Directional Antenna Basics
Nov 1, 2011 2:00 AM, by Jeremy Ruck, PE
There is no way to cover all the important concepts of directional antennas in a single column. There have been many voluminous tomes written about this subject. This month we will look at some of the basics of AM directional antennas, and in subsequent months delve deeper into the directional array.
The first thing to remember about antennas is that every one is, in some way or another, directional. The only truly non-directional antenna is the isotropic radiator, a theoretical construct, which would look like a point source in space with no preferred direction of radiation. Although the sun, at a distance, is isotropic, an antenna with that quality cannot be constructed. Beyond the disturbance caused by the feed point, an isotropic antenna would ultimately violate Maxwell’s Equations.
The ground plane antenna, which is the general topology in modern run-of-the-mill AM systems, is for practical purposes, non-directional in a given horizontal plane. Although the local environment and construction of the antenna will tend to induce some directional characteristics, we can consider such an antenna essentially non-directional in nature. In the vertical plane, the situation is very different. The sinusoidal distribution of current in the radiating element results in directional characteristics in the vertical plane. This directionality can cause nighttime interference to other facilities. Conversely, this vertical plane directionality can also be used to prevent interference in certain regions by varying the radiator height.
An inline array has two or more towers in a straight line.
The directional AM antenna is comprised of two or more active elements. The topology of the directional array usually falls into four broad categories. The simplest, in-line array, consists of two or more elements along the same azimuth.
A dogleg array has an odd number of towers oriented in a right angle.
Similar to the in-line array is the dogleg. The dogleg is typically a three-tower array on two separate azimuths. The center tower is common to both azimuths, and the result is a “bend” in the middle of the tower line. Ultimately, a dogleg would not necessarily have to be limited to three towers, but does have to be based on an odd number of towers.
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Directional Antenna Basics
Nov 1, 2011 2:00 AM, by Jeremy Ruck, PE
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.
A parallelogram array has a quadrilateral shape that ranges from a square to 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 50O 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.
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Directional Antenna Basics
Nov 1, 2011 2:00 AM, by Jeremy Ruck, PE
At the outputs of this network, or networks, the RF will typically enter the transmission line feeding each tower. At the far end of the transmission line is the ATU. This portion of the system takes its RF at the transmission line impedance and transforms it to the tower drive point impedance using a specific phase shift.
The drive point impedance of each tower is the impedance of the tower when the array is active. It is derived from the self-impedance of the tower and the coupling that occurs with other elements in the array when the array is active. Certain designs can result in a negative tower, which is a condition in which the resistance of the drive point impedance is less than zero. Instead of delivering power to the array, the negative tower is parasitic and absorbs power delivering it back to the phaser. Contrary to popular wisdom, a negative tower is not a nine-headed Hydra unless the impedance is within a couple of ohms of zero. Such low impedances, regardless of the sign, tend to cause stability issues.
Now that the power is out to the tower, we have to have some way of measuring what is actually there. This is where the sampling system comes into play. Typically, current transformers or sample loops are used to feed a voltage sample back to the phase monitor. Section 73.68 of the Commission’s Rules provides the requirements for approved sampling systems. In addition to the sampling system, many arrays still have ways of measuring the base currents. Although no longer mandatory, maintaining the ability to measure base currents is highly recommended as it provides an additional health check on the antenna.
Next time I’ll delve deeper into the directional antenna array and look at tee networks.
Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.
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