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.
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.
· 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:
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.
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.
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.
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.
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.