Beam Antennas

Beams are also called parasitic arrays or subtractive arrays as the elements that make them up are fractions of a wavelength apart and closely coupled. Beams come in many forms and are the most common form of rotatable directive antennas used in the amateur service at HF through UHF. While there are specialty antennas like parabolics and phased arrays that may have more gain, they are usually bulkier, or only designed to be fix mounted. The basic beam is a driven element with either a reflector (typically a longer element) to reinforce the signal in the opposite direction or a director (typically a shorter element) to reinforce the signal toward it. Elements can be dipoles (these are called Yagis), or loop forms (called Quads). The Quad has the advantage in front-to-back ratio for two element beams, at the expense of a larger, more complex, mechanical structure.

The performance of a beam antenna is related to its boom length, at least for near-optimal designs. The number, size, and spacing of the parasitic elements are selected for the specific performance optimization, which is usually front-to-back ratio (the ability to provide preferential gain in the desired direction to any lobes in the general rearward direction), forward gain, or bandwidth. We haven’t spoken about bandwidth to now – most of these antennas exhibit optimal performance only at a single frequency, and reasonable performance over a rather narrow bandwidth, perhaps 3%.

Most of the design choices in optimizating one parameter give up something in another. As can be seen from the figure above, the 2 element quad has a better front-to-back ratio than even the 3 element yagi, but noticeably less gain. Designing a 5 element yagi to provide a much improved front-to-back ratio results in an antenna that has just a small increase in forward gain.