I know the TitanDX has been around for a long time, but I thought I'd pass this along. I had this thing kicking around the garage for about 4 years and my son and I finally got around to putting it together and getting it up this summer. It might be a solution for those of you who want a good HF antenna but don't want to put up massive beam antennas and don't have the space, or trees or other structures to hang up a dipole or wire antenna. I have it tucked away behind the garage and a tree and it isn't even visible except to one of my neighbors but he doesn't care because he's a radio geek too. From the street it can't be seen at all, and otherwise it's completely invisible except for a few feet of a slender aluminum rod sticking up over the roofline of the garage. It isn't a "stealth" antenna but if you're careful about where you put it, hardly anyone will know it's there. It's mounted on a pipe driven into the ground, with about 3 feet above ground, so total height of the antenna is about 26 feet.
All of those stubs you see there are actually hidden behind the roof of the garage and only about the upper 5 feet are visible.
It does not use radials (I don't have room for them and we're avid gardeners here and are always digging up the yard anyway so radials wouldn't be an option) but it does require a counterpoise which takes up some space.
It's a bit awkward to work around but not too difficult to deal with. Even my wife doesn't mind (but then again she's a ham too).
Guying is strongly recommended. I've heard people claim that it doesn't need it. Don't listen to them. We had 75 MPH winds roll through here not long after we put it up and if we hadn't put guy lines on it it would probably have ended up in the scrap pile. We have it guyed with UV resistant, non-stretch, light weight paracord at the moment and that seems to be working well.
Seems to work remarkably well, too. On receive it's 2 full S units better than my OCFD, and results on transmit have been equally impressive. I've made contacts all over the world with it. PSK Reporter shows I'm hitting most of the world using 80 watts or less on FT8. (Interestingly, Africa, India and the middle east seems to be a complete dead zone for me, but that was true when I use the dipole as well).
Putting it together isn't hard. It took me and my son about 2-3 hours to assemble it. You will need a couple of sawhorses to support it while it's being assembled. It's top fed, so the supplied coax is threaded down from the top, inside of the aluminum tubing and comes out the bottom. A PL-259 connector is supplied that has to be soldered onto the coax after assembly. It's a male connector, so you'll either have to replace that with a female one or get an adaptor. That goes to an Alpha Delta grounding/lightning arrestor system before it goes into the house.
They claim it can handle full legal limit, 1,500 watts, but the most I've fed it is 180 watts. Usually I operate 100 watts or less (usually a lot less).
SWR is a bit high on some bands. If you look at that first photo you may be able to see that there are movable sections on the ends of some of the stubs that permit fine tuning. One of these days I'll probably get around to tweaking it, but for the time being I'm just using an automatic antenna tuner.