Directional Antenna Basics
Next in complexity is the parallelogram. The parallelogram consists of an even number of elements and is characterized by a quadrilateral shape. This shape may range from a square, through a rectangle to a parallelogram that looks like a box squashed almost flat. Typically, any design where the array looks like a four-sided figure with the opposite sides equal in length will be referred to as a parallelogram.
The fourth and final broad category includes those designs that do not nicely fit into the other three categories. They include designs with several tower lines, odd inconsistent spacings and strange orientations. For instance, there is an array in Southern Illinois that started out as a three-tower in-line to which two additional towers were added. These two towers were added on separate lines, so from the air the array looks almost like a trapezoid.
Staying in phase
The geometry of the array defines the basic shape of the pattern that will result. Included in the geometry is the orientation of each of the towers as well as the spacing between the elements. In addition to these values, each tower also has an associated field ratio and phase relative to a reference value. These parameters, along with the input power and afore discussed height, define the theoretical parameters of the pattern.
The creation of these various field ratios and phases is the job of the phaser and the ATU (antenna tuning unit), which you may sometimes see as ACU or other similar term. As the RF enters the phaser cabinet, it will usually encounter a common point trim circuit, which transforms the impedance of the actual common point into something the transmitter will like. Typically, this is an impedance of 50Ω resistance and a few ohms or less of reactance. In a directional antenna system, the current and impedance at this location is what defines the input power to the directional antenna. Unless subsequently modified, the input power for a directional antenna is 8 percent above nominal powers of 5kW or less, and 5.3 percent above nominal powers greater than 5kW.
The actual common point lies beyond the trim circuit. This location is common to the various networks of the array. Immediately downstream from this location is where the power division and initial phasing will take place. In distributed systems, there may be additional distribution occurring at each tower, but such topologies are the exception rather than the rule.
- continued on page 3
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