In May/June 2008 NCJ Al Christman, K3LC, suggested adding a fifth element at the center of a standard 4-square array to improve the pattern. In his scheme the 4-square was transformed into two reversible 3-element arrays at right angles to each other. In the 3-element array, one element was parasitic and two were driven. The remaining two elements were inactive.
After reading the article I realized I had already used a very similar 3-element array on 160 meters, and I can say that it works as advertised. The pattern is very good indeed. My array had one important difference, however. Only the center element was driven; all other elements were parasitic. With this configuration pattern nulls were not quite as deep as the K3LC version, but they were very close. In exchange I had an antenna that eliminated all of the transmission lines and phasing networks associated with a driven array. These components were replaced with a small tapped inductor and a SPDT relay at the base of each parasitic element and a simple tapped inductor at the base of the driven element.
Incorporating this idea into a 4-square and eliminating the transmission lines and phasing networks should reduce losses substantially and save a lot of time, not to mention the expense of phasing networks and transmission lines. The new arrangement is also much easier to adjust for peak performance than a standard 4-square or even K3LC's version. There is, of course, the disadvantage of an additional element in the case of a modified 4-square, but if you already have four elements in place, the center element does not have to be self-supporting. It could be a wire suspended from the other four elements. A number of similar arrays use a tower as the driven element, with the parasitic elements suspended from the tower.
The N6LF 160 Meter Array
Figure 1 is a sketch of the 160 meter 3-element array. Each element is 80 feet of 4-inch aluminum irrigation pipe, top loaded with two 40-foot lengths of #12 wire sloping downward at about 45 degrees. The length of each loading wire was adjusted so the elements - without base loading - were individually self-resonant at about 2.0 MHz. This made it possible to adjust the final resonant frequencies by adding a small tapped inductor (about 5 uH) in series with the base of each element.
For director operation a tap point was selected that made the element resonant at 1.95 MHz; for reflector operation a tap was selected for resonance at 1.8 MHz. The self-resonant frequencies of the parasitic elements were adjusted with the other two elements open circuited. After both parasitic elements were tuned, one was set to be a reflector and the other a director. At this point the driven element was tuned to resonance (1.83 MHz in my case) and matched to the feed line by varying the tap on the base coil.
The change from director to reflector was done using a SPST relay and two taps on the base inductors for the parasitic elements (see Figure 2). The inductors were made a bit larger than the minimum required size so the two values of inductance required could be reached simply by moving the taps. The bottom end of the coil is not connected to anything. This was just a matter of convenience during adjustment.
To match the feed line to the driven element I also used a small inductor with two taps, but no relay was needed. One tap was connected directly to ground and used to resonate. The other tap was adjusted for minimum SWR on the input feed line - very close to 1:1 at 1.830 MHz. Figure 3 shows an example of a base inductor and relay; I used a vacuum there only because it was handy. A simple open contact relay would be fine for this application, as long as the contacts can carry the current. One small trick was to invert the NO and NC contacts between the two parasitic elements so that with no power to the relay, the antenna would fire east. With power applied it would fire west. In my case I powered the relay via the coaxial feed line, using RF chokes and capacitors to isolate the RF from the dc voltage for the relay.
For the complete version of this article as published in the NCJ, view the pdf version.