As always, when building an antenna it pays to do your math first. I find that 300 million kilometers per second divided by the frequency you want to use the antenna on then divided by 2 for 1/2 wavelength, then multiplied by .95 (percent of) the speed of light gets you pretty close. The Q of this antenna is low enough that no adjustments should be necessary to hit your target frequency if these instructions are followed. Also as with any ladder-line fed antenna, any final adjustments may be made by adjusting the length of the feed line.
Doing the above equation, for smack dab in the middle of each standard ham HF band you will need wires of at least:
1.9 MHZ 160 meters 75,000mm 246 feet 3/4 inch
3.75 MHz 80 meters 38,000mm 124 feet, 8 inches
5.332 MHZ 60 meters (channel 1) 26,725mm 86 feet, 8 & 1/8 inches
7.150MHZ 40 meters 19,930mm 65 feet, 4 & 1/2 inches
14.175MHZ 20 meters 10,053mm 32 feet 11 & 3/4 inches
21.225MHZ 15 meters 6,713mm 22 feet & 1/4 inch
28.850MHZ 10 meters 4,939mm 16 feet 2 1/2 inches
BUT DON'T cut the wire yet!
I say at least because you will want to add about 6 inches above and beyond the lengths indicated here so that at either end of each wire you have enough to feed through the insulator and you don't know how any variations in the feed wire will necessitate some adjustment. Just keep these measurements in mind for the time being.
For the third wire, the one that is fed at the center, use the same formula 300 divided by frequency then divided by 4 then multiplied by .95 You will need several extra inches for feeding through the insulator, and the same 6 inches at either end for the end insulators.
1.9MHZ 160 meters 37,500mm 123 feet & 5/8 inches
3.75MHz 80 meters 19,000mm 62 feet 4 & 3/8 inches
7.150MHZ 40 meters 9.965mm 32 feet 8 & 3/8 inches
14.175MHZ 20 meters 5,027mm 16 feet 5 & 7/8 inch
21.225MHZ 15 meters 3.357mm 11 feet & 1/8 inch
28.850MHZ 10 meters 2470mm 8 feet 1 & 1/4 inches
Cut 2 wires of equal length accounting for the added extra you need to feed through the center insulator, the end insulators, and maybe even some extra for wiggle room.
The best place to start construction of a multi-wire folded dipole is at the center. My favorite design is from the November 2010 issue of QST,
It should look similar to this when done.
You may have noticed that the center insulator here differs slightly from the one linked to on the "Supplemental" page. Namely the extra hole for the eye-bolt is omitted, I deemed it not necessary, but you could include it if you decide your ladder line is long/heavy enough to need extra support. Simply tie a string from the eye-bolt to the upper spreader bars. Also, I arbitrarily changed the spacing of the middle holes slightly.
Feed each of the short wires through the center insulator and twist back on itself as shown.
Cut the 2 the long wires from above. Make sure they are extra long. Longer than 300/f/2 and the six inches at each end I mentioned. We are only doing an approximate length right now and NEED extra.
Find the midpoint of each of these 2 wires and clamp to a board with the wire that has the center insulator as shown.
Ensure that the wires are laid parallel and secure at regular intervals. I am using black electrical tape. Twists of soft copper wire, zip-ties or any other reasonable method would work fine.
Measure the length of all 3 wires from the center of the feed insulator. The midpoint of the feed insulator MUST be exactly in the center of the assembly. Trim wire ends and adjust as necessary.
We now have 3 wires exactly the same length with the feed point exactly in the middle.
Measuring out the desired length of wire from the center, mark the length you need (the black electrical tape), then add the extra needed to go through the insulator. Repeat for both ends.
Feed the wire bundle through the end insulator and fold over as shown. Now is a good time to ensure that all wires continue to be parallel to each other. Repeat the for both ends.
Using soft copper wire, take two equal lengths about 18-20 inches long and bind each end in a tight coil of wire. Ensure both ends are nearly as equal in all measures as possible. Copper-clad steel does not lend itself to adjustments. All aspects of the antenna should be considered permanent from here on out.
Once satisfied, solder solid. As there are only 6 wires in the coil, and each wire is fairly stiff, they will tend to arrange themselves equally around the inside of the coil. You may want to add a small 1-2 inch piece of wire to go down the center, to better make a solid mass, but it is not required.
I'd recommend using a 80-100 watt soldering iron. They are readily available at many craft/hobby stores. Mine came from a stained glass shop. If you feel uncomfortable relying on the bond of soldering lead to the copper cladding, I have had luck with a 180 degree bend of any extra antenna wire back over the coil for added mechanical strength. I did not allow enough to do that for this demonstration. If I had I would have factored in a minimum of 1/2 inch extra into the calculations.
Remove the antenna to an outdoor location, and suspend or place under a light tension. Remove the tape or zip-ties, and spread the wires ensuring no tangles or crosses. The last thing you need is any shorts in the parallel circuit you have created. Any crossed wires must be corrected before continuing.
