It is not at all uncommon in the antenna world to deal with absorption and reflection. Though both absorption and reflection can keep power from reaching its desired destination, the two are actually quite different.
Reflection
Reflection of RF power is similar to the reflection of light with a mirror; when light or RF energy hits a reflective surface, it glancea off. RF-reflective surfaces are conductive surfaces. The amount of reflection is determined by the conductivity — in general more conductive surfaces reflect better. This is why encasing an antenna in a metal box tends to prevent RF energy from escaping or entering — the RF simply glances off the metal and does not pass through.
Sometimes, reflectors are not necessarily solid sheets of metal. A microwave oven, for instance, uses a perforated metal screen in the window of the door. The holes in the screen allow the user to see in, but, at the same time, keeps the high-power RF energy employed in cooking the food contained within the microwave. The trick behind a perforated reflector is that the holes are much smaller than the wavelength of the RF being reflected; a good rule of thumb is to use a hole size one tenth of the wavelength or smaller. When this criterion is met, the RF bounces off of the perforated surface as if it were a solid one.
Absorption
Absorption occurs when a surface attenuates the RF that interacts with it as opposed to the energy merely glancing off. This absorbed energy is converted into heat, rather like when a resistor is used in conventional DC circuitry to drop voltage.
Carbon is commonly used in a resistors, and, as it happens, carbon is also an excellent absorber of RF energy. For this reason, commercial RF absorber typically comes in the form of carbon-impregnated foam.
Though carbon ranks as one of the most effective absorbers of RF energy, many other materials absorb RF as well. This can become a particular problem in PC board antenna designs, for many inexpensive PC boards also consume a notable amount of RF energy, especially at high frequencies. Often, small amounts of carbon in the materials can cause RF absorption.
Water can be good at absorbing RF. Water absorbing RF energy becomes a particular problem at Bluetooth frequencies, as these frequencies happen to be very close to the inherent resonant frequency of water. This is why small personal devices can be a challenge to design, because the human body largely consists of water, with, incidentally, a healthy amount of carbon as well.
Coping with Absorption and Reflection
The easiest way to cope with reflection is to keep metal objects away from any and all antennas. RF absorption due to the materials associated with the antenna and its feed is mostly a problem at higher frequencies, and is often due to inexpensive PC boards. Unfortunately, it may become necessary to purchase more expensive PC boards, especially as cheaper PC boards have low quality control, which can also alter the tuning of the antenna.
There are only limited ways to cope with external absorbing influences. Positioning the antenna where it will be the least likely to be interfered with by external influences helps. However, in the end, the best results will be had by increasing power where possible, and making the antenna more efficient so as to utilize the available power most effectively.
As a whole, consider where RF needs to travel for the antenna to be effective in its application. In this path, you want neither reflective nor absorbing materials.
ARSI 