For me there has always been something magical about antennas. Unfortunately, I never really did all that well in my antenna theory classes in school and now that I’m a bit wiser, I realize it would take hundreds of thousands of words to do antennas justice. But, I’m not usually one to back away from a challenge, so let’s see how far we get in a few hundred words. First, remember that the job of an antenna is to convert electro-magnetic waves to an electric current matching the same pattern. This current is then amplified, filtered and decoded in the radio’s receiver resulting in some communication - it might be music, it might be data, it might be voice. So the job of the antenna is simple - maximize the conversion of the right electro-magnetic waves to an electric current and minimize all the conversion of all the other electro-magnetic noise. Even though the really hard work is in the receiver, don’t underestimate the importance of a good antenna.Think of an antenna as a catcher of electro-magnetic waves of a certain frequency. I think a good metaphor is pouring muddy water through a strainer. Most of the water goes through, but particles of a certain size get trapped. The size of the trapped particles is related to the size of the holes in the strainer in that they are always bigger. Well, while it’s not a perfect metaphor, we might say that the strainer is “tuned” to the size of the particles that we wish to trap. Think of an antenna working the same way - only instead of holes, it is tuned to the wavelength of the electro-magnetic wave it is trying to “capture.” Now for those of you confused about “wavelength” remember that the wavelength is the distance between the start and end of one wave cycle. It is effectively the inverse of frequency. So, if an electro-magnetic signal has a frequency of 2.4 Gigahertz (2.4 billion cycles per second), then the wavelength is the speed of light (300 million meters per second) divided by the frequency of 2.4 billion cycles per second. This is about 12.5 centimeters or 5 inches. So, one might theorize that in order to catch a 2.4 gigahertz signal (WiFi or Zigbee), a 5 inch antenna might do nicely - this would be a one wavelength antenna.
It turns out, of course that the antenna and the waves we are trying to catch can effectively be bended back on itself, making it just as effective at half the length or half of a wavelength. This is cool because our 5 inches now becomes 2 ½ inches. Further, we have learned that really large ground planes can reflect half the wave back if they are big enough. This means that if we put the antenna orthogonal (fancy word for 90 degrees) to a large enough ground plane, we can cut it in half again to ¼ of the wavelength (1 ¼ inches - a pretty nice length. Think of the ground plane as a giant mirror. Of course there are many variants of antennas and they now come in all shapes in sizes the goal of squeezing them into small places or focusing the direction.