19 July 2017 | Reading Time: 3 minutes
Antennas are a core component of modern passive infrastructure. Understanding how to specify, design and install them is crucial to maintaining successful network performance the design of an antenna can have a substantial impact on signal strength.
Because of this, below we’ve outlined key antenna design considerations to increase signal strength.
Vertical and horizontal antenna radiation patterns:
All antennas, regardless of polarization, have three-dimensional radiation patterns. If the pattern is extended in all directions equally, the resulting shape would be a sphere, with the antenna at its center. The polarization of the antenna determines which portion of that sphere represents an antenna’s actual pattern.
Slicing the sphere vertically yields a vertical circle while a horizontal slice would reveal a horizontal circle.These theoretical descriptions of the two polarization patterns appear to be omnidirectional within their planes, but that’s not quite the case.
In practice, there are no truly omnidirectional antennas. Our example half-wave dipole antenna, for instance, reveals the truth (figure 1). The pattern appears circular, like a doughnut, on a horizontal plane, but forms a figure-8 in the vertical plane.
As we will see later in this blog post; most real-world antennas consist of a vertical array of radiating elements and elevation pattern shaping has become quite important for interference minimization.
ANTENNA GAIN IN SIGNAL STRENGTH
Using our half-wave dipole as our reference, we measure gain in decibels (dB). Decibels are used to compare one quantity of power to another. So, taking our dipole as our starting point, we say it has a gain of one, or that it has a value of 0 dB, as it has “zero difference” from itself. If we have an antenna with double the power of our reference dipole, we find that its power ratio of 2.00 yields 3 dB in gain (figure 2).
BOOSTING ANTENNA SIGNAL STRENGTH
Theoretically, there are two ways to increase antenna signal strength:
We can increase the power or current density in the antenna so it will radiate its pattern with greater intensity. But as we discovered earlier, we cannot raise the power without fundamentally altering the mechanics of the antenna, cable and transmitter.
Alternately, we can change the radiation pattern, tightening its focus, so more of the existing power is directed where it will be used. This can be done without changing out hardware.
Consider again the circular, doughnut-shape pattern of our dipole antenna (figure 3). By “squashing” the doughnut vertically, we produce a denser, flatter, rounder pattern. As a consequence, the circle also grows larger as vertical space is traded for horizontal space. Since verticality is rarely of importance, this is a very profitable exchange.
OMNIDIRECTIONAL PATTERN GAIN ANTENNAS
To achieve a greater increase in this circular (or omnidirectional) pattern, we can stack multiple vertical dipole antennas above each other, as shown in figure 3. This increases the vertical size of the antenna. Then, we feed power to the dipoles in such a way that they add together at a distant point – again, with transmission lines matching their radiation power limits for greatest efficiency.
By feeding equal amounts of power that arrive at each dipole at the same instant, the dipoles radiate “in phase,” or in synchronicity, for improved gain by virtue of its pattern.This type of antenna is called a vertical collinear phased array, and it is the most commonly used type of base station antenna.
Aperture, or beam width, determines the gain of an antenna. Like an adjustable nozzle on a garden hose, aperture describes the degree to which the signal is focused: the tighter the focus, the greater the gain within that area of focus.
SPACING OF DIPOLE ELEMENTS
In a vertical collinear array, each dipole or sub-array of dipoles is connected in parallel to the common feed point by a separate transmission line. This means it’s possible to locate the dipoles so that their vertical separation tightens overall aperture to boost gain. This separation is usually something less than a single wavelength of the assigned frequency being transmitted. Anything less tends to reduce the improvements in antenna signal strength.
THE BOTTOM LINE OF ANTENNA DESIGN
We’ve just covered only some of the basics of antenna design, but it’s important to keep in mind that there are no magic formulas or one “ideal” antenna configuration. Improving one aspect of operation always comes at the expense of another aspect. The best design is always one that is driven by the specific operational objectives of the antenna.