Many people assemble their loop antennas using copper pipe and 45 degree angle connectors to form an octogon. This approximates a circle well enough, but introduces some electrical resistance to the loop. I want to bend a piece of copper into a continuous loop for peak efficiency.
I started by buying a 10ft length of copper from Home Depot. I attempted to see if one of the local muffler shops would bend it into a loop for me, but both of the shops I talked to said they weren't able or willing to bend copper.
Next, I bought a conduit bender like this one. I was able to bend the copper into a loop by marking equal spaces along the length of the loop and bending a little at a time. Unfortunately, the copper became quite brittle and tended to buckle as I bent it. (it looked more like a 20-sided polygon than a circle).
Copper is known to work-harden. This means that copper becomes more brittle and less malleable as it is bent and formed into different shapes. Annealing is the processs of heating the copper up to a point where the internal stresses are relieved and the copper is ready to be formed again. Annealing copper is simple – I bought a blowtorch and heated the entire length of the copper until it glowed red. For a 10ft length of pipe I was able to heat up the entire length of pipe in only a few minutes.
The copper changes color and becomes much easier to bend/work after the annealing process.
After annealing, I can easily fold the copper into a loop with my bare hands. It is remarkable what a difference it makes.
With a little more care, I think I can get a nearly perfect circle for the antenna.
If you are building your own loop antenna, I recommend using a blowtorch to heat up your copper piping before bending. The copper will bend easier and have less tendancy to buckle during the bending process.
As a next step, I think I can fabribate (maybe 3d-print) a series of three rollers which will let me bend a copper tube to a specific radius. I'll post an update once that is finished.