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Chapter 4: Antennas Patterns and Downtilt

A final real-world factor to consider in designing a wireless link is that not all antennas are created equal. Omni-directional antennas cover all directions, while sector antennas may only cover a 60 degree swathe of countryside. Additionally, how far up (or down) you point your antenna can make a huge difference. This section will discuss antenna patterns, downtilt, and how to use them to your advantage.

A final real-world factor to consider in designing a wireless link is that not all antennas are created equal. There are two major types of antenna:

Different antenna devices all fall into one of these two basic types, as well as providing different performance characteristics. Manufacturer data-sheets typically include a plot of antenna performance characteristics.

For example, here are the antenna characteristics of a Ubiquiti AMO-5G10 omni-directional antenna:

This chart is a little hard to read (I recommend using Radio Mobile with antenna pattern files where possible; as described in the Radio Mobile section of this book). The two graphs are attempting to represent the two major axes on a three-dimensional output graphic. The Horizontal Azimuth and Horizontal Elevation charts show antenna output if you were looking at the antenna from above. On the “azimuth” chart, it is showing that in all directions emanating out from the sides of the antenna it provides very strong coverage. The “elevation” chart is showing that at the vertical center of the antenna you will receive the strongest signal, while directly above or below the antenna you will receive very poor coverage. This type of disk pattern is common among omni-directional antennas.

A sector antenna has a more interesting pattern:

Looking at the Horizontal Azimuth, you can see that the front of the antenna has very strong signal sensitivity. There are two “lobes” to either side of the antenna that have medium sensitivity. There is also some sensitivity to the rear of the antenna, which can be the source of self-interference (RFArmor.com shield kits largely eliminate the side lobes and rear sensitivity). This is to be expected: your sector receives best from its front, within the 90 degree arc specified by the manufacturer (but with some drop-off at the edges). What you may not expect is the vertical pattern: the antenna has 2-3 degrees of “down-tilt” built into its design. That is, optimal performance is actually slightly below the front of the antenna. You may achieve better performance by pointing the antenna slightly upwards if you are aiming for a distance site!

Every reputable antenna maker provides sensitivity charts of this type, and you should always consult the data-sheet for this type of information before purchasing and deploying an antenna.

Side-Lobe Issues

Sometimes, when you are aiming a device at a sector you get decent performance – but not what was expected. This is often a sign that you are pointing to the “side lobes” of the antenna (the secondary receiver patterns on the chart). Adjusting your aim towards the center of the antenna can often resolve this problem.

Down-tilt

Many antennas, such as the 90-degree sector antenna pictured above feature a degree of built-in down-tilt. In this case, simply aiming for the front of the antenna may not give the strongest possible signal: you also want to be somewhat below the antenna (at an angle of 2-3 degrees). This isn’t an issue for most tower deployments in which the sector is high above the clients, but can be a problem for shorter-range links where you point two antennas directly at one another and expect great performance! It is always a good idea to check for built-in down-tilt.

Additionally, when aiming antennas you need to remember to consider both horizontal and vertical aiming. You will often need to adjust the vertical alignment at either end of a link to ensure that the radios can see one another.

« Chapter 4: Real World Path Loss Estimation Up To Contents Chapter 4: Practical Examples »

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