Halo Antenna for Amateur Radio

Halo antenna is a center fed half wave dipole antenna which has been bent into a circle, with a break opposite to the feedpoint. Halo antennas are often mounted horizontally and has an almost omnidirectional radiation pattern with horizontal polarization. Vertical radiation can be minimized by adding more capacitance to the ends, which is useful for terrestrial VHF communication avoiding wastage from upward radiation into space. For VHF and higher frequencies, physical size is small enough to be used as a mobile antenna. The cover image shows a halo antenna for 6m with a triple loop. An advantage cited for halo antenna on mobile operation is that it picks up nearby electrical spark interference like vehicle ignition noise less than monopole and dipole antennas.

Though the ends of the dipole in a halo antenna are close to each other, they do not touch each other. Small circular plates may be fixed at the ends to form an air capacitor and the spacing between the plates can be adjusted to tune the antenna. Feed cable in the sketch is seen as a thick black vertical line. The small black box contains a trimmer capacitor for tuning the gamma match. Tuning the gamma match can also be done by changing the length of the gamma arm. That is how impedance matching at the antenna feedpoint is obtained. This is similar to gamma match in Yagi antennas with an undivided driven element of half wavelength.

Ends of the halo antenna, that is near the break, has zero current and peak RF voltage. Connections of the shield and central conductor of the coaxial cable are opposite to the break in the halo loop. There is no break at the feedpoint. Instead there is a distance between shield and inner conductor connections and inner conductor is connected through a gamma capacitor rather than directly. Gamma capacitor is a variable capacitor used for tuning the antenna to resonance. These details are well seen in the public domain image from Wikipedia shown here.