The Cubical Quad or Quad antenna has traditionally not been as widely used as its more popular cousin the Yagi. However the cubical quad beam antenna is able to provide some advantages in a number of circumstances.
Since its first introduction in the 1940s, the quad antenna has found uses in a number of areas. It has been used for some television antennas, but it has also seen a lot of use within amateur radio applications for use at HF and VHF / UHF.
Although the cubical quad antenna is not nearly as widely used as the Yagi, it still has advantages and can be used to good effect in many instances.
Quad antenna basics
The basic quad element can be seen to be derived from two dipole elements stacked one above the other and fed in phase. This arrangement in itself gives gain because of the phasing effect between the two dipoles.
The next stage in the evolution is to retain the two separate dipoles but bend the ends together. It is found that the voltages at the ends of the antennas are in phase with one another.
As a result it is possible to connect these ends together and remove one of the feeders to create the basic quad element. As such the loop forming the element is a full wavelength with each side being a quarter of a wavelength.
Often the cubical quad is constructed by having an ‘X’ frame made from insulating material, even wood, and then using this to support the wire or tube that forms the conducting radiating element. In some instances where antennas are small enough and the tubing is sufficiently robust, it is possible to rely onth e strength of the tubing alone for the mechanical strength. This is normally only achieved at VHF or better still UHF.
Although the format of the horizontal square for the cubical quad is the more obvious format and the more common, it is also possible to turn the square format through 45°. When this format is used it should be remembered that the width of the antenna is equal to the diagonal of the square, whereas when the horizontal square is used, then the width is equal to only bottom of the square.
Sometimes the diamond style construction can provide a attractive option for construction. One point to note when using this configuration is that the two side mounting points labelled ‘a’ for the wires are at the voltage maximum points. Points marked ‘b’ form the current maximum points. For higher power levels it may be necessary to utilise some form of proper insulator at the voltage maximum points.
Both the diamond and familiar horizontal square version of the cubical quad antenna produce a horizontally polarised signal. To generate a vertically polarised signal the whole antenna should be rotated through ninety degrees so that the feed point is at the central point of the vertical section of the "square" version or in one of the "side" corners of the "diamond" version.
Quad antenna current & voltage distributionFrom the development of the basic quad element it is fairly easy to deduce where the current and voltage maxima will be. As the current maximum is at the feed point the same is also true for the quad. There is also another current maximum on the opposite side of the loop to the feed point i.e. where the second feed point was.
The voltage maxima appear at points a quarter of a wavelength away from the feed point i.e. where the two ends of the dipole would have been. In view of the position of the voltage points it is often advisable not to position any fixings at these points.
Cubical quad advantages & disadvantages
Since the introduction cubical quad beam antenna there has always been considerable debate about its advantages compared to the more familiar Yagi.
As with any antenna there are always advantages and disadvantages that need to be balanced to see what is the most attractive option for any given situation.
Cubical quad advantages:
- Gain: The cubical quad beam antenna offers about 2 dB over a Yagi of a similar length. This means that the quad compares to a pair of stacked Yagis as there is always some loss in the feed arrangements for stacking the two antennas.
- Low angle radiation: The fact that the quad antenna is effectively a pair of stacked dipoles, it offers a lower angle of radiation than that provided by a single Yagi. It compares to a pair of stacked Yagis.
- Effects of close objects: The quad antenna is less affected by nearby objects, giving it an edge over the Yagi in many installations, especially those that are inside, possibly in the loft or attic.
- Internally stackable: One advantage of the cubical quad antenna is that it is possible to internally stack separate quad antennas for multiple bands under some circumstances. This option is used mainly on the HF bands.
Cubical quad disadvantages:
- Construction: The construction of the quad antenna is not as straightforward as a simple Yagi. It requires a more complicated form of construction.
- Cost: The more complicated construction often results in higher costs for a quad than for a Yagi. This has meant that for commercial VHF / UHF operation it is rarely used. That said for amateur radio HF operation, it enabled home made construction with wire to provide some cost effective installations.
- Robustness: The quad antenna is larger than its equivalent Yagi and the ‘spider’ arrangement used on many HF bands is often not as robust as a Yagi.
The characteristics of the cubical quad mean that it is used in a number of situations. It was popular some years back as a UHF television antenna, but now it is less widely seen. It is still used for amateur radio operations both at HF and VHF / UHF where its features bring several advantages.
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