The quad antenna is without doubt the best DX antenna available. It has a higher gain than any other antenna of similar dimensions and is also very cheap to construct. One of the greatest advantages of the quad is that it exhibits a lower angle of radiation than a yagi at heights of less than one wavelength. This makes it a great antenna for working skip as more of the power is radiated at low angles even if you cant get them mounted as high as you would like.
One of the reasons for the effectiveness of the quad is the fact that it not only exhibits more forward gain but also it has a larger physical capture area than a yagi of the same boom length. Each element is a full wavelength in size compared to a yagi which has elements of only one half wavelength making the quad a far more effective antenna for both TX and RX.
As with any antenna the true performance is only achieved when all elements are resonant at their correct frequencies and also when the feed point is matched correctly. I learned this from bitter experience over the years and I encourage you to take the time to do it right if you decide on building a quad.
The image below shows the basic structural design of a typical 4 element quad and the associated dimensions. These dimensions must be adhered to as accurately as possible. The idea being that the Driven element is tuned to resonance at the middle of the band which it is designed for. Once cut to the correct dimension then the feedline can be attached and the element can be finely adjusted by either a dip meter or some kind of antenna analyzer.
The Formulas for calculating quad element lengths (in centimeters) are as follows:
Reflector Element ( L ) = 78.5/Frequency ( Mhz )
Driven Element ( L ) = 74.5/Frequency ( Mhz )
Director Elements ( L ) = 70.8/Frequency ( Mhz )
If you don’t have a dip meter then once all the elements are in place the driven element may be adjusted in size for the lowest SWR. The reflector element is tuned to resonance at a frequency 5% lower than the driven element and the directors are both tuned to a frequency 5% higher than the driven element. Once again a dip meter is extremely handy to make sure that all of the elements are tuned to their corresponding frequencies.
If the resonant or tuned frequency of any of the parasitic or “non driven” elements approaches that of the driven element then the SWR will rise sharply and the gain will drop substantially. This is the reason that it is critical to adjust the length of each element as accurately as possible to obtain the highest gain and relatively low SWR.
For bands with a large bandwidth ( such as 10 meters ) the parasitic elements may be tuned to as far as 7% from the driven element to obtain a low SWR over the entire band. This will work fine however the maximum forward gain will be sacrificed slightly to gain the wider bandwidth.
Quad Antenna links
Quad hardware – Italian based company manufacturing quad antennas for Ham operators and also selling quad parts for the do it yourself antenna builder.