Absorption

When electromagnetic waves go through 'something' (some material), they generally get weakened or dampened. How much they lose in power will depend on their frequency and of course the material. Clear window glass is obviously transparent for light, while the glass used in sunglasses filer out quite a share of the light intensity and also the ultraviolet radiation.

Often, an absorption coefficient is used to describe a material's impact on radiation. For microwaves, the two main absorbent materials are:

• Metal. Electrons can move freely in metals, and are readily able to swing and thus absorb the energy of a passing wave.
• Water. Microwaves cause water molecules to jostle around, thus taking away some of the wave's energy.

For the purpose of practical wireless networking, we may well consider metal and water perfect absorbers: we will not be able to go through them (although thin layers of water will let some power pass). They are to microwave what a brick wall is to light. When talking about water, we have to remember that it comes in different forms: rain, fog and mist, low clouds and so forth all will be in the way of radio links. They have a strong influence, and in many circumstances a change in weather can bring a radio link down.

There are other materials that have a more complex effect on radio absorption.

For trees and wood, the amount of absorption depends on how much water they contain. Old dead dry wood is more or less transparent, wet fresh wood will absorb a lot.

Plastics and similar materials generally do not absorb a lot of radio energy-but this varies depending on the frequency and type of material. Before you build a component from plastic (e.g. weather protection for a radio device and its antennas), it is always a good idea to measure and verify that the material does not absorb radio energy around 2.4GHz. One simple method of measuring the absorption of plastic at 2.4GHz is to put a sample in a microwave oven for a couple of minutes. If the plastic heats up, then it absorbs radio energy and should not be used for weatherproofing.

Lastly, let us talk about ourselves: humans (as well as other animals) are largely made out of water. As far as radio networking is concerned, we may well be described as big bags of water, with the same strong absorption. Orienting an office access point in such a way that its signal must pass through many people is a key mistake when building office networks. The same goes for hotspots, cafe installations, libraries, and outdoor installations.

A commonly held myth is that water “resonates” at 2.4GHz, which is why that frequency is used in microwave ovens. Actually, water doesn't appear to have any particular “resonant” frequency. Water spins and jostles around near radio, and will heat when in the presence of high power radio waves at just about any frequency. 2.4GHz is an unlicensed ISM frequency, and so was a good political choice for use in microwave ovens.