A quad antenna is a type of radio antenna frequently used by amateur radio operators. Like a Yagi-Uda antenna ("Yagi"), a quad consists of a driven element and parasitic elements; however in a quad, each of these elements takes the form of a loop, which may be square, round, or some other shape.
The quad antenna is a development of several inventions.
In 1924, Moses Jacobson patented a loop antennas with rhombic shape.
In 1938, George Brown et al. patented a loop antenna with rhombic shape and quarterwave sides.
In 1951 Clarence C. Moore, W9LZX, a Christian Missionary and engineer at HCJB (a shortwave missionary radio station high in the Andean Mountains) developed and patented a two-turn loop antenna that he called a "quad". He developed this antenna to resolve issues caused by large coronal discharges while using a beam antenna in the thin air of higher altitudes. This is not the antenna termed "quad" today. Moore describes his antenna as "a pulled-open folded dipole" and describes the time when it was developed:
We took about one hundred pounds of engineering reference books with us on our short vacation to Posoraja, Ecuador during the summer of 1942, determined that with the help of God we could solve our problem. There on the floor of our bamboo cottage we spread open all the reference books we had brought with us and worked for hours on basic antenna design. Our prayers must have been answered, for gradually as we worked the vision of a quad-shaped antenna gradually grew with the new concept of a loop antenna having no ends to the elements, and combining relatively high transmitting impedance and high gain.
Moore's design eliminated interference from coronal discharge. "End effect", which is inherent with the Yagi, is absent in a quad because its elements have no ends. But other advantages appeared. The higher impedance mentioned in the quote above translates to lower current and thus lower loss on the transmission lines, and gain is higher than that of a Yagi.
In 1957 James Sherriff McCaig patented what we now as a "cubical" (two-element) multi-band quad antenna.
In 1960 Rudolf Baumgartner patented the Swiss quad
Advantages over a Yagi-Uda
A Quad is a 3 dimensional antenna so maintenance can be difficult. Even with a tiltover tower tall ladders or a bucket truck may be needed. The QuadLock is a device that will let you tilt your tower all the way to the ground to work on your cubical quad antenna, rotator, or tower. It works by letting the quad loops swivel out of the way. When the tower is in the operational position the elements are locked into position. The locking mechanism is powered by gravity. More information at: 
The 2-element quad has almost the same gain as a 3-element Yagi: about 7.5 dB over a dipole. Likewise, a 3-element quad has more gain than a 3-element Yagi. However, adding quad elements produces diminishing returns. Quoting from William Orr, "Whereas parasitic beams having twenty or thirty parasitic directors are efficient, high gain antennas, it would seem... that maximum practical number of parasitic loop elements for the quad array is limited to four or five." (Orr, p. 48)
Radiation resistance is affected by antenna height above ground, element spacing, and environmental conditions. However, values will be higher than for a Yagi and more closely matched to a 50-ohm coaxial feed.
Lower boom height
"A two-element, three-band quad, with elements mounted only 35 feet above ground, will give good performance in situations where a triband Yagi will not."
William Orr's book shows a 10-15-20 meter, 2-element Quad with boom length of 6'10".
Interaction between antennas of a multiband quad are quite low, even when fed with a single feed line. (Orr, Pg. 63)
Lower radiation angle
According to K0SR the claim that quads "open the band earlier", which suggests that they exhibit a lower angle of radiation than Yagis, has persisted for 50 years in spite of the fact that computer models disagree. He posits that the vertical sides of each element actually radiate the low angle component.
The bandwidth for a 3-element quad antenna tuned for maximum gain is limited because deviation from the design frequency unbalance the near resonance condition of the parasitic elements. However, lengthening the director elements, thereby sacrificing approximately 1 dB gain, allows for much broader bandwidth.
- U.S. Patent 1747008, Antenna. http://www.google.com/patents/US1747008
- U.S. Patent 2207781, Ultra high frequency antenna. http://www.google.com/patents/US2207781
- U.S. Patent 2537191. http://www.google.com/patents/US2537191
- GB Patent 850974 Improvements relating to composite aerials. http://www.puertobalsillas.com/radio/GB850974A.pdf GB850974A
- Swiss Patent 384644, Vollgespeiste Richtantenne für Kurz- und Ultrakurzwellen. http://www.puertobalsillas.com/radio/384644.pdf 384644
- Orr, William I, Stuart D. Cowan. All About Cubical Quad Antennas. Radio Publications, 1959. ISBN 0-933616-03-1, ISBN 978-0-933616-03-5, LCCN 82080282, ASIN B0007EX2M2.
- ARRL. The ARRL Antenna Book. 17th edition, Chapter 12, p. 12-1 to 12-13.
- ARRL. National Contest Journal, Jan/Feb 2008. p. 5.
- Antenas Cúbicas Quad antennas page info by EA1DDO, in Spanish.
- Quad Antenna Gallery Large Quad Antenna gallery with +200 photos.
- N8PPQ N8PPQ's circular quad antenna.
- N6ACH N6ACH quad antenna calculator.