Antenna theory as it applies to Apple Airport wireless systems

This white paper should help Apple users understand and increase the performance of their Airport wireless systems. It touches on the design and implementation of low-power RF radiating antenna systems. There is a lot of confusion on how to install a Wi-Fi system optimally. Here are facts, theory and recommendations.

Wireless Issues

1. Physical Location

Mount your wireless Airport Base Station and/or antenna as high as possible. Try mounting the Base Station on the wall, vertically aimed at the intended wireless use area, especially if you are working in a location with multiple floors. Mount the Base Station away from metal surfaces that will reflect RF energy. Also keep it away from any objects that will absorb the RF energy. Keep as few walls and human bodies from coming between your computer and the Base station as possible. The human body absorbs a lot of energy, just like food in the microwave. Mount Base Stations away from heating ducts and metal studs because they block and reflect RF signals, just like radar. When that happens, those signals confuse the receivers, and some of the system’s ability to transmit and receive data is lost.

2. Receiver and Transmitter Sensitivity

It doesn’t do any good to have a lot of RF power, if the energy gets diverted, absorbed or blocked. The key is that for the wireless system to work well, it takes RECEIVER SENSITIVETY and TRANSMITTER POWER. People who buy wireless cards based just on transmitter power are deluding themselves.

3. Diversity Antennas

As you may know, Apple has elected to put diversity antennas in all computers with wireless capability. This means there is one antenna on each side of the PowerBook. The reasoning is to allow the antenna system to pick the antenna that is getting the strongest signal in different locations. In theory (working inside plastic computers) this works well.

4. Antenna Gain (dbi)

Antenna gain is also very misunderstood. Antenna gain does not mean that the antenna actually amplifies signals coming in or going out. All it means is the gain measured against the isotropic base line sphere shaped antenna. That is what the “i” means after the db. In English that means a perfect sphere equals 0Dbi, it will transmit and receive signals equally in all directions. The dbi number will go up as the sphere is flattened into a donut shape.

5. Directional and Omni Antennas

Even what is called omni directional antenna has to be aimed as many of these types of antennas have a built-in downward tilt.

6. Radiation Patterns

There are basically two issues at work here: horizontal and vertical polarization of the radiation pattern. You should try to mount both the Base Station and the PowerBook antenna to match their respective polarizations. Think about it this way, if your Base Station is located on a different floor than where you use the computer, then maybe you need to use a directional antenna, pointed toward your usage area. But if you want to use the computer upstairs from the Base Station, maybe you should use a horizontal polarized Base Station antenna. The top of the radiation donut should be pointed at upstairs rooms [your usage area]. The 0 dbi antenna radiation pattern looks like a sphere with the antenna in the middle. The 5dbi antenna takes that same sphere and squashes it into a donut, with the antenna in the middle.

The more gain an omni antenna has, the flatter the radiation donut will be.

7. Common Mistakes

Not making sure the RF connectors are correctly seated.

8. Unheralded Solutions

Making sure the Antenna is polarized correctly.
Get a directional antenna and point it correctly.

We always hear this question: “How much increased distance will I get by adding an antenna?” This question’s answer is never as easy as it seems. Our first question back is, “What is the physical layout of your usage area?” Before we can tell you which antenna type to consider, we need to know if you have vertical or horizontal usage area considerations.

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