Attenuation
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Attenuation
- As RF signals travel through space and matter, they lose signal strength (attenuate).
- Frequency and wavelength properties of an RF signal do not cause attenuation.
- Distance is the main cause of attenuation.
- All antennas have an effective area for receiving power known as the aperture.
- The amount of RF energy that can be captured by the aperture of an antenna is smaller with higher frequency antennas.
- As a signal travels through our atmosphere, the signal will attenuate to amplitudes below the receive sensitivity threshold of a receiving radio. Essentially, the signal will arrive at the receiver, but it will be too weak to be detected.
- In Figure 2.5, you see a comparison of a single cycle of the two waves generated by different frequency WLAN radios. Figure 2.5 2.45 GHz wavelength and 5.775 GHz wavelength.
The length of a 2.45 GHz wave is about 4.8 inches, or 12 centimeters. The length of a 5.775 GHz wave is a distance of only about 2 inches, or 5 centimeters.
Higher frequency signals will generally attenuate faster than lower frequency signals as they pass through various physical mediums such as brick walls. This is important for a wireless engineer to know for two reasons.
Higher frequency signals will generally attenuate faster than lower frequency signals as they pass through various physical mediums such as brick walls. This is important for a wireless engineer to know for two reasons.
- First, the coverage distance is dependent on the attenuation through the air (referred to as free space path loss or FSPL).
- Second, the higher the frequency, typically the less the signal will penetrate through obstructions.
For example, a 2.4 GHz signal will pass through walls, windows, and doors with greater amplitude than a 5 GHz signal. - Think of how much farther you can receive an AM station’s signal (lower frequency) versus an FM station’s signal (higher frequency).
Wavelengths of the different frequency signals are different because, although each signal cycles only one time, the waves travel dissimilar distances. In Figure 2.6, you see the formulas for calculating wavelength distance in either inches or centimeters.
- All radio devices have what is known as a receive sensitivity level. The radio receiver can properly interpret and receive a signal down to a certain fixed amplitude threshold.
- If a radio receives a signal above its amplitude threshold, the signal is powerful enough for the radio to sense and interpret the signal.
- If the amplitude of a received signal is below the radio’s receive sensitivity threshold, the radio can no longer properly sense and interpret the signal.
- The signal must be louder than any background noise.
- In addition to the radio being able to receive and interpret a signal, the received signal must be not only strong enough to be heard but also strong enough to be heard above any RF background noise, typically referred to as the noise floor.
- You must make sure that the RF signal will not attenuate below the receive sensitivity level of your WLAN radio simply because of free space path loss, and you must make sure that the signal does not attenuate near or below the noise floor.
Attenuators
- It may be necessary to decrease the amount of signal that is radiating from the antenna.
- In some instances, even the lowest power setting of the transceiver may generate more signal than you want. In this situation, you can add a fixed-loss or a variable-loss attenuator.
- Attenuators are typically small devices about the size of a C-cell battery, with cable connectors on both sides.
- Attenuators absorb energy, decreasing the signal as it travels through.
- Fixed-loss attenuators: Provide a set amount of dB loss.
- Variable-loss attenuators: have a dial or switch configuration on them that enables you to adjust the amount of energy that is absorbed.
- They are often used during outdoor site surveys to simulate loss caused by various grades of cabling and different cable lengths.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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