Simply put, antenna theory is boring. If I were to go into great technical detail about how antennas work, your eyes would glaze over and you'd eventually start snoring. Because it's not my goal to create a 400-plus-page sleep-aid, I'm going to keep this description very simple. Of course, simple is a relative term. Still, the more you understand about the basic workings of antennas, the better you'll be able to choose the best antenna(s) for your WLAN.
When alternating current is applied to an antenna, electrons inside the antenna vibrate. The vibrating electrons create an oscillating electric field. Each electron has its own magnetic field, and as a result, the oscillating electric field, which is composed of electrons, creates an oscillating magnetic field. This gives us electromagnetic waves, or radio waves.
Radio waves radiate out from an antenna at the speed of light. The wavelength of a radio signal is determined by measuring the distance between wave peaks. This distance is also known as a wave cycle.
The frequency of a radio wave is the number of wave cycles that pass a given point in a fixed period of time. We measure frequency in hertz (Hz); one Hz is equal to one wave cycle passing a given point in one second. Because it takes longer for a long wave cycle to pass any given point than it does for a short wave cycle, we can say that low-frequency signals have long wavelengths, and high-frequency signals have short wavelengths. So, our 802.11b signal has a short wavelength, which gives it a high frequency (2.4 billion cycles per second, to be exact).
At the receiving end, the radio signal causes the same thing to occur in reverse. The electrons in the antenna vibrate in reaction to the radio waves, and this is transferred to the radio receiver as an electrical signal, which can be heard or interpreted as data if the receiver is tuned to the same wavelength as the signal.
Are you sleepy yet? No? That makes one of us then, but let's continues.When I was discussing signal amplifiers, I mentioned that a high-gain antenna may be a better choice than an amplifier. There are two reasons for this. First, unlike an amplifier, an antenna does not add noise to a signal. This is because an antenna is a passive device; it doesn't provide any additional power to a signal. An antenna can only radiate power applied to it. Second, an antenna improves reception of weak signals from WLAN clients as well as increases broadcast strength. An amplifier does nothing to improve reception of weak signals from WLAN clients.
This is where the subject of antenna gain can start to get confusing. I defined gain as the amount that a signal is amplified. This is true for any electrical circuit where there is an increase in output power as compared to input power. It may seem that an antenna actually produces power itself and then broadcasts the stronger signal. An antenna cannot magically produce power; it's a simple device made of simple materials, usually metal.
An antenna produces gain by focusing the electromagnetic radiation into a tighter directional signal. A simple omni directional antenna has no gain; it doesn't focus the radio waves, and the signal retains the same power that it had coming into the antenna, minus any loss that occurs along the way due to resistance.
Think of signal gain from an antenna like tight from a lighthouse. If a lighthouse didn't use focusing lenses (designed by our friend Augustin-Jean Fresnel), then the light would shine equally in all directions. This wouldn't be as effective for protecting ships. The lens focuses the tight into a tighter and brighter beam. The lens is passive; it doesn't create additional light. Instead, it merely directs the existing light into a more concentrated and powerful signal.
A high-gain antenna works the same way. It doesn't create additional energy; it merely focuses the existing RF energy into a more powerful signal resulting in gain. High-gain antennas are directional. The amount of directivity of an antenna is its beam-width.