The Concept of Cellular Base Station Antennas

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Cellular antenna concepts:

Cellular antennas are a familiar feature in nearly every corner of the world. In many cases, these are cellular networks that bring new connectivity where it had never before been possible – and these connections depend on cellular base station antennas.

  1. Omnidirectional antennas, which we defined in previous blogs as antennas that exhibit a circular radiation pattern and operate in virtually all directions, and
  2. Directional (or sector) antennas, which operate in a specific direction, most commonly covering an arc of 120 degrees or less, depending on capacity requirements.

In these cases, two Receive antennas were required per sector to support Receiver diversity. For modern sites, a single Dual-pol (±45° polarization) model with the appropriate horizontal beamwidth supports Receiver diversity.

CELL REUSE

What makes cellular networks different from other types of communications is the principle of cell reuse. Cell reuse is a way of increasing network capacity by “reusing,” or reassigning, individual frequencies on the fly within a particular cell.

To see this process in action, consider the shape of cells and how they fit together. Typically, cells are represented as interlocking hexagons, as seen below [figure 2]. Depending on the density of the area served, these hexagons can be miles across or cover just a few hundred feet.

As a result of this incredible flexibility, channel sensitivity is limited by external interference rather than noise issues, as older radio communications have traditionally been.

The specialized pattern shaping available with directional antennas, both in azimuth (horizontal direction) and elevation (vertical space), allows incredibly precise coverage that doesn’t interfere with neighboring cells.

ANTENNA CHARACTERISTICS

A cellular base station antenna is the most critical consideration in an efficient cellular network, and it all depends on choosing the antenna with exactly the right physical characteristics for a specific application.

These characteristics relate to radiation pattern, antenna gain, front-to-back ratio and a number of other critical factors.

In the real world, defining, choosing and testing these characteristics requires a great deal of technical expertise and mathematical skill, so for the purposes of this discussion, we will cover the basics with a far more generalized approach than an engineer would use in an actual evaluation.

CELLULAR ANTENNAS ON A PRACTICAL LEVEL

When we move beyond the drawing board of theoretical antenna design to the real world, we soon discover that the laws of physics are not the only limiting factors affecting an actual installation.

These issues include everything from tower weight and wind limits to local zoning board approvals for antenna size, shape, height and appearance. In most installations, compromises are necessary to satisfy all the competing interests.

Most cellular antennas are produced in a variety of physical sizes to offer the best performance while conforming to other requirements. Chances are that you’ve seen cellular antennas mounted in a number of ways, featuring diverse sizes and designs, such as the commonly used lengths of 4, 6 and 8 feet.

MATERIALS AND ENVIRONMENT

Cellular base station antennas are only as reliable as the materials that go into their construction, and the construction of their arrays. When it comes to working with the physical limitations of an antenna’s location, matching the right materials to the environment is a critical consideration.

Here are just a few examples:

IN THE ANTENNA ARRAY ITSELF:

  • Aluminum alloys offer lightweight strength, but can be vulnerable to the elements.
  • Pressure cast aluminum is well suited to bases, sockets, mounts and clamps, where its hardness and resistance to corrosion are critical.
  • In circumstances where weight is not a serious factor, copper and brass are used for their easy plating and conductivity properties.

ANTENNA RADOMES:

  • High-strength, low-RF loss materials such as fiberglass offer protection from the elements.
  • Materials must offer UV protection to prevent deterioration due to sunlight exposure.

TOWER APPEARANCE:

  • For purposes of appearance and zone compliance, non-metallic paint can be applied to the entire structure.
  • For better wear, smooth surfaces should be roughed prior to painting.

These are just a few of the more obvious physical considerations. Other matters in cable selection, connector choice and termination options demand close attention as well.

MORE CAPACITY WITH FEWER ANTENNAS

As mentioned earlier, cellular antennas are directional, often covering 120 degrees, or one-third of a complete circle. Mounted together on a triangular tower, three sets of these antennas can cover all directions.

But in densely urban areas that require more capacity, narrower focus-antennas (called a six-sector scheme) can handle additional traffic – along with the cost of adding more antennas.

Having so many antennas in a single location makes it more likely to run afoul of local zoning codes.

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