4-1 Read/Write Antenna and ID Tag
4-2
(4) Influence of external objects
• Radio wave absorbers: water, human body, water screen, water-absorptive material, etc.
A radio wave (microwave) penetrates anything (solid body and liquid) other than metal, but it is attenuated while penetrating. In partic-
ular, water absorbs a radio wave extremely. When a radio wave penetrates the water, the radio wave is absorbed considerably. Also, the
radio wave is attenuated remarkably in a human body which contains much water. So, any solid body and liquid must not exist between
the antenna and tag.
A general-purpose plastic plate with thickness of or glass plate a few millimeters does not absorb the radio wave, and the radio wave
attenuation is not a serious problem in this case. However, the radio wave attenuation varies depending on a type of material and/or
thickness of external objects which the radio wave penetrates. Execute the communication experiment in a working site in advance.
When the communication is performed through the plastic plate or glass plate which absorbs the radio wave so much, such plastic plate
or glass plate may be covered with water due to rain. The radio wave may be attenuated by this water screen and the communication
may fail. Execute the communication experiment in a working site in advance and take great care not to get out of the communication
range during an operation.
Dry wood and paper do not attenuate the radio wave so much. However, wood and paper absorb water easily. The wet wood and paper
may attenuate the radio wave considerably. Execute the communication experiment in a working site in advance using both of dry
materials and wet ones.
• Radio wave reflectors: metal, ground, etc.
Metal reflects a radio wave (microwave) like a mirror reflects light. If there is a metal near an antenna communication area, the commu-
nication area is affected by the metal. If a metal is put between an antenna and tag, the communication between the antenna and tag may
fail. Metal, whether metal plate or wire netting, may affect the communication. Also, the ground affects the communication like metal.
As shown below, a radio wave absorber or reflector can be used to interrupt a radio wave. When you interrupt the radio wave, execute
the communication experiment in a working site in advance.
Example of radio wave absorber: ECCOSORB AN75 (61 x 61 cm, E&C Engineering)
Radio wave transmitted
from an antenna
A part of radio wave
is reflected.
Object
Absorbed in an object and attenuated.
Tag
Radio wave transmitted
from a tag.
Outgoing
Incoming
Antenna
Communication
area
Tag that you want to
process data
Tag that you do not want to process data
Radio wave interrupt