I’ve got a strange fascination with folded dipoles. Single or stacked and phased, I just can’t seem to shake the need to build one from my head. So toady I have made a simple jig to help bend up a batch of folded dipoles.
You will need
First off, you will need to know the dimensions of the dipole that you are making. I started with my element length using this tool set to calculate both sides. On 434.000 Mhz that gave me a element length of 329mm.
Then I went here and played around until I got my input impedance to 200 Ohms. Change the dimensions at the bottom of the page to see the results at the top change. I ended up with D1 & D2 = 0.4 (4mm is the size of my elements), S = 4.4 and an Assumed Dipole Impedance = 50 Ohms.
Once you have your dimensions, get the materials and find a round former to match your S value minus the diameter of your elements (so 40mm for me).
A few offcuts of wood
Some kind of round former (40mm for me – I used an old wallpaper seam roller that had broken off its handle)
A few screws
Screw the thinner piece of wood along the back of the thicker piece. The thicker piece is the base of the former and the thinner piece becomes the back.
I use 4mm steel rod (just because I have a large stock of it) for my elements so I measured 20mm from the end of the base and 24mm (my round former is 40mm so half of that + 4mm for the element) from the back. I then screwed my former to the base.
I’m building folded dipoles centred on 434.000 Mhz so I measured out the correct length and marked it on the base. for me this was 329mm from end to end. Remember to take into account the size of your element. I also marked the centre (164mm) on the base as well as 154mm and 174mm to give me a 20mm gap at the ends of the element.
Start bending. Place the end of your element at the 174mm mark and slowly bend it round the end former. When done, turn the element around placing the end at the 154mm mark and complete the other end.
There we have it. A nice, well formed folded dipole. Next time we will look at building a 4:1 coax balun and feeding it with 50 Ohm coax.