Transmission Line Feeding for Antennas: The Four-Square Array
Since version 8.50, the AN-SOF Antenna Simulator has enabled us to implicitly model transmission lines. This capability allows us to define a transmission line by specifying its characteristic impedance, velocity factor, length, connection ports, and losses. One valuable application of this transmission line modeling technique is in modeling the feeding system of phased arrays. An illustrative example of a versatile phased array that utilizes transmission lines in its feeding system is the four-square array. This configuration consists of four vertical elements, each measuring 1/4-wavelength in height and arranged in a square pattern. It serves as a powerful tool for both radio enthusiasts and professionals seeking a straightforward phased array for controlling the main lobe direction of the antenna radiation pattern.
The figure below illustrates the layout of the four-square array and its corresponding radiation pattern. When treating the four vertical elements as a 4-port network, calculations dictate the addition of an 18 Ohm resistor at the base of each monopole to achieve the desired directional radiation pattern. Additionally, the feeding system of this array involves six transmission lines, meticulously configured for length and interconnections, all detailed in Chapter 8, Section “The Four-Square Array,” found in the 19th edition of the ARRL Antenna Book.
Here are some of the properties that make the four-square array a compelling choice for antenna enthusiasts and professionals:
1) Forward Gain: 3.3 dBi, considering an average ground.
2) Beamwidth: The array offers a 3 dB beamwidth of 100°.
3) Horizontal Front-to-Back Ratio: 20 dB or better over a 130° angular range.
4) Symmetry for Directional Switching: Thanks to its symmetric design, the four-square array allows for directional switching in 90° increments.
By employing the feeding system outlined in this model, the four-square array showcases excellent performance characteristics, with any limitations primarily influenced by environmental factors. Furthermore, the array’s design lends itself to the implementation of a remote switching mechanism, enabling effortless adjustment of the array’s direction as needed.
Whether you’re a ham radio operator, a DXer, or a professional in the field, the four-square array is a fascinating option to consider for your next antenna project.