location and etc.
We are ready to tune this beast. It is much easier than it may appear.
1.
Drop
the center and connect ONLY the lowest band antenna. The pigtails from the center and the antenna element can be
twisted but not taped.
2.
Hoist
this center (or end) back up and check for resonance.
3.
Check
the SWR from bottom to top of the band.
4.
Now
can figure (calculate) how much to shorten the legs.
5.
Drop
the ends and “fold back” the tuning stubs onto themselves. For now a short piece of electrical tape
will hold it in place.
6.
Repeat
steps 2 thru 5 until the desired results are achieved.
For each higher band repeat this
procedure. Remember – connect OLNY the
band being tuned.
After all have been tuned, drop the center
insulator, connect all of the pigtails and hoist it back up. Now verify all SWR reading are satisfactory. You may need to do some minor tweaking. This is where the “tuning stubs” are a tremendous
asset also.
When all is well, solder and tape the
pigtails. Also, tape or cut the tuning
stubs. Personally, I would just tape
these securely.
Last and important is to wrap tape around
each spacer where the wires pass thru.
This will prevent the spacers from slipping/sliding on the wire.
Do not
forget to have a “drip loop” in the pigtails to keep water out. Just look at your house’s electric service
entrance (outside) to see a similar “drip loop”. I also taped over the holes where the pigtails exit the center
insulator.
Other
Notes.
ALWAYS, ALWAYS, have retrieval lines on you
end insulators. If the wire breaks you
need to be able to pull down to reattach.
Otherwise the insulator and rope may be lost.
For vertical supports at the ends, tall
trees are the best. But some may need
to us a pole. One suggestion I have is
to go to the local building supply and get a 20 foot oak stair rail. These can be purchased for about $30. It may
be a cheaper alternative to a fiberglass pole.
For the center insulator, the extra eyebolt
on the side serves the purpose of attaching a retrieval line.
ADDITIONALLY, you can “strain relieve”: you
coas by taping it to this retrieval line.
I taped it every 18 inches. BUT
for 18 inches of rope I had 20 inches of coax.
This slack will further avoid damage to the coax if the rope does
stretch.
Your coax should be strain relieved!!
Below
are some of the preliminary SWR curves for these 2 fan dipoles. This is prior to the extended “tweaking”.
Remember that the directions indicated are the broadsides.