Make the spreader bars of a non-conductive material. I have successfully used 11/16 X 3/4 Hemlock trim moulding available from the local hardware store in 8 foot lengths for around $5-6 each. 3/4 Oak or Poplar dowels, either square or round would work too for considerable extra cost. Any PVC pipe would work too, but Schedule 40 UV resistant PVC electrical conduit is intended for outdoor use, and is what I actually recommend. The choice is yours.
During previous antenna experiments with fat dipoles, I discovered a point of diminishing returns in spreading the radiating elements beyond a distance of about a foot apart or so on 40 meters. So keeping this in mind, I've arbitrarily decided that the elements should be spread 2 feet apart, so that when looked at on edge there would by 2 elements 2 feet apart with a third element (visually) down the middle. I have no idea if this is good, bad, or indifferent. Given the broad bandwidth of this design, generally satisfactory performance, I've never felt the need to experiment further in this arena. I am open to input from other experimenters on the results of bandwidth v/s spacing results.
Edit: I am also told that the antenna need not be constructed in a 3 dimensional cage type fashion. Meaning all 3 wires side by side parallel to each other in a flat configuration would be just as effective. The only issue I can think of is the fed element in the center would be of a different length than the others and would therefore behave differently somehow. I have not investigated this proposition and claim to be no authority on the matter.
Install the spreader bars at each end. I like to install the ones at the end such that the 2 foot bar is 2 feet from the end so a nice visually appealing cone is formed. The flatter the "cone" the greater the effective tension on the wire segment between the spreader and the insulator will be. Placing the load at 0 (or 180) degrees (pulling straight in line with the wire axis)=100 percent load rating available and would be ideal. Since this design is in 3 dimensions, some angular force is necessary. Consequently, some de-rating of the wire strength is necessary. At a 65 degree angle, the wire is under 110% load. At 45 degrees, the wire is under approximately 150% load. At a 30 degree angle, the wire is under 200% load. Granted, there is 3 wires, so theoretically 3 strands of 12 gauge copper-clad steel would have a rating in excess of 2,700 pounds when pulled at 0/180 degrees, which is plenty, but this quickly drops to 1350 pound rating at a 30 degree angle. Still plenty strong, for this case, but worth keeping in mind if any other gauge, type of wire, or significant span or suspension weight is used.
To achieve equal spacing of all spreader/supports, measure the distance between the end supports you just installed and divide by the number of spaces you intend to have. 4 support sections will have 3 equal spaces between. 6 supports gives 5 spaces, 10 supports gives 9 equal spaces.
Take the total distance you measured, divide by the spaces between, and that is the distance from each other that the middle supports should be.
Install the remaining spreaders, adjust and secure tightly. The black tie wraps are UV resistant and oriented so that the pull the wire into the pocket. If using wood spreaders, observe how the wrap of soft copper wire goes through the center of the triangle and pulls the antenna wire deeper into the pocket.
Place the antenna under tension and orient the center insulator so that it remains parallel to the ground in a neutral state. It may be necessary to grip the wire wrap on either side of the feed insulator and twist, to accomplish this. Solder the wire wrap at the feed insulator. Then install the feed line per the article on the "Supplemental" page.
To make the spreader bars: Simply measure out equal lengths of your desired material. Select a drill bit the same diameter as the wire you intend to use, then drill each end centered, perpendicular and in alignment with holes in the the other end of the spreader bar. A drill press will aid in being straight and centered in the round stock, and a section of wire placed in the far end will aid in making sure the holes you are drilling are in alignment with the holes you just drilled. The distance from hole to hole should be the same for all spreader bars made.
Place the spreader bar in a vice and cut just up to the holes you drilled with a narrow saw blade. A hacksaw, or narrow jig saw will work well. The idea is the wire will "snap" into the pocket you have created. If you are using wooden spreaders, cutting in-line with the grain instead of across is the way to go.
As we have gripped, twisted, bent, formed, hammered(!), straightened and otherwise stressed the wire, good practice dictates a close and careful inspection is warranted. A dab of solder any place the copper cladding may have chipped, or any steel wire is exposed will prevent rust from forming. Steel wire, especially steel wire under a load, will rust through, weaken and get very brittle very quickly.
After the inspection is complete, hoist the antenna into its permanent installation.
Connect the coaxial feed line to the balun and measure the match with an antenna analyzer. If these instructions were followed it should be near 1:5 or better. Trim the feed until satisfied with the match. I am satisfied with the match here. Click on the SWR sweep video to see why.
Route the feed line to either the 9:1 or 12:1 balun. Then, erring on the side of caution, cut the feed line at approximately an odd multiple of 1/8 wavelength of the frequency the antenna was designed.
When an antenna is not in use, or when there is a risk of dangerous weather it is recommended that it be disconnected from the radio. It is at this point that Anderson Power Poles connectors may be added. You should also notice that the baluns are out of the weather, properly grounded, and there is a lightning arrestor properly grounded and installed outside the shack for extra nth degree safety/redundancy. The power pole connectors also make a convenient break point to add in other sections of feed line of various lengths to make the antenna system "resonant" on other bands.