Quad antenna

Quad antenna. A Quad antenna is a type of directional antenna popular among amateur radio operators and for some commercial applications. It consists of one or more square or diamond-shaped wire loops, each approximately one wavelength in circumference, mounted on a supporting boom. The design, often compared to the Yagi–Uda antenna, is known for its relatively compact size for a given frequency and its distinctive performance characteristics.

Design and construction

The classic design, known as the Cubical Quad, features loops constructed from a wire or aluminum tubing frame, with each side being roughly a quarter-wavelength. These loops are supported by a non-conductive crossarm or spreader made from materials like fiberglass or bamboo, which are attached to a central metallic or insulating boom. A two-element Quad typically has a driven element and a single parasitic element, which is usually a reflector, though director elements can be added for increased directivity. The feed point is commonly at the center of the bottom or side of the driven loop, with coaxial cable or ladder line used for connection to the transmitter or receiver. Pioneering work on this design is attributed to Clarence C. Moore, who promoted its use after World War II.

Operating principles

The Quad antenna operates as a closed loop antenna, where each loop functions as a full-wave loop. When the perimeter is approximately one wavelength, the loop resonates, supporting a standing wave of RF current. The parasitic elements, similar to those in a Yagi–Uda antenna, interact with the electromagnetic field from the driven element through mutual coupling. This interaction, governed by the spacing and tuning of the elements, re-radiates the signal to create a directional radiation pattern. The reflector element, typically slightly longer than the driven loop, causes constructive and destructive wave interference to enhance signal strength in one direction, providing front-to-back ratio and antenna gain. The polarization of the radiated wave is primarily linear, determined by the orientation of the feed point.

Variations and configurations

Beyond the standard Cubical Quad, several variations exist. The Delta loop antenna uses a triangular shape instead of a square, offering mechanical differences. A Bi-Quad uses two full-wave loops for a single element to increase bandwidth. For multi-band operation, designers create trap Quads or use separate sets of elements for different amateur bands like the 20-meter, 15-meter, and 10-meter bands on a single boom. Configurations can range from a simple single-element directional antenna to large multi-element arrays used in contest stations or for moonbounce communications like EME. Some designs incorporate a ground plane or are mounted as vertically polarized arrays.

Advantages and disadvantages

A primary advantage is its relatively compact turning radius for a given gain compared to a Yagi for the lower HF bands, making it suitable for limited spaces. It often exhibits a good front-to-back ratio and lower angle of radiation, which can be beneficial for long-distance communication via the ionosphere. The closed loop design can also provide a degree of noise reduction. Disadvantages include greater mechanical complexity, with multiple spreaders and wires being susceptible to damage from ice or high winds. Its bandwidth for a perfect SWR is often narrower than a comparable dipole or Yagi. Construction and tuning can be more intricate, requiring precise adjustment of element lengths and spacing.

Applications

The Quad antenna is predominantly used within the amateur radio community for DXing and in radio contests on the HF and VHF bands. Its directional properties make it effective for point-to-point communication in specific commercial and military applications, such as fixed radio station links. Some specialized applications include use in amateur satellite ground stations and for meteor scatter communications. While largely supplanted by the Yagi and log-periodic designs in many commercial arenas, it retains a dedicated following among enthusiasts for its unique performance profile on bands like the 40-meter band.

Category:Radio frequency antenna types