How important is loop circumference? How important is the size of the conductor diameter?
Loop circumference relates directly to efficiency and performance (also, loop image: see below). The larger the circumference of the loop, the better your loop will work — however, if you exceed 1/4 wavelength, things seem to get mysterious. It’s a balance, I try to get about 85-90% of the wavelength in circumference, and that seems to be the magic number. However, I’m not making any definitive statements because I have found loops above 1/4 wavelength that seem to work well. There are other factors that effect the total loop size, and that is loop pipe diameter. I have built loops that were 16 feet in circumference and made out of 2 inch copper. The loop worked poorly on 20 meters (where 16.5 feet would be 1/4 wavelength), and worked better on 40 meters. What I assume is that if you use pipe that exceeds a certain size, the total loop size (not circumference), but total loop image, in other words the electrical size is too big for the intended frequency — maybe. You end up exceeding
