To help you pick the right antenna for the job, first survey the site to get an idea what coverage pattern you want, and what degree of signal loss you are currently encountering. Start by mapping out the site on a piece of graph paper, noting possible locations for the high-gain antenna. Then place the access point (with the existing antenna) in a possible location, and walk around with your laptop to the areas you want to cover. If you have NetStumbler on your laptop, you may be able to use it to measure the signal-to-noise ratio (SNR) in each desired coverage location.

The higher the SNR, the better. Lacking that, just note whether the laptop can connect to the network, and if it can, how long it takes to perform some common tasks, such as transferring files. See the basic site survey document in Figure 4-6. If nothing else, this will give you a point of comparison, to determine how much boost you get from your high-gain antenna. In addition, you’ll probably get a better idea what kinds of transmission losses you’re experiencing.Typically, you may see three kinds of losses: propagation losses, multipath losses (signal fading), and interference.

Propagation loss represents the “resistance” of whatever the RF has to pass through. Every medium, including air, has a particular loss at a given frequency. The loss is proportional to

the distance that the signal has to travel through the medium. For example, the signal will probably lose about 6 dB going through a wall with 2-by-4 wood studs and sheetrock on both sides. Beyond about 20 feet from the access point, propagation losses can increase at up to 30 dB per 100 feet, depending on building construction and layout. You’ll get losses not only from walls, ceilings and floors, but even from furniture and people.

Visualize the signal as half a football, with the antenna at the point. Beyond 20 feet or so from the access point, anything that pokes into that area (called the Fresnel Zone) will degrade the signal. See Figure 4-7 for an example. Fresnel Zone incursions are a secondary factor when it comes to interference. The most critical factor is objects that are in the direct line-of-sight (LOS) between the access point antenna and clients. However, objects below the LOS but in the Fresnel Zone can have a significant effect, as well. That’s why, in long-distance outdoor installations, antennas are usually positioned higher than is necessary just for LOS. Outdoors, it’s standard practice to calculate the size of the Fresnel Zone and use that to determine how high the antenna should be.

Indoors, antennas may be installed in the attic or on the ceiling, in order to avoid Fresnel Zone incursions. If that’s not possible, you just have to get a more powerful antenna, in order to overcome the losses. Whether you can clear the obstacles, or whether you just have to compensate for them, it’s good to know what your Fresnel Zone Clearance is. There are simple calculators available on the Web that will allow you to calculate this very quickly. (For instance, www.thirdheight.com/support/downloads/wireless/SOM_calcs.xls.)

This calculator will help you not only with Fresnel Zones, but with your overall “link budget,” a calculation to determine whether the signal transmitted from your antenna is powerful enough to reach the intended receivers. A link budget calculation takes into account a variety of factors, including TPO, transmit antenna gain, losses in the transmission antenna cable, free space propagation losses, Fresnel Zone losses, losses in the receive antenna cable, receive antenna gain, and receiver sensitivity. Other factors such as connectors and lightning arresters in the cables can also be taken into account. (Figure 4-8 shows the variables involved in a typical link budget calculation.)

A common rule of thumb is that the signal should be 10 dB stronger than the minimum required for communication. This 10 dB is referred to as the System Operating Margin (SOM).

Multipath losses occur when signals take different paths to a receiver. If one path is significantly longer than the other, the peaks and troughs of the two signals will be significantly out of sync with one another (“out of phase”) when they get to the receiver. Signals that are totally out of phase (180 degrees) will completely cancel one another, as the troughs of one signal cancel the peaks of the other (see Figure 4-9). In practice, the phases of the two signals will usually drift constantly, so the signal cancellation changes, resulting in a constantly fluctuating signal (“signal fade”). Having two receiving antennas usually helps substantially. Phase cancellation will be different for the two antennas, because of the differences in the length of the two paths required to reach the antennas.With properly spaced antennas, the signal at one antenna can be strong when the other is weak.

When you attach a high-gain antenna to an access point, you lose the advantage of two antennas, and you tend to increase multipath losses. The high-gain antenna should more than make up for this, but you may want to take this effect into account in calculating your antenna gain requirements. If your connection seems to continuously fade in and out, suspect multipath losses.

Interference may come from any other equipment running at 2.4 GHz (including other Wi-Fi networks), particularly if they’re using the same channel as you are, or equipment generating broad spectrum RF. Interference is often “bursty” in nature, as the interfering equipment is seldom operating full-out all the time. If your network experiences sudden and seemingly random attacks of deteriorating performance, look for a source of interference. You may be able to change channels or locate your network equipment and antenna to get away from the offending source. Otherwise, you’ll have to up your power to overcome the interference.