A halo antenna is a horizontally polarized, omnidirectional, 1⁄2 wave dipole, bent into a loop with a small break on the side of the loop directly opposite the feed point.
2m FM uses vertical polarization by convention, whereas 2m SSB uses horizontal. Using the wrong polarization results in about 20dB loss.
A stacked halo configuration uses two identical antennae oriented one above the other, fed in parallel through a phasing harness. The stack adds gain in the horizontal plane with a corresponding reduction in the vertical plane. The realizable gain is around 8dBi.
A directional antenna such as a Yagi can provide a little more gain (9+ dB), but requires an antenna rotator.
The halo antenna characteristics — omnidirectionality and horizontal polarization — make it a good choice for 2m SSB.
Mike Fedler, N6TWW, has documented several of his halo builds. His web pages and videos are extremely detailed, and capture the entire process including dimensional calculations, materials, fabrication, assembly, and tuning.
Single halo made from soft copper tubing and PVC pipe.
Stacked halo made entirely from aluminum bar, rod, and tube stock…. 15 videos, amazing level of detail here!
Videos about DIY halo antennae.