The quad antenna is a type of directional antenna that typically has its elements configured into four-sided square loops. Similar to a Yagi antenna, it usually possesses a driven element that sends or receives radio current through a feed line to a receiver or transmitter, and one or several parasitic elements, which modify the driven element's pattern, direction, and gain. Where a Yagi dipole, or rabbit ear-type double antenna, uses one charged active element and one field-enhancing parasitic element, the quad uses director and reflector elements. Directors help guide the signal's main direction, and reflectors strengthen a signal by occupying its opposing angle. This type of radio antenna reduces unwanted noise and permits clear reception even at low heights, making it a favorite among amateur citizens band (CB) and ham radio enthusiasts.
In 1940, the quad antenna was invented by Clarence Moore, a radio operator in Quito, Ecuador. This higher-gain antenna focuses its radiation pattern in a certain direction, permitting stronger signal reception less susceptible to interferences and unwanted noise. Some elements are stacked into cubical formations of anywhere from two to 16 elements in a single pattern. Easily constructed, products and configurations vary from small handheld radio antennas to dishes and rooftop arrays.
Cancellation of undesirable radiation contributes to an antenna's gain while making power consumption and signal transmission more efficient. This cancellation occurs in the quad, as each side consists of short lengths requiring low current and less net radiation to function well. The square or cross-shaped sides are out of phase with each other, and upper and lower halves are also out of phase. This results in greatly reduced radiation and less interference from unrelated signals.
Performance of a transmitter-receiver like a CB radio relies a great deal upon the quality of its antenna. Factors include an antenna's gain and type, height, range, and operating efficiency. Weather also influences performance; a high-voltage gradient between air and antenna creates coronal discharges that create static hisses and pops in a receiver. The quad antenna was designed to address this issue by minimizing protrusions and placing a blunt side higher toward the charged areas. Even with this design, it can maintain a pattern and gain consistent with a dipole element, that is, a double or T-style antenna.
Quad antenna technology is a favorite of hobbyists due to its effectiveness and the ease of building one from scratch. Homemade antennas can be made from wire on supports like fiberglass or polyvinyl chloride (PVC) tubing. These antennas are usually mounted on rotating fixtures to permit aiming in a desired direction. Other components include insulators, coils, and tuners.
Certain characteristics of the quad antenna provide numerous advantages over Yagi and dipole types. Quads tend to provide more gain than dipoles of similar size, and better performance at lower heights, or boom lengths. They may be configured in numerous sizes, stacks, and multiband arrays, and may be foldable for portable use. Users typically report quieter operation. Their bandwidth can be tuned for maximum gain by lengthening their reflector and director elements; this has the effect of greatly expanding the available bandwidth at a diminished level of gain.