Weird & Wireless: What’s up with Antennas?

cc-antenna.jpgWelcome again to the wonderful but sometimes weird world of wireless comms, written by Joel Young, CTO of Digi International

What’s up with Antennas? Some are short, some are long, some are round, some are straight – what gives?

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 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 2½ 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.

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(Picture  – Antenna by vivianejl, under Creative Commons Attribution Licence)


joel-young-2009-150x150.jpgJoel Young, VP of Research and Development and CTO at Digi International, has more than 22 years of experience in developing and managing data and voice communications. He joined Digi International in June 2000 and in his current role he is responsible for research and development of all of Digi’s core products.

Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.

He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.

Tags: wavelength

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