Notes on Cubic Quads

Cubic Quads

KQ6RH

(C) 1998, 1999, 2000

Ray Jurgens

(Up-Dated 2/25/2000) 

  Cubic Quads

  The cubic quad is a very popular way to get reasonably high gain and excellent front to back ratios as well as low angles of radiation for without going to extreme heights. Here I present several designs that that achieve the great performance that hams have associated with this antenna for years. Data are presented for 2 and 6 meter quads and a combined 2 and 6 meter quad that is optimized. The 2 meter 3 element design gets a great 9.5 dBi gain coupled with a F/B ratio of 23 dB.

  Light weight portable cubic quads can be constructed rather easily from fiberglass tubes supported by central hubs. You should be familiar with the material presented in the Quad Loop and Pfeiffer Quad sections of the Antenna Magic page. Cubic quads for wavelengths shorter than 15 meters are easily constructed, however, you should be aware that the weight of these structures is larger by a factor of about 3 relative to most of the planar designs presented in the main menu. Because of this, a heavier mast must be used to support the structure in most cases. Also, be aware that the space needed to assemble and raise a full cubic quad is larger than for the planar designs, and this may be a significant limitation imposed in some locations. In my own case, the backyard associated with my town house is barely large enough to assemble a cubic quad with spreaders of 8′ in length. Wires and guy cords get tangled in the fruit trees, and spreaders hang over into neighbor’s yards. Anything larger 8′ with extended spreaders is essentially impossible to assemble without working above the level of the fence and fruit trees. For that reason, I shall present only two designs which are more or less typical of what can be done easily. The two designs presented are for HF and VHF and should be useful to a wide audience. The HF design is a two element quad for 10, 12, and 15 meters while the VHF design is a two element design for 6 meters with three elements for two meters. A specific advantage of the standard quad design is that multi-band operation is easily accommodated.

 

2 and 6 Meter Quad

  The spreader length necessary to support a 6 meter quad is less than 3.5 feet, so the standard 8′ lengths of fiberglass tubing can be cut in half to make 4′ sections. It is also possible to telescope shorter sections of 1/2″ and 1/4″ tubing to make a slightly lighter weight design. In that case, the 8′ sections could be cut in quarters and the overlap of about a half a foot would be entirely adequate for the telescoping leaving spreaders of about 3.5 feet. The boom length for a full quarter wave spacing is less than 4.75 feet, so a single 8′ piece of 1″ fiberglass tubing is more than ample.

  Looking quickly at the 2 meter requirements, the spreaders need be no longer than 1.25 feet and a three element wide spaced boom requires no more than 31.25″, thus this can be easily tucked between the 6 meter 2 element quad. In fact, it is necessary to stretch it out a bit. So, a common design requires beginning with an optimized design for 6 meters and accommodating the 2 meter design to the locations of its two hubs. The third 2 meter hub occupies a space between the two 6 meter hubs.

  Looking at an optimized 2-element 6-meter design, the following parameters give excellent performance:

  The 6 meter parameters are given in Table 2, which is a simple two element design:

  The next step is force the 2 meter 3 element quad to have a total boom length identical to that of the 6 meter quad. Note that the 2 meter 3 element easily fits within the same space as the 6 meter 2 element, that the 2 meter quad will have a longer boom than is considered optimum.

….

10, 12, and 15 Meter Quad

  The construction of 2 element cubic quads for 10, 12, and 15 meters is not very difficult, but the structure requires some guy strings to keep the light weight elements from bending. The bending actually would not degrade the performance very much, but the nice square structure clearly looks better, and it will probably hold up better under high winds. In order to keep the spreaders from bending due to gravity loading and wire loading, you will need to have a place to guy them from two directions. The antenna wires can serve as structural elements to help reduce the bending in the plane of the loops. However, bending is also a problem perpendicular to the plane of the loops, and guy strings are necessary to stabilize that direction as well. The easiest way to provide a place to connect the guys is to use a boom extension. The extension does not need to be any longer than 3 feet for 8′ spreaders. In the case of 15-meters, the boom can be about 8′, and the two extension increase the boom to 14′ or a turning radius of 7′. The actual turning radius of a quad depends upon whether it is set up as a diamond or square configuration, the square being the smaller of the two.

 Article originally available at http://www.antenna-magic.com/antenna/cubic_qu.htm