A halo antenna, or halo, is a horizontally polarized, omnidirectional 1/2 wavelength dipole antenna, which has been bent into a loop with a small break on the side of the loop directly opposite the feed point. The dipole ends are close but do not meet, and may have an air capacitor between them as needed to establish resonance. Earlier halo antennas  used two or more parallel loops, based on a 1943 patent.  This was a folded dipole  bent into a circle. The two loop design broadens the SWR bandwidth and helps with impedance matching. More recently halo antennas use a single continuous conductor, fed with a gamma match capacitor. The newer approach uses less material and reduces wind load, but may be less mechanically robust and needs a balun to prevent feed line radiation.
The halo antenna is distinct from the magnetic loop antenna, which is similar, but quite a bit smaller, and the full wave loop, which is larger and in which the element is a complete loop. Smaller halos which require significant capacity to resonate them approach the small magnetic transmitting loop in design, but unlike most small magnetic transmitting loops are used in the horizontal configuration.
Advantages of a halo antenna
- When constructed correctly, the antenna will present a good match to 50-ohm coaxial cable with a low SWR.,
- Towards the horizon, the pattern is omnidirectional to within 3 dB or less. Making the loop smaller and adding more capacity between the element tips evens out the gain while reducing upward radiation.
- The radiating element of the halo is grounded, which tends to reduce static buildup, an advantage shared by many antennas fed with a gamma match.
- On the VHF bands and above, the physical diameter of a halo is small enough to be effectively used as a mobile antenna.
- Halos may be stacked, narrowing the vertical radiation pattern, but having little or no effect on the azimuthal pattern.
- Halos pick up less ignition noise from vehicles when used in mobile stations. 
Disadvantages of a halo antenna
- Horizontal halos have very little vertical polarization components. One can expect 3 to 20 dB of signal loss when working with stations using vertical polarization. 
- For mobile use, the halo is rather conspicuous compared to the much more common vertical whip antenna, and may attract unwanted attention.
- The halo is a rigid structure and may suffer damage from tree branches or other obstacles in mobile service.
- Stites, Francis H. (October 1947). A Halo for Six Meters. QST, p. 24.
- Danzer, Paul, Sept 2004 QST, p.37
- Tildon, Edward P., Polarization Effects in V.H.F. Mobile, Dec 1956 QST, p.11
- Tildon, Edward P., Polarization Effects in V.H.F. Mobile, Dec 1956 QST, p.13
- www.kr1st.com/2mhalo Construct a halo antenna for two meters.
- fedler.com/radio/2m_loop Another two meter vhf loop design.