Now that I have a butterfly capacitor working, it’s time to select my magnetic loop material.
Materials used for magnetic loops must be highly conductive metals. Almost always, this means copper or aluminum. These materials might be formed into pipes or tubes, foils or flashing. To minimize electrical losses, it is advantageous to use a continuous length of material, rather than soldering or braising shorter pieces together. You also want to maximize the magnetic loop material diameter. The lower your frequency, the higher the diameter required.
There are two neat things about radio current flowing along conductors. The first is called “skin effect”. No matter how thick your material is, the radio current flows along the outer surface, or the skin. So, for my frequencies of interest, the power stays in the outermost 0.005” (5 mils) of the material. This means that you can use the thinnest wall copper pipe, or even substitute copper foil wrapped around a plastic (PVC) pipe. The second neat thing is that there is an equivalence between flat metal strap or flashing, and round piping. Using flashing can work just as well as pipe and can give you a weight advantage.
So, I evaluated all of these loop material choices against my design goals. Basically, I want my first loop to:
- cover 7 to 15 MHz (including the 40, 30 and 20 meter ham bands)
- work with my home made butterfly capacitor with a range of up to 150 pF and within voltage handling limits
- keep the diameter of the loop at 5 feet or less
- perform similarly to regular dipole antenna
Magnetic Loop Material Selection – By The Numbers
My first option is a five foot loop made of half-inch copper tubing (see left picture). This material is readily available and easily formed into a circle that can be mounted on a PVC pipe. Tubing is substantially lighter than pipe and does not require soldering.
The second option is wrapping thin copper foil around a plastic form with say 2” diameter (see middle picture). Unfortunately, foil of the proper thickness (say 5 to 10 mils) is hard to find in Calgary, and shipping it in from elsewhere is expensive.
Lastly, I could buy a roll of 10” aluminum flashing from Lowest quite cheaply. This would give me the equivalent of a 2” diameter pipe (see right picture). But I would need to build a complicated frame to mount the flashing and keep it steady.
All three of these options would meet my criteria. The copper coil magnetic loop material is slightly less efficient than the other choices, but the easiest to build. So, that is where I will start.