Building a Quad Antenna
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.
For more information on quad antenna dimensions you can use this Quad Antenna Calculator. The quad calculator is a very useful tool which not only calculates element dimensions but also works out the ultimate element spacing for maximum gain and gives you the spreader arm lengths as well. Note: this calculator is designed to give maximum gain and does not take into consideration the feed point impedance. To feed directly with 50 Ohm coax and a 1:1 balun spacing needs to be kept around 0.12 wavelengths between elements. If you are using some kind of impedance matching device then these dimensions are fine and will give you maximum gain.
Quad Antenna Feed Systems
One of the most crucial aspects of the quad design once the actual elements are all tuned and sorted is the matching system. The idea of the matching system is to provide a relatively close impedance match between the feedline and the driven element feedpoint.
The feedpoint impedance varies according to the spacing between the driven element and the parasitic elements. At spacing of 0.15 wavelength or less the impedance is very close to 50 Ohms and can therefore be fed directly with a 50 Ohm coax.
As the element spacing increases the effects on the feedpoint impedance lessens and the impedance rises closer to what it would be as a single element at around 120-140 ohms. At this wider spacing a quarter wave 70 ohm matching stub is often utilized. The stub is an electrical quarter wavelength rather than a physical wavelength and is approx 0.66 of the physical length.
This matching stub provides a close match to the driven element’s impedance at the wider element spacing. Other than matching the impedance we also need to make sure that there is no current flowing on the outer shield of the coax. This can often be an issue when making a direct connection to a balanced antenna using an unbalanced coaxial cable.
The solution to this RF flowing on the outer shield of the coax is to use a balun of some sort. A balun is simply a device which transforms the line currents from an unbalanced state to a balanced state resulting in no or very little RF on the shield of the coax.
The most simple form of balun is formed by simply looping 3 turns of your coax through an iron fer-rite core one electrical quarter wavelength from the feedpoint. At an electrical distance of one quarter wavelength from the feedpoint we already have the highest impedance point so by adding a balun here we create an even higher impedance which pretty much stops all RF on the braid from travelling along the coaxial.
An even simpler form of balun which also creates a very high impedance on the outer braid is to cover the coax using a metallic sleeve one electrical quarter long. This sleeve is terminated to the outer braid of the coax at one electrical quarter wavelength form the feedpoint. This is a very easy solution and stops any stray RF from travelling on the shield of the coax.
Please pay special attention to the matching system and balun as this will ultimately determine the overall performance of the antenna. Without a balun the radiation pattern and F/B ratio is often less than optimal.
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.
All About Cubical Quad Antennas – This is the full book by William Orr. One of the best known books on construction of quad antennas.
Hy-gain Big Gun 2 – the full instruction manual for the hy-gain 4 element quad, perhaps one of the most legendary CB antennas ever built.
Lightning Quads – if you cant be bothered building your own then Lightning Quads sell them already made.
Big Signal Antennas Systems – quad antennas, quad hardware and baluns. Quads for ham and 27Mhz.
Cubex Quad antennas and hardware – Cubex is another supplier of quad hubs, arms and pre-built quad antennas.