Halo antenna

A gamma matched halo

A halo antenna, or halo, is a horizontally polarized, omnidirectional 1/2 wavelength dipole antenna. 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.^[1] 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, 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 coaxial cable with a low SWR.
• The antenna provides a low-angle omnidirectional pattern.
• 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 higher 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.

Disadvantages of a halo antenna

• Due to their narrow pattern, halo antennas perform poorly for NVIS and skip propagation.
• Halos have very little vertical polarization components, and will perform poorly for vertically polarized signals.

References

US 2324462, "High Frequency Antenna System", issued July 14, 1943 

External links

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