Antenna Selection

Now that you have your old antenna removed and know its connector type, how do you select a new one? There are four main factors to consider:

Operating Frequency
Because of the way that radio waves work, antennas are designed to work over specific frequency ranges. Generally, the higher the operating frequency, the narrower the frequency range of an antenna.

For 802.11b/g and the 2.4 GHz version of draft 802.11n, you need an antenna designed for 2.4GHz operation. This antenna won't work for 802.11a purposes or the 5 GHz version of draft 802.11n, or course.

To complicate matters, draft 11n is also bringing back dual-band Wi-Fi, which throws the complication of dual-band (2.4 / 5 GHz) antennas into the mix. The concept of a dual-band antenna might seem to contradict my first statement above. But dual-band antennas are actually combinations of two single-band antennas into one assembly.

2.4 / 5 GHz antennas are hard to find, and high-gain versions even harder. This is why you tend to see business class APs with separate antennas for multiple radios.


Gain
As noted in Part 3, the simple dipoles that come with your AP have a gain of about 2.2dBi. And no, the two antennas on your AP don't provide a total 4.4dBi of gain, but are there to support antenna diversity, which can improve your WLAN's performance through a different technique.

This is the main thing you're trying to improve by changing antennas and you'll probably spend the most time agonizing over this spec.


Radiation Pattern
This factor is as important as gain in determining whether a specific antenna is right for you. It determines the directivity or coverage area of the antenna, and if chosen incorrectly can make your wireless connection worse!


Packaging/Mounting
It used to be that the antenna type determined the physical form-factor of an antenna. However, you can now get both omni and directional antennas in different physical forms. Especially handy for overcoming a spouse's objections to your wireless improvement plans.

The art of antenna selection can get fairly involved when you're trying to get a long-range outdoor link to work. But for indoor or short-range (house to workshop, across a street, etc.) use, selection is fairly easy if you keep two rules of thumb in mind:

Rule of Thumb #1: It takes at least a 6-dB (dBi) increase in gain to improve performance over what you get with a simple dipole antenna.

Rule of Thumb #2: The higher an antenna's gain, the higher its directivity or narrower its area of coverage will be. This effect is similar to what happens with binoculars or telescopes. The higher the binoculars' power, the narrower the field of view.

TIP: An antenna's directivity is also commonly referred to as its "beamwidth"

With these rules in mind, let's look at the types of antennas made for indoor use and their merits for specific applications.

Tags: antenna, How To, WiFi,

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