Here's what I want in an HF antenna. I want a compact antenna that I don't have to put high up off the ground. I want a low radiation angle. I want something that works for lots of different bands. I want something that's easy to tune up. I want decent efficiency. I don't want ground radials or, well, ground anything.
Okay, I don't know how to get all of that, but I can come close. The low radiation angle close to the ground means a vertical antenna, but ground radials kind of goes against that. (It doesn't really, but more about that in a minute.) Lots of different bands also conflicts with the desire to have an antenna that doesn't take endless tweaks to get it to work.
For a long time, I envisioned a vertical dipole that would be a full half wave at the lowest frequency and that frequency would be 1.8 or 3.5 MHz and so we're talking about an antenna that was supposed to be 100+, maybe 200+ feet in the air. To avoid feed problems, I would feed it at the end which leaves the difficulty of figuring out how to get the signal actually into the antenna. (Halfwave antennas which are really fed at the end are actually fed by local electrical and magnetic fields usually generated by a parallel feedline stub.) Each band would be made resonant with stubs or parallel radiators or something. Later on, I imagined a vertical monopole with raised resonant radials for each of the bands and each wire running to a different corner of a tetrahedron, and possibly holding the whole thing up with air pressure.
I might still do the pneumatic antenna support structure thing, for it is a cool idea, but I want to do some operating now, dangit. That would be more in the nature of a science experiment. I don't even know what kind of gear I would need. Not to mention I still really haven't figured out how to do the radiators for the individual bands and I've never heard of using traps or whatever to tune the radials and the thing would still be 150 feet tall (and wide!) So, I'm at a loss.
Or rather, I was at a loss. You see, I've had an idea. It's long been known that having enough wire is important, but that the wire need not be in a straight line. So, while I need 40 meters (say 1600 inches or perhaps 135 feet) of wire to make a quarter-wave radiator for 160 meters, I can arrange that wire any way I want. If I was to wrap that wire in a spiral around a two-inch tube with two turns every three inches, then I could make the structure perhaps 15 feet tall. The helix would not add much inductance. I hope. Of course, I would need to lay out ground radials, but radials on the ground aren't really sensitive to length (beyond "longer is better".) What about tune up? What about multiband? Well, I've had an idea that should solve both of those, too.
I'm going to use a remote antenna tuner as part of the antenna. That will provide an impedance match between the antenna and the feedline and will prevent feedline losses from being too large. I may have to make some adjustment to keep the antenna impedance within limits, but it should all mostly just work. So, I think I'm going to move forward with this. I'm going to build the first version out of PVC and attach it to the side of my house with an antenna wall-mount. Parts are on-order, and we'll see. I'll let you know if I ever actually build the thing, but that's what I'm thinking right now.