It is a 2-meter halo antenna as built by Mike Fedler N6TWW: the image appears to have been lifted from his page describing the build. It's 40" in circumference, and the boom length (slightly more than the diameter) is 14.5" long. It's tuned for 144.25 MHz. The name "halo" comes from a combination of the antenna's appearance and an abbreviation for half wave loop.