Double Quad for VHF UHF by I5NZR

By I5NZR Renzo (SK)

This antenna made me know, IW5ARM Luigi Del Turco Rosselli of Pisa in the 70s. He used it in his country house in Fabbrica di Peccioli, making it rotate on itself with a series of references made from bicycle chains and gears to vary the polarization. I think he made the first one out of vineyard wire. I later found this antenna published and developed in various magazines.

A few years ago, in the early 1990s, I saw it used by RTF (French TV) to keep contact between the cameras on the bikes and the repeater helicopter at the Tour de France. A directive version with a curtain reflector and Yagi directors is used as a TV receiving antenna. I have built and used two examples for VHF and UHF.

The structure of the antenna is very simple. It consists of two identical square loops, whose sides measure a quarter wave (?/4), connected together at one end to form a double lozenge, as can be seen in the drawing. The system for calculating the size of the single loop is identical to that of the Quad.

To calculate quarter wave, use 75 / Frequency, like for all Quad is not necessary to calculate any shortening.

The antenna is fed at the centre, at the point where the two loops (x – y) join, directly with 50 ? coaxial cable. At that point the antenna has a characteristic impedance of approx. 50 ? as it is an array of two Quad loops, fed in parallel, where each individual loop has a characteristic impedance of approx. 100 ?. Gain is 2.8 Db on the dipole. (1.4 dbd per loop, 4.5 dbi).

The polarisation of the antenna is vertical when the elements are placed horizontally (as in the drawing) and horizontal when the elements are placed vertically. Without parasitic elements the radiation is bidirectional and perpendicular to the antenna plane. The shape of the radiation lobe is identical to that of a single-element quad or dipole and is the sum of the two lobes emitted in phase by each of the quad elements.

The antenna can be made directive with parasitic elements. The reflector can be made either with a curtain of only 2 Yagi elements, but 4 or 6 is much better, or more conveniently with a wide-meshed wire mesh structure: The Yagi elements of the curtain or the short side of the wire mesh curtain must be between 5 and 7% larger than ?/4.

The long side of the curtain should also be slightly longer than the space occupied by the two loops side by side.
The spacing between the reflector and the radiator, as in Quad antennas, is between 0.10 and 0.20 of ?, depending on whether you want to enhance the gain or the forward-back ratio, bearing in mind, however, that bringing the elements very close together lowers the impedance of the antenna and is more difficult to achieve a low ROS. The highest antenna gain with the reflector alone is obtained by spacing the two elements by 0.12 ?.

Any conductors should be Yagi elements and should be calculated using the Yagi antenna method, both for size and spacing. I’ve seen a design with two rows of directors in the middle of each loop. I think this only serves to complicate things, both mechanically and in terms of antenna tuning. A single row of Yagi directors in the centre of the antenna is enough to give very good performance: In fact, I could see that such a 5-element antenna offers a higher gain than an equal Yagi element.

An antenna structured in this way for 2 meters can present mechanical problems due to the size of the radiator and reflector that will always need a support, but in UHF, where the antenna is self-supporting, certain problems can be easily overcome.

Recently (December 2011) Loris IK5GFC has realized it as a mobile antenna, with only the radiator placed inside the car, applied behind the headrest of a rear seat of the FIAT Panda HI HI !

Article by I5NZR (SK) and freely translated by IW5EDI