A halo antenna, is omnidirectional half wave dipole with horizontal polarization.
It is basically a loop, that can be square or circular, featuring a small break on the side of the loop directly opposite the feed point.
The antenna is usually one continuous conductor, fed with a gamma match capacitor, featuring a good match to 50-ohm feed line.
Read more on Halo Antennas on WikiPedia or browse for more Halo Antenna Projects
Seven Halo Antenna Projects
This halo antenna for 50 MHz is made with a true Gamma Section this time and is fashioned from aluminum
A well documented article about construction and analysis of a horizontally polarized halo antenna for 6 meters band by Dr. Carol F. Milazzo, KP4MD
2 meter & 70 centimeter mobile halo antenna project, by N6TWW Mike Fedler
2 Meter Halo Antenna project by Mike Fedler with many detailed pictues and detailes homebrewing instructions so you can build your own
Halo antenna for 4 meters band with dimensions, pictures and assembling instructions