But what about beaming a signal 5, 10, 20 miles, or more? Wireless is a natural replacement for land lines, T1s, DSL, and other high-speed data when needed in a remote location. Or even a location that’s not so remote, but where DSL or cable Internet may not be available. Figure 13-1 shows a prime example. A long-distance Wi-Fi link creates a high-bandwidth connection to the mountain operating at the speed of light.

Creating a long-distance link gathers many of the essentials of wireless and adds a healthy dose of physics to overcome the obstacles of a longdistance, free space link.

This chapter is a compilation of practical guidelines designed to enable you to establish a long-distance Wi-Fi link of your own . For your convenience, we’ve condensed the most essential aspects of strategy, design, and experimental deployment for you here.We’ll start with site selection, then take on design considerations including antenna location considerations, and work our way through important hindrances—such as Fresnel zones, path loss, and the Earth’s curvature—many of which can be mathematically determined.We’ll move on with a discussion on link planning and actual deployment strategies, and conclude the chapter with tips and recommendations for creating a successful link.

A typical long-distance Wi-Fi link will require:

One of the most important fundamental aspects of setting up a long-distance wireless link lies in the matter of site selection. Choosing the proper location of your links can mean the difference between a quick and easy setup and a long day of problems when it finally comes time to establish the link.

The time you spend on initial site selection can be drastically reduced by using topographical mapping software. This easy-to-use software can show the terrain profile of a line drawn between two or more points (see Figure 13-2). From that line, you can quickly gauge whether or not line-of-sight is possible given the terrain.

In the case of large obstructions blocking your path, you’ll need to seek an alternative. One alternative is to employ a repeater, as was described in Chapter 9 (see Figure 13-3). Other solutions are to shift the site requirements slightly. You can run Ethernet cabling up to 100 meters from network equipment, and fiber cable can be run for several kilometers. The possibility of stretching the wired portion of the link horizontally or vertically to a suitable transmission point is apparent.

Software is only the first step. You’ll need to make an actual site visit to determine if foliage, buildings, or other obstructions will interfere with the link path. One of the best tools for this is a spotting scope (see Figure 13-4). Binoculars will also help, but the magnification level is not as high as a spotting scope.

When you set up your long-distance Wi-Fi link, there are several factors to consider, including background research and testing. Through the course of this section, we’ll work our way through the most commonly used types of antennas, followed by antenna location, and finally review potential obstacles and impedance problems and how to deal with them accordingly.

We’ll only focus on three types of antennas you could deploy in your outdoor WLAN as a link between two points or point-to-multipoint: the dipole antenna, coaxial antenna, and the dish antenna. Albeit interrelated, each type has its own design strengths.

radiating element. The element can be oriented in any fashion, although it is usually vertical.

the energy converges on the driven element. If the horn or dipole is connected to a transmitter,

the element emits electromagnetic waves that bounce off the reflector and propagate outward

in a narrow beam.

A long-distance Wi-Fi link is not an easy accomplishment. There are many factors working

against successful communications such as distance, open space, interference, obstructions, and

inherent equipment limitations.To start building a strategy, you should consider location very

carefully. Your radio signal path must have a clear, line-of-sight path—end-to-end—and a clear

Fresnel zone (covered in more detail later on). Be sure to use GPS and a spotting scope to visually

map and sight your path over long distances. Incidentally, Fresnel zone losses of up to 6 dB

can be avoided by ensuring that there are no objects large enough to act as diffracting edges

within the first 0.6 Fresnel zone. If a large, rounded object is in your path, losses may exceed 20

dB through several Fresnel zones. This will force you to mount your antennas on towers or

buildings at a significant height. Unfortunately, microwave frequencies can also be affected by

too much antenna height, and the signal can be degraded due to ground reflections canceling

out the signal. Signals will propagate through a few obstructions such as trees or small buildings,

and the radio signal will slightly extend over the line-of-sight horizon, but you shouldn’t

always count on it.

For all practical purposes, it’s safe to assume that if light can’t penetrate a stand of trees,

microwave losses will be unacceptable. Consider Table 13-1.

Although typically microwaves are not affected by the ionized layers in our atmosphere because

these layers are higher than the normal line-of-sight transmission of the signals, temperature

inversions can still prove to be a problem. This is because as the hot air rises, moisture rising

within the air causes attenuation of the signal. One might assume that lower microwave frequencies

are affected by water vapor and oxygen, but this is not the case.

