The basic design of the antenna is a fairly simple folded dipole.
To construct the antenna you will require between 3m & 4m of 1/2″ (15mm) copper pipe, 8 x elbows, 4 copper saddles and a few nuts & bolts.
Construction shouldn’t take much more than an afternoon, you will require a tape measure, some solder, flux, blow torch and a pipe cutter.
Pipe cutters can be found fairly cheaply from DIY stores. I use the type that clamp around the pipe, which is then cut by rotating the cutter.
The unspecified lengths merely state “adjust length to balance on mast” I’m mounting mine in the loft by hanging it from the trusses, but I would guess at a length of about 14-15″ for these pieces.
You will also require a very short piece to create the 2.5″ (64mm) “U” shape – the exact length required will depend on the elbows being used.
Note: All measurements are taken from the centres of the pipework, so you will need to calculate the lengths to cut the pipes allowing for the size of the elbows used.
As an example, the elbows I used required me to subtract 1/2″ (12mm) from each measurement shown in the diagram.
Thus for the 27.56″ length, I had to cut the pipe at 27.06″ as each elbow added approx 1/4″ (6mm) – x 2 elbows = 1/2″ (12mm) to maintain the correct centre to centre distance.
Once you’re happy that everything is correct, you can go ahead and do the final assembly by soldering the joints.
If you don’t have much experience of sweating copper pipework joints, I suggest you use solder-ring elbows to make the job a lot easier.
If you’ve never soldered copper pipe before, here’s how to do it – it’s fairy simple!
Start by cleaning the ends of the copper pipe using wire wool, fine sandpaper etc. Apply a thin film of plumbers flux to the end of the pipe and push the elbow into place.
At this point remove the heat and let the joint cool. Remove any flux residue with a damp cloth.
Note that when soldering elbows, both ends will heat fairly uniformly. This means that you need to have both pieces of pipe in the elbow before heating, otherwise the solder will simply flow out of the open end of the elbow.
I started at one end and worked my way around, your method may differ. Whatever order you decide to solder the joints, make sure the antenna lays flat and you don’t end up with one side or piece drooping.
When the soldering is complete and the copper has cooled, you can install the “shorting bar” and connect the coax feeder.
I used copper saddle clamps folded around the pipe with nuts & bolts as my method of attaching these.
Place the antenna on something non-conductive and check the SWR.
Moving the connection towards the “U” has the effect of lengthening the dipole, while moving away from the “U” shortens it.
You may need to adjust the spacing between the ends of the two elements. My antenna spacing ended up about 20mm wider than the plans.
I used a piece of wooden dowelling at the gap between the two dipole ends, and a couple of cable ties to help prevent the dipole gap closing. This resulted in a final SWR of 1.4:1 at 50.5MHz