Phoenix Contact FL WLAN 24 AP 802-11, Руководство пользователя

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Functions and wireless technology
7190_en_02 PHOENIX CONTACT
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The superposition of waves and their effect on the signal form is known as interference.
As the causes of interference are difficult to determine and often cannot be prevented,
receivers with antenna diversity have been developed to help with this problem.
In this solution, the receiver has two receiving antennas, which are positioned at intervals of
approximately one quarter of the wavelength. In this way, one of the two antennas (almost)
always receives a signal of sufficient quality. In the diagram below, a signal with a high level
reaches receiving antenna 2, whereas antenna 1 does not receive a usable signal.
Figure 1-2 Antenna diversity (diagram)
1.2.2 Attenuation of radio waves
Attenuation is a measurement of the reduction in signal output on a medium. The
attenuation is strongly dependent on the frequency and can only be acquired using
measuring instruments. The unit of attenuation is "dB" (decibel). The lower the dB value, the
lower the attenuation.
The decibel value indicates the logarithmic relationship between two values and this
enables the determination of gain/attenuation along a transmission chain through the
simple addition of individual attenuation/gain values.
The unit dB is therefore a factor and does not provide information about the absolute value.
Absolute values ( levels ) are specified in relation to a fixed reference variable. To indicate
the reference variable that the relevant level refers to, a letter is added to dB for
identification. Common variables are dBm <-> 1 mW, dBµ <-> µV or dBi <-> isotropic
omnidirectional antenna. The level indicator therefore indicates the factor by which a value
is greater than or less than the reference variable. Example for 20 dBm:
PWR POE HOST 1 2 3 4
PWR HOST 1 2 3 4
PWR POE HOST 1 2 3 4
PWR HOST 1 2 3 4
T ra ns m itte r
Si g n al w it h a h i gh l e v e l o f inte rf e r ence
Si g n al w it h
a h i gh l e v e l
Si g n al w it h a
l o w l e v e l
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2
Recei v e r
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