10 ELEMENTS ON A 4 METRE BOOM WITH FOLDED DIPOLE FEED by Dave, VK2JDS (October 2006) This antenna utilises a commonly available boom length of aluminium tubing from harware stores and allows 10 elements to be added using galvanised fencing wire or recycled tv antenna parts. The example in the pictures has been assembled for over 10 years using the fencing wire and tiewire mounting technique for the directors. Originally this antenna was part of a slot-fed phased array which was mechanically cumbersome and difficult to rotate hence the remanufacture to make it a workable yagi. In this experiment I decided to attempt to make a folded dipole out of straight aluminium scraps using pliers and drills that anyone would have in their tool collection. It looks a bit wobbly but it works briliantly and proves it can be made simply from recycled parts. Dimensions: Element lengths: Reflector 980mm Driven element 925mm overall for the folded dipole. Refer to the folded dipole detail. Director 1 890mm Director 2 870MM Director 3 870mm Director 4 860mm Director 5 850mm Director 6 845mm Director 7 845mm Director 8 840mm Spacings: Reflector to driven 405mm Driven to director 1 305mm Director 1 to 2 300mm Director 2 to 3 330mm Director 3 to 4 500mm Director 4 to 5 520mm Director 5 to 6 520mm Director 6 to 7 520mm Director 7 to 8 520mm Driven element dimensions: overall length from tip to tip is 930mm to cover the segment 144 to 147 MHz The driven element is constructed from scrounged tv antenna parts comprising a 2 piece insulator section and 2 bent pieces of aluminium tubing. The elements fold back along themselves and are cut to length then drilled and bolted into the plastic mounting. Note that there is a strap of aluminium joining the ends together that are at the boom but not being connected to the feedline. Refer to the picture of the unassembled feedpoint.The spacing between the folded elements is 50mm. Balun at the feedpoint dimensions: Because the folded dipole is approximately 300 ohms impedance we need a 4:1 balanced to unbalanced transformer which is easily made from some rg213/u coaxial cable, the same as you are using for the feedline. Cut a piece of coax 670mm long then cut the plastic back from each end and peel the braid out so the length of the sheilded section is 630mm. Find or make some eyelets for the inner copper of the cable to attach to, then solder and trim the overall length including the eyelets to be 675mm. The centre conductor of your feedline attaches to one terminal on the driven element as does the centre conductor of one end of the balun, the other end of the balun has its centre conductor attached to the other terminal on the driven element. Join the 3 braids together as neatly as possible and then waterproof it all with silicone etc. Note that the braids dont attach to the antenna at all.
1. assembled feedpoint
2. the directors
3. the reflector
4. finished yagi up and mounted on mast