In the design of Cubical Quad antennas there are several competing goals or constraints that control the final results. Three important parameters are Forward Gain, Front to Back ratio and Source Impedance.  Sure, there are other factors such as boom length and number of elements, but they are more related to materials and their mechanical properties.  Variations in these three parameters are modified by the choice of the number of elements, their dimensions and element spacing. Unfortunately, the final result is both a compromise and an “optimization” effort to achieve certain goals and fit both mechanical and cost considerations.

One of these variables is the Source Impedance – At Cubex Quads we strive to design for either 50-ohm or 112.5-ohm source impedance. This allows common element location and the resulting equal physical spacing for all bands. We do not find any advantage to the so-called “boomless” design and there may even be some disadvantages to that approach. One disadvantage is the boomless quad requires longer spreader arms, and the compound angles add even more structural stress, especially under icing conditions.

The result of the Cubex Quads approach is that the higher frequency bands (10,12 and 15m ) exhibit higher source impedance’s averaging around 100-ohms. This allows for a simple ‘series’ transformer using 75-ohm coaxial cable to transform the terminating impedance down to ~50-ohms, providing a good match for those bands.

The feeding of each band with separate feedlines is the preferred method offering the following benefits:

• Broader bandwidth
• Deeper side nulls
• Virtually no power limitations

This approach does require separate runs of coax and series matching cables from the antenna feed points to a remote antenna switch but ensures maximum efficiency of transmitted power and lower attenuation of received signals.  Once upon a time, Cubex Co. did offer a matching transformer that allowed for a single feedline, but this idea was scrapped as an unnecessary point of failure.  Smoke the transformer or experience water ingress and ALL bands are rendered inoperable, not to mention the added complexity and power limitations with such a device.  Our goal is maximum efficiency and peak performance, so the transformer was rendered obsolete years ago.

Cubex Quads offers 1/4-wave or 3/4-wave cables cables for all bands where impedance matching is necessary.  Likewise, 50-ohm patch cables of appropriate lengths are offered to make the connection from the feed point to the remote antenna switching unit.  Why two different lengths?  Simply because 1/4-wave lengths are adequate to reach a mast mounted antenna switch on a 3-element model where the driven element is close to the mast, but are too short for wider space of the 2-element and 4-element HF multi-banders.