RF Antenna Concepts

There are several concepts that are essential knowledge when implementing solutions that require RF antennas. Among those that will be described are:

• Polarization
• Gain
• Beamwidth
• Free Space Path Loss

The above list is by no means a comprehensive list of all RF antenna concepts, but rather a set of must-have fundamentals that allow an administrator to understand how wireless LAN equipment functions over the wireless medium. A solid understanding of basic antenna functionality is the key to moving forward in learning more advanced RF concepts.

Knowing where to place antennas, how to position them, how much power they are radiating, the distance that radiated power is likely to travel, and how much of that power can be picked up by receivers is, many times, the most complex part of an administrator's job.


Polarization
A radio wave is actually made of up two fields, one electric and one magnetic. These two fields are on planes perpendicular to each other, as shown in the following figure.


The sum of the two fields is called the electro-magnetic field. Energy is transferred back and forth from one field to the other, in the process known as "oscillation." The plane that is parallel with the antenna element is referred to as the "E-plane" whereas the plane that is perpendicular to the antenna element is referred to as the "H-plane." We are interested primarily in the electric field since its position and direction with reference to the Earth's surface (the ground) determines wave polarization.

Polarization is the physical orientation of the antenna in a horizontal or vertical position. The electric field is parallel to the radiating elements (the antenna element is the metal part of the antenna that is doing the radiating) so, if the antenna is vertical, then the polarization is vertical.

• Horizontal polarization - the electric field is parallel to the ground
• Vertical polarization - the electric field is perpendicular to the ground

Vertical polarization, which is typically used in wireless LANs, is perpendicular to the Earth’s plane. Notice the dual antennas sticking up vertically from most any access point - these antennas are vertically polarized in that position. Horizontal polarization is parallel to the Earth. In the foolowing figure illustrates the effects polarization can have when antennas are not aligned correctly. Antennas that are not polarized in the same way are not able to communicate with each other effectively.


Gain
Antenna gain is specified in dBi, which means decibels referenced to an isotropic radiator. An isotropic radiator is a sphere that radiates power equally in all directions simultaneously. We haven't the ability to make an isotropic radiator, but instead we can make omni-directional antennas such as a dipole that radiates power in a 360-degree horizontal fashion, but not 360 degrees vertically. RF signal radiation in this fashion gives us a doughnut pattern. The more we horizontally squeeze this doughnut, the flatter it becomes, forming more of a pancake shape when the gain is very high. Antennas have passive gain, which means they do not increase the power that is input into them, but rather shape the radiation field to lengthen or shorten the distance the propagated wave will travel. The higher the antenna gain, the farther the wave will travel, concentrating its output wave more tightly so that more of the power is delivered to the destination (the receiving antenna) at long distances. As was shown in the figure, the coverage has been squeezed vertically so that the coverage pattern is elongated, reaching further.


Beamwidth
As we've discussed previously, narrowing, or focusing antenna beams increases the antenna’s gain (measured in dBi). An antenna’s beamwidth means just what it sounds like: the “width” of the RF signal beam that the antenna transmits. The following figure illustrates the term
beamwidth.

There are two vectors to consider when discussing an antenna’s beamwidths: the vertical and the horizontal. The vertical beamwidth is measure in degrees and is perpendicular to the Earth's surface. The horizontal beamwidth is measured in degrees and is parallel to the Earth's surface. Beamwidth is important for you to know because each type of antenna has different beamwidth specifications. The chart below can be used as a quick reference guide for beamwidths.


Free Space Path Loss
Free Space Path Loss (or just Path Loss) refers to the loss incurred by an RF signal due largely to "signal dispersion" which is a natural broadening of the wave front. The wider the wave front, the less power can be induced into the receiving antenna. As the transmitted signal traverses the atmosphere, its power level decreases at a rate inversely proportional to the distance traveled and proportional to the wavelength of the signal. The power level becomes a very important factor when considering link viability.
The Path Loss equation is one of the foundations of link budget calculations. Path Loss represents the single greatest source of loss in a wireless system. Below is the formula
for Path Loss.