Overshoots and Close-In Coverage
     

This is the third in a series of articles about FM transmission systems.

In the Oct. 23 installment, we looked at the question of antenna gain vs. transmitter power. In this segment, we will discuss methods of dealing with the problem of overshooting the target coverage area and problems with close-in coverage.

Consider that with an FM antenna, we achieve "gain" by increasing the number of elements in the antenna. Antenna "gain" is referenced to the field produced by a horizontally-polarized half-wave dipole in free space.

A horizontally-polarized antenna with one bay, for example, would nominally exhibit a power gain of one; adding an additional element to the antenna would increase its power gain to two. Additional bays would likewise continue to increase the gain of the antenna.

Circularly-polarized (CP) antennas would exhibit half this nominal gain, as half the power of the antenna is vertically polarized (i.e., a two-element CP antenna has a nominal power gain of one). As a rule, only the horizontally-polarized mode is considered when discussing antenna gain.

A single-element antenna exhibits a large vertical-plane lobe with the radiation being distributed evenly both below and above the horizon. As additional elements are added to increase antenna gain, the vertical plane lobe is narrowed, focusing the main lobe energy on the horizon.

Spotlight analogy

Think of a single-element antenna as a floodlight that fairly well lights up an entire room with a relatively low light intensity. Increase the "gain" of the floodlight by focusing it with a reflector, turning it into a spotlight. The same amount of light energy is emitted by the bulb, but now it produces a much greater intensity in a much smaller area. Continue to focus the light until it is a brilliant pinpoint and you have done in essence the same thing as stacking 12 or more elements in an FM transmitting antenna.

There are several places where this analogy falls apart, but it illustrates the general principle.

Maximum coverage is achieved by focusing the "beam" of the vertical-plane lobe on or just below the horizon. Increasing the antenna height naturally pushes the horizon out by allowing the antenna to look slightly beyond the curvature of the earth.

If a tight vertical-plane "beam" of a multi-element antenna mounted on a tall tower is focused on the horizon (i.e., perpendicular to the tower and antenna), most of the energy radiated from the antenna will overshoot the target coverage area, being wasted out into space.

This isn't much of a problem in cases where the antenna has few elements or is mounted less than 500 feet above average terrain. However, in cases where a multi-element antenna is mounted 1,000, 1,500 or even upwards of 2,000 feet above average terrain, steps must be taken to mitigate the overshoot.

The seemingly obvious solution would be to tilt the antenna mechanically so that its vertical-plane "beam" is focused in the desired location, just below the horizon. While this technique does work, it has a serious drawback in that it only lowers the beam on the side toward which the antenna is tilted and it raises the beam an equal amount on the opposite side, in effect robbing Peter to pay Paul.

In certain situations, this may be an acceptable course of action, for instance where an antenna is located with all the populated area to one side or where the antenna has a significant amount of terrain shielding on the side away from the populated area. In both such cases, the loss of signal on the opposite side would not matter.

A better way of achieving "beam tilt" is electrically, by delaying the currents to the lower antenna elements. This is achieved by simply inserting a "delay line," a short, additional length of transmission line between the power divider and the lower antenna elements. Electrical beam tilt has the advantage of lowering the vertical-plane lobe equally in all directions. Typical values of electrical beam tilt are 0.5 to 1 degree.

Null impact

Another byproduct of increasing antenna gain is that of elevation plane nulls.

Any antenna with two or more elements will, in addition to the main vertical-plane lobe, have other secondary lobes both above and below the horizon. Along with these secondary lobes come vertical-plane nulls. Nulls above the horizon are of no consequence because they have no effect on coverage. Nulls below the horizon are a different story as it is below the horizon where the desired coverage area lies.

With an increasing number of elements, the elevation angle of the first null (i.e., the first vertical-plane null below the horizon) increases. As a result, the distance from the tower to the point on the ground where the first null lands increases.

This null area, even though relatively close to the tower, often is an area of real signal problems. With virtually no direct-path signal, all the remaining signal comes from reflections and refractions and the net signal is plagued with multipath effects. If there is significant population within the null area (or if there is a major thoroughfare through the area), something must be done to mitigate these effects and make the station listenable there.

The best solution is to take steps in the antenna design to prevent the signal in the null from going all the way to zero. A small amount of null fill, usually 5 percent or so, is more than adequate to provide plenty of direct-path signal to overcome the reflections and refractions.

Null fill does not come without a penalty, however. That power put back into the vertical-plane null must come from somewhere, and it mostly comes from the main vertical-plane lobe. As such, null fill generally slightly lowers the gain of the antenna.

How does one determine the distance to the first null? A quick method of determining the approximate distance is the following. Please note that this formula is different than the one that appears in the print edition of this article; the following is correct:

First Null Radius (mi.) = [Height (ft.) X No. Bays] / 5280

The antenna manufacturer can provide a complete vertical-plane pattern for the antenna being considered, and with this you can find not only the location of the null but its width. Overlaying that on a map can help you determine whether or not null fill is even needed.

As we continue this series on FM transmission systems, we will focus next time on antenna bandwidth and some of the problems that can result from inadequate antenna bandwidth.

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