The cubical quad aerial is another of my favourite aerials which are very easy to build with a little time and patience. The Quad aerial has been used for many years by Radio Ham's to increase their output power from their transmitters and because an aerial has the properties of amplifying the received signal, weaker signals can now be received where they may have been buried in the noise before building your aerial. I have also found the Ham radio Quad to be better, a quieter aerial due to the fact that it is a closed loop.
A Quad also has a good front to back ratio, but what does that mean and how can this property be utilised. Quite simply pointing the back of the Quad, the reflector, to the direction where stations may be causing some interference to the signal you wish to receive reduces their signal strength.
Your quad aerial will reduce these signals by a certain degree, this percentage of attenuation of the signal is dependent on the gain which is produced in the forward direction may be four signal points down. Element spacing forms the ratio of forward gain.
It may seem confusing and new radio Hams often struggle with these concepts, after all Radio is Physics, but stay with your studies and you will become proficient in this fascinating hobby.
Building the Quad aerial is not too difficult and it can be made using material sourced from local stores specialising in Do It Yourself products. The Boom of the Quad aerial can be made from plastic tubing, the type used for plumbing or wood. Wood is a good choice because it is stronger than plastic. The spreaders used to form the squares to form the cubical quad can be made from plastic tubing or wooden slats one inch square, or a combination of both. Spreaders made from plastic allow you to bend them to add a little tension when fixing the wire to form the square loops.
An advantage of building an aerial is that it not only works out cheaper but you learn from building, as you make the aerial you start to understand how the theory of aerials work, after all it is a well-known fact that knowing comes from doing . Building gives you more control too, you decide on the design frequency, how many elements you wish to use and how you will space the elements, spacing affects the gain of an aerial and building puts you in control of this factor. Elements are spaced between.14 and.25 of a wavelength, wavelength is calculated by dividing the frequency into 300,000,000, the speed of light.
The Quad unlike a dipole is made from square loops of wire which are approximately a full wave long as opposed to a dipole where we use 468 feet to calculate element length. There are many calculators on the internet now where you simple input the frequency of where you wish the Quad aerial to work and it calculates the dimensions for you.
When I sat the radio Amateurs Examination, it was a requirement to know certain formulas for calculating aerial elements and other electronic components but now you have the internet where formulas are built into online calculators. The Quads that I have built successfully over the many years I have been interested in Ham Radio have all been built using these easy to learn calculations.
The reflector on a Quad aerial is calculated by dividing the deign frequency into 1030 feet, if you are considering building a Quad aerial for the two metre band or 144 Megahertz, your reflector length will be 1030 feet divided by 144.5, this equals 7.12 feet.
The driven element is the next element you construct; this is number two element if you are making a three element Cubical Quad. By diving 144.5 into 1005 feet we get a figure of 6.95 feet for the driven element, this is the element where we connect our 50 ohm coax.
The next and final element is called the director and it is calculated from 975 feet, dividing 144.5 gives the answer of 6.74 feet. To make life easier with this number, multiply them by 12 to give inches they are easier to work with. 6.74 feet now becomes 80.88 inches, using a tape measure marked off in tenths of an inch makes it easy to measure your wire for the Ham Radio Cubical Quad aerial, .88 becomes approximately 9 tenths of an inch, this is easy to see on a tape measure.
Spreader lengths are calculated using simple trigonometry, the quad has four equal sides, drawing a line from the top right hand corner of a square to the bottom left hand corner forms a triangle this is called the hypotenuse.
Place your square piece of wire on the floor it is easier to measure these lengths. If we call the equal sides, side A and side B the hypotenuse is the line you have drawn from corner to corner and this is the length of your spreader.
The formulae is A squared plus B squared, the hypotenuse is the square root of this answer. this calculation gives the answer of 30.24 inches for our spreader, using our reflector length of 7.12 feet, 7.12 feet multiplied by 12 equals 85.44 inches, or approximately 85 inches and four tenths of an inch. 85.44 divided by 4 equals 21.36 inches per element side.
Play around with these numbers, put different frequencies into your formulae, for example a local air-band frequency and calculate the loop sizes. I have constructed many three element Quad aerials for the air-band, if you turn the quad where the feed point of the aerial is on one of the sides, it becomes vertically polarised, turning it again to a point where the feeder is along the bottom or top of the quad changes the polarity to horizontal polarisation.
Ham radio beacons are usually horizontally polarised and found in the lower part of the 144 Megahertz spectrum, experiment with your two metre or air-band quad aerial by turning it through ninety degrees, you will hear that the signals increase or decrease depending on polarity of the transmitted signal.