Quad antenna

Lore & Background

The quad antenna is a development of several inventions. In 1924, Moses Jacobson patented a loop antenna with rhombic shape, and in 1938 George Brown et al. patented a similar design with quarterwave sides. The modern quad was developed in 1951 by Clarence C. Moore, a Christian missionary and engineer at HCJB in the Andean Mountains. Moore sought to resolve issues caused by large coronal discharges while using a beam antenna in thin air at high altitudes. He described his antenna as 'a pulled-open folded dipole' and patented a two-turn loop design, though the patent also included a mention and illustration of a two-element unidirectional quad. Moore's design eliminated corona because the loop elements have no ends, avoiding the 'end effect' that causes corona on dipole-based Yagis. The higher impedance of the quad translates to lower current and lower loss on transmission lines, and gain is higher than that of a dipole Yagi.

Later developments include James Sherriff McCaig's 1957 patent of a 'cubical' two-element multiband quad, Rudolf Baumgartner's 1960 Swiss quad, Werner Boldt's 1969 DJ4VM quad, and Hans F. Ruckert's 1971 'Mono-loop tri-band cubical quad'. The quad antenna is used by radio amateurs on the HF and VHF amateur bands.

Reader's Guide

The quad antenna's significance lies in its practical advantages over the dipole-based Yagi, particularly for amateur radio operators. Rigorous testing shows that a 2-element quad has almost the same gain as a 3-element Yagi (about 7.5 dB over a dipole), and a 3-element quad has more gain than a 3-element Yagi. However, adding quad elements produces diminishing returns, with the maximum practical number of parasitic loop elements limited to four or five. The quad offers higher radiation resistance, better matching to 50 Ohm coaxial feed, and a lower boom height requirement—a two-element, three-band quad mounted only 35 feet above ground can outperform a triband Yagi. It is also easier to design as a multiband antenna, and higher frequency loops can be nested inside lower frequency loops for multiband operation. A false claim has persisted for 50 years that quads 'open the band earlier' due to a lower radiation angle, but computer modelling fails to show any such difference; the vertical sides of each element may actually radiate the low angle component. Disadvantages include limited bandwidth when tuned for maximum gain, and difficult maintenance due to the three-dimensional structure. In 2008, Daniel Mills designed the E-Z-O antenna, which uses flexible dielectric tubes and claims slightly higher gain over the quad due to its roughly circular form.

Did You Know?

• The quad antenna was developed by Clarence C. Moore in 1951 to solve corona discharge problems at high altitudes in the Andes.
• The maximum practical number of parasitic loop elements for a quad array is limited to four or five.
• The claim that quads 'open the band earlier' due to lower radiation angle has been disputed by computer modelling.