Also consider the temperature effects on paths such as: reflections, refractions, diffractions,

transmission “ducts” and even tropospheric reflections and scattering. These atmospheric conditions

can cause a link to fail even though you have visual line-of-sight. A basic understanding

of these conditions may help you when troubleshooting a long-distance link.

Other sources of performance degradation in frequency hopping systems are spectrum background

noise, received signal fading, interference from other services in that frequency range, random

FM components in the signal, “click” noise resulting from the phase discontinuities between

frequency hops, errors in receiver synchronization, or even the wind moving your antennas.

The antennas will also have to have the same RF signal polarization. The polarization of the

signal will depend on the direction the actual antenna is positioned. If it’s up/down, the polarization

is vertical; if the antenna is left/right, the polarization is horizontal. If the antenna is

diagonal (45 degrees usually), you’ll have diagonal polarization. By not having the same polarization

on your network’s antennas, you can receive a 20 dB loss of signal strength. This is an

enormous loss, but can also be very useful. By changing antenna polarization, you can help

eliminate certain types of radio interference, or allow many antennas in one location.

Horizontal antenna polarization at microwave frequencies will generally provide less multipath

and may also provide lower path loss in non line-of-sight situations, but you should always

experiment with different polarizations.

FCC or PerCon frequency databases for the coordinates to transmitter locations in your area. You

polarization, so if you need to locate your antenna site near one, use vertical polarization. Other

things to look out for at your antenna site are high-power PCS wireless cell phone transmissions in

the 1.8–1.9 GHz band, broadband noise from high-power colocated transmitters, harmonics from

mobile radio and paging transmitters, and other nearby microwave links.

The proper Earth grounding of your antenna tower is essential for lightning protection and

static discharge. Many towers are inadequately grounded by using only a few grounding rounds

and large gauge round copper cables. This is not correct. The small number of grounding rods

are inadequate, and round copper cable has a relatively high impedance to an instantaneous rise

in electric current (lightning hit). Extremely high voltages will develop across these cables and

instead of going to ground, these charges will go directly into your building equipment. A minimum

of four ground rods per tower leg with some sort of chemical grounding material should

be used. The chemical grounding material will help to lower the ground rod resistance. Copper

straps should be used to connect the ground rods to the tower due to their low inductance. In

areas with sandy soil or excessive wind-generated static, it’s advisable to use a more elaborate

grounding method. Most likely a radial grounding system like that found in AM radio.

You should also try to have all your transmission line runs inside the tower column. This will

help shield them from lightning if it hits the tower. You should also securely bond the lines to

the tower every 15 meters or so. Use the recommended bonding kits that your tower manufacture

approves of.

Antennas mounted on very high towers may need to take into account beam tilt. Beam tilt is

needed when a radiating signal’s vertical beam width is narrowed (by using high-gain antennas),

and the areas near the tower location lose service because most of the signal is wasted by

broadcasting into open air. The beam must be tilted either mechanically or electrically to steer

the signal back into its proper location.

Mechanical beam tilting is relatively easy. The antenna can be mounted slightly less than 90

degrees from the horizontal plane so the tilted beam illuminates the desired service area.

However, in the opposite direction, the signal will be pointed toward the sky, reducing the

effective service area in that direction of the antenna.

If the signal needs to be “bent” downward in all directions around the antenna site, an electrical

tilting method must be used. This is commonly referred to as “null fill”. Electrical tilting is produced

by controlling the current phase in the antenna itself. Thus, it must be done during the

antenna’s design stages by an engineer with expensive equipment.

Finally, consider potential weather problems. Ice buildup on antenna elements will result in an

increased SWR (impedance mismatch, standing wave ratio) that will de-tune a transmitter sys-

tem, significantly reducing its output power. Ice can also cause severe transmission line damage,

and falling icicles can kill. The easiest way to prevent ice buildup is with special antenna heaters

or by covering the antenna system with a fiberglass radome. Radomes will increase the wind

load on the tower and antenna heaters can be expensive. For more information, visit the Web at:

www.teletronics.com/tii/documents/Antennas/2.4%20GHz/Antennas_Omni.pdf.

There’s a great guidebook to building your own custom WLAN antenna on the Web at

www.saunalahti.fi/elepal/antennie.html.