A halo antenna, or halo, is a horizontally polarized, omni-directional 1/2 wavelength dipole

Dipole

In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

 antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

. It is shaped like a loop with a small break on the side of the loop directly opposite the feedpoint, so that the dipole ends do not meet. The antenna is usually one continuous conductor, fed with a gamma match capacitor, although a double-halo system may be utilized, with a larger radiating loop at the top, and a smaller loop on the bottom that is fed directly. The double loop system was the feeding method described in the original patent, with the gamma match feed being more predominate with modern halos. The effective difference is that the double loop is balanced, while a gamma match is unbalanced.

The halo antenna is distinct from the magnetic loop antenna, which is similar, but quite a bit smaller, and the full wave loop

Loop antenna

A loop antenna is a radio antenna consisting of a loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line...

, which is larger and in which the element is a complete loop.

Advantages of a halo antenna

• When constructed correctly, the antenna will present a good match to 50-ohm coax with a low SWR
  Standing wave ratio
  In telecommunications, standing wave ratio is the ratio of the amplitude of a partial standing wave at an antinode to the amplitude at an adjacent node , in an electrical transmission line....
  
  .
• The antenna provides a low angle omni-directional pattern.
• The radiating element of the halo is grounded
  Ground (electricity)
  In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
  
  , which tends to reduce static buildup, an advantage shared by many antennas fed with a gamma match.
• The physical diameter of a halo is small enough to be effectively used as a mobile antenna on the higher vhf bands and above.
• Halos may be stacked, narrowing the vertical radiation pattern, but having little or no effect on the azimuthal pattern.

Disadvantages of a halo antenna

• Due to their narrow pattern, halo antennas perform poorly for NVIS
  Near Vertical Incidence Skywave
  Near Vertical Incidence Skywave, or NVIS, is a radio-wave propagation method that provides usable signals in the range between groundwave and skywave distances . It is used mainly for military and paramilitary communications and by radio amateurs...
  
   and skip propagation
  Radio propagation
  Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
  
  .
• Halos have very little vertical polarization components, and will perform poorly for vertically polarized signals.

External links

• www.kr1st.com/2mhalo Construct a halo antenna for two meters.
• fedler.com/radio/2m_loop Another two meter vhf loop design.