I've been fascinated for a while by the prospects of large loop antennas. For one thing, my shack is on the second floor (the room with the windows next to the umbrella in the picture below), and this height makes it a bit hard to install a really good station-level RF ground. A balanced antenna doesn't need an RF ground, and a loop is (ideally) a balanced antenna. In addition, many have sung the praises of "skywire loops" as flexible, well-performing multiband antennas. (See for example some of these sources.)
Beginning in August 2011 I built a "temporary" Delta Loop in my back yard, as an experiment. It was essentially this antenna by KL7JR which is itself a variant of WB8IMY's in May 2002 QST. (The earliest reference to this design that I've since found is on pages 13 and 21 of SGC's "Smartuners for Steath Antennas" from 1997).
Rather than using a bottom-center feedpoint with balun and coax feedline as in the first two references above, my loop is fed in the corner, directly by the SGC-237 autotuner - no feedline (as in the SGC book). I believe locating the feedpoint in the bottom corner yields a vertically polarized signal. A diagram of the design is below along with photos of the two installations made so far.
At home the loop worked great, and the SGC-237 easily tuned it to < 1.5 SWR on 80-6 meters. It seemed much quieter than the "inverted vee longwire" I had been using at home, and it propagated wonderfully. Overall, I worked all around the US and the world with it using 5-100W.
In the end I actually left the loop up for about 6 weeks. We only dismantled it in October so we could set it up portable as our antenna for the Illinois QSO Party, in Lodge Park, Monticello, IL. In the park we used essentially the same configuration, and again it worked beautifully. With 90 watts we worked the following DX (among many more domestic and Illinois contacts): JQ6, HP1, and T32C on 10 meter CW; VA1 and F8 on 15 meter CW; and DL6, G3, LA8, LY5, PA3, SA8, SM3, SP5, and YV5 on 20 meter CW. We worked 150+ stations total on 7-28 MHz. CW & SSB, in about four hours of operating. Quick retuning across the bands during the contest was a snap with the autotuner and the loop.
As soon as I get a chance I'm going to re-install the loop at home and take some real measurements of impedance vs. frequency, SWR curves, etc. to give some harder data on how it's really behaving.
Also "temporary" but now up for 2 years...