I looked at this cool Seattle Wireless page with wonderful pictures of a manufactured directional Yagi antenna and then I did some math and built one as much like it as I could afford. When I was done, I had the basic model for the antenna, but 36" long. Both are similar in construction and materials, and identical in theory. I just scaled down the materials and made a smaller one, and the potato crisp can just happened to be about the right size and it worked out. Another good point to mention, is that for smaller pcmcia powered devices, the smaller antennas worked better.

Simple Alternatives

A simple way to make an antenna is to build a "di-pole". These are simply a piece of wire that with a length that's a even multiple of the wavelength. These can then be cut in the center and attached to a piece of coax which is run to your wireless card via the nifty-but-expensive "pigtails". I have used many di-pole antennas for HAM radio projects. A disadvantage is that they can take up large ammounts of space. On the other hand, they are very easy to make.

Buy one. Yup, if you want to have an antenna and you don't want to build it, you should buy one. I'm sorry, but I won't build one for you. :-) There are many places out there that make antennas for this kind of thing. This is a page for the adventurous person who wants to build an antenna. Buying an FCC approved device is also a very good idea.

Simple Antenna Theory

Waves
Imagine a boat making a wake in a river. As the wake laps up against the poles of a pier, it imparts a vibration of the same frequency to the flat boards in the pier itself. If you put your ear on the pier, you can hear them quite loudly because the pier amplifies the sound at the water level. The frequency is very low, because the length between waves is long. It can be up to a few seconds between waves. This length is called the period of the wave. With wireless, the waves are much shorter, the period is less than a second, much less. It is because of this that we name the wave by it's frequency. For 802.11b, it's called 2.4GHz, because the wave repeats 2,400,000,000 times every second.

Resonance
An antenna is an amplifier in a sense. It is the antenna that catches the signal from the air, much like a sail of a ship catches the wind. Any piece of metal will serve as an antenna, and the bigger the better if you use the sail/wind analogy. But regardless of size, some work much better than others. Why is this? Resonance. Some bits of metal are the same size in one dimension as the radio wave it's catching. This causes the wave to be felt stronger by the metal, resulting in the a stronger electrically induced current. It actually works well if you are some even multiples and fractions of the wave's size. Quater wave length segments are very common and useful.

Frequency Bands
We need to know the electrical characteristics of our signal in order to understand an antenna. These are the wave's frequency, wavelength and amplitude. The frequency is inversely related to the wavelength. 802.11b uses several frequencies that are close together. The range as a whole is called a band. Because they are all close to 2.4GHz, it is generally referred to as the 2.4GHz band. It is the begining of a band of frequencies on up to 2.4835GHz. A useful fact here is that one antenna can and does serve for more than one frequency, but generally the ones in the middle will work better than the ones on the ends. You should design your antenna around the middle of the band you are working with so that it will work more efficiently.

Light Speed Ahead
The frequency is the number of times your carrier signal cycles or repeats in one second. The wavelength is the distance your signal goes in one cycle. To calculate wavelength for 802.11b, we must know the speed of the signal. Electromagnetic waves, our radio waves, travel at the speed of light. That means the velocity is about 3.0 x 10^8 meters per second. It's a constant. This means that distance it goes per second will always be the same. One can imagine a sine wave flying through the air. This picture shows one cycle of a sine wave. No matter how long the cycle of the wave, it always goes the same speed, and therefore the same distance in one second as any other electro-magnetic wave. Now that we know the velocity , the frequency and the band or our signal, as well as a little bit about resonance, we are ready to do some calulations.

 Yagi 14 dbi

 Wajan 25 cm (in Indonesia)

 Now, I have 14 dbi + 7 dbi...yyuummiiiii....

My Daughter "Debian" with Yagi Wajan (Yajan)

see u next subject.....