Directional Antenna Basics

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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.

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.

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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