The basic quad antenna element consists of a loop of wire a wavelength long in the form of a square as already described. As with the Yagi, parasitic elements can be added to make the antenna more directional.
Like the Yagi antenna the quad beam only ever has a single reflector, although more than one director provides additional gain.
For the Cubical quad beam antenna, the reflector and director work in exactly the same was as for the Yagi, using induced currents in the parasitic elements. This enables the signal in the forward direction of the quad beam antenna to be reinforced whereas the signal in the reverse and other directions are reduced.
Quad beam reflector and director
A reflector can be added behind the driven element to make a cubical quad beam antenna. To give the right phasing of the currents in the elements of the beam antenna for it to reflect, it should be made inductive, and this can be accomplished by tuning it below resonance. This can be achieved in a number of ways. The first is to make the reflector slightly longer than the electrical full wavelength. Typically it is made between 3 and 5% longer.
An alternative method is to insert a short circuit stub. This has the advantage that the element can be made exactly the same size as the driven element, and this may have some mechanical advantages for the quad beam antenna.
Directors can also be made. They need to be capacitive to have the right current phasing and this can be achieved by tuning the element above resonance and making the element slightly shorter than the electrical full wavelength. Similarly directors can also use a stub to give the required characteristics, but in this case an open circuit stub is used.
Quad beam antenna element spacing
In the same way that the element spacing played a large part in the design of the Yagi beam antenna, the same is found for the quad. In general a spacing of around 0.15 to 0.2 of a wavelength is used. This conveniently gives a feed impedance of around 50Ω.
If a two element quad beam antenna has a spacing of just over a quarter of a wavelength then the feed impedance rises to around 75Ω. This is convenient for designs used for VHF FM broadcast reception. If the spacing is reduced below about 0.15 wavelengths then the impedance falls. If this is done then some form of impedance transformation would be required to enable the antenna to be fed by standard coax.
The spacing also has some effect on the gain. However as in the case of the Yagi the effect is fairly small and the impedance matching is the major requirement whilst adjusting the spacing for the cubical quad beam antenna.
Cubical quad beam antenna gain
As with many antennas the level of gain for the cubical quad beam antenna is an important factor of its overall performance.
The basic cubical quad driven element on its own has a gain of about 2dB over a dipole because it is essentially a pair of stacked dipoles. It is generally about 2dB.
The addition of a reflector adds about 5dB gain and a director about an additional 2dB. Further directors average out at giving very approximately one dB each. This means that a quad having the same number of elements as a Yagi will have about 2dB further gain. In fact the comparison should be made between antennas having a similar length and designed to have optimum element spacing.
It is found that adding a large number of quad directors produces diminishing returns, and the limit is generally accepted to be about four or five. If much greater gain and directivity is required then it becomes more viable to use the Yagi approach or the hybrid quagi design which typically uses the driven element and reflector from a quad beam antenna and the directors in the form of those used by a Yagi..
When deciding upon a high gain cubical quad beam antenna design it is worth remembering that quad antennas are more prone to wind damage, and as a result they have tended to fall out of favour. Any that are built should be sturdily constructed if they are to be used externally.
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