This article explains how to improve the front-to-back (F/B) ratio of the classic 4-square phased vertical array. While this modification adds another element to the antenna, the completed array fits into the same space as the original. Computer simulation indicates that this change increases average F/B ratio by nearly 20 dB, while reducing peak forward gain by less than 0.4 dBi.
For many contesters, a 4-square array represents the state of the art in antennas for 160 meters, and ofter for 80 meters as well. Figure 1 shows a bird's-eye view of a 4-square with an AM broadcast-style ground screen. Each quarter-wave element utilizes a ground system comprising 60 radials with a maximum length of 0.25 wavelength - just under 65 feet at 3.79 MHz. Buried three inches deep, these radials are not allowed to overlap one another where they cross. Instead, they are truncated wherever they intersect, and are bonded to four buried "bus wires" oriented north, east, south and west on the drawing. In this configuration, the radial system for each monopole consists of approximately 3300 feet of wire, but all four elements actually share the entire ground screen because of the many interconnections the bus wires provide.
This array was modeled using EZNEC ver 4.0 with a double-precision calculating engine. In this simulation, all conductors were assumed to be #12 AWG copper and the antenna installed over average soil with a conductivity of 5 mS (millisiemens) per meter (or 5000 micromhos) and a dielectric constant of 13. When the monopoles are driven with equal-amplitude currents using a progressive 90 deg phase shift, the elevation plane pattern is as expected (see Figure 2). The peak forward gain is 6.04 dBi at a take-off angle of 23 deg with a F/B ratio of 18.17 dB in the elevation plane. As shown in Figure 3, the principal azimuthal plane pattern has a F/B ratio of 23.82 dB and a half-power beamwidth of 100.2 deg.
Improving Front-to-Back Ratio
One way to increase the performance of this array would be to somehow shrink the undesired lobe of radiation at the back of the beam. This can be accomplished by converting the antenna into a pair of identical 3-element arrays that are perpendicular to one another. To do this, a fifth monopole is placed in the exact center of the existing "square." This additional element then becomes the center radiator of a 3-in-line array. Figure 4 depicts a plan view of the modified system, which includes the added radials for the new (fifth) monopole. In the computer model, I first removed about three feet from each of the four bus wires at the point where they intersected in the center of the array. This opened up a spot to install the new element. Next, I added 60 quarter-wave radials at the base of the fifth monopole, buried only two inches deep so they wouldn't touch the wires already present. Four of these extra radials - the ones oriented exactly north, east, south and west of the new element - turned out to be directly above and parallel to the four existing bus wires and separated from them by a vertical distance of only one inch. So I reduced their length to about three feet, sloped them slightly downward and connected their tips to the inner ends of the bus wires, which are part of the original radial system. This served to bond together the ground screens of all five monopoles.
When the 5-element array is operational, the two inactive monopoles must be open-circuited at their bases to make them electrically invisible. A binomial (1:2:1) current distribution is used, with the center element receiving twice as much current as the front and rear monopoles. With the base current of the central radiator used as a 0 deg reference, the phase angles of the input currents to the two remaining elements were then continually adjusted in an effort to maximize the front-to-back ratio in the elevation plane. The best F/B ratio - 37.22 dB - was achieved with phase angles of +130 deg to the rear monopole and -130 deg to the front radiator. The forward gain did slip a bit - from 6.04 to 5.66 dBi, but the F/B ratio increased by slightly more than 19 dB. This seemed like a worthwhile trade-off. Unfortunately, the feed-point impedance of the front element was now 0.31 ohm + j77.86 ohms - a very small resistance combined with a large reactance - which is undesirable.
For the complete version of this article as published in the NCJ, view the pdf version.