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I have a long history of building multi-band dipoles. My favorite method involves stripping the center conductor and dielectric out of RG-58 coaxial cable and using that to wind traps around PVC pipe cores of various diameters. The PVC core size, and therefore coil diameter is determined by frequency. There is a program called "Coax Trap" written by Tony Field VE6YP that allows you to calculate the length of wire needed for HF band traps. For best efficiency the ratio of coil diameter to coil length should fall into the range of .45 to .55. Using this program 80 Meter traps usually work best when wound on 3 inch PVC, 40 Meters on 2 inch PVC, higher frequency traps can usually be made on 3/4 or 1/2 inch PVC without too much loss. The photo above shows a completed 80 meter trap wound on 3" PVC and wrapped with electrical tape to make it weatherproof. Also shown in the photo is a 40 meter trap wound on 2" PVC, but without the electrical tape to show construction detail.