Halo antenna

This section contrasts halo antennas with loop antennas which are electrically dissimilar, but can be confused as they all share the same circular shape, and can have sizes that are indistinguishable, when built for frequencies twice as high or half as high as the halo's design frequency.

In the radiation diagram (left) the square, light grey full-wave loop has maximum signal (magenta) broadside to its wires, with nulls off the left and right sides of the diagram; the small loop is the light grey octagon, with its maximum signal within the plane of the antenna-wire octagon, with nulls (black center point) broadside to them.

Moreover, the halo ends are often pressed even closer together, to increase their mutual capacitance and the ends then cut even shorter to compensate, in order to make the radiation pattern even more nearly omnidirectional, and to produce even less wasteful vertical radiation[c] (for a horizontally mounted halo).

Early halo antennas[2] used two or more parallel loops, modeled after a 1943 patent[1] which was a folded dipole bent into a circle, similar to the illustration to the right.

More recent halo antennas have tended to use a single turn loop, fed with a one-armed gamma match.

[d] The newer approach uses less material and reduces wind load, but has narrower bandwidth, may be mechanically less robust, and usually requires a current balun to inhibit feed-line radiation.