# Forum

## Wavelength and antenna size

1 posts by 1 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: April 29, 2009:
• It is hard for me to explain it without a whiteboard (I love whiteboards), however, I will give it a try.

Mike -- You probably know much of what I have in my response, but I wrote it for completeness so sorry for the rambling.

Radio frequency is power over the air. Yep, electric. Just like power in our homes, it has a frequency. Now, RF communication systems like Wi-Fi are in the 2.4 and 5 GHz frequencies.

Wavelength is computed with a simple math problem with one constant and one variable; the speed of light and frequency.

The analogy I use in class is this: If you were counting how many cars past a point per minute, that is frequency. Now, imagine that every car will be going a constant speed (70 MPH). At 5:00 a.m. you count 6 cars per minute. Now, at 8:00 a.m. you count 60 cars per minute. Ok, the speed is constant (like light speed) but the frequency has gone up. What else changed? The distance between the cars. Higher frequency with the same speed equals a smaller (shorter) wavelength.

So, on to antennas. In order for a radio to receive data, it must have points of reference. In any modulation type, AM, FM, or PM, the only way to receive data is to be able to sense change.

If you were to look at a 360 degree wave (this is where I need my whiteboard), at what points in that wave would you want to "sample" the wave? Well, if you did it on the beginning and end (0 degrees) that would tell you how the wave started, but would you know what happened between the beginning and end of the wave? Nope! So, we have to sample at more points. We at least should sample the beginning and the middle. Hey, even better, we should sample the beginning, 1/4, 1/2, and 3/4 of the wave. This way, we sense change in the power. If we just look every 360 degrees we don't get a good picture of what happened. So, we need to look at every 90 degrees (1/4 wavelength) in order to properly receive data.

Now, when we build an antenna, if we build it on the 1/4 wavelength design, we design it with a certain number of wavelengths. An antenna is designed with a certain number of wavelengths in mind. A 5 wavelength antenna at 5 GHz will be much shorter than a 5 wavelength antenna at 94.1 MHz.

I've rambled enough, but I'll leave you with a fun fact. The Navy's ELF (extremely low frequency) antenna operates at 76 Hz and has an antenna system that is 28 miles in length. :)

GT

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