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Difficulty: MediumTroubleshooting Wireless Connectivity and Signal Issues

A network technician is troubleshooting a wireless point-to-point bridge between two campus buildings situated 250 meters apart. Following a storm, maintenance crews re-anchored the directional antenna on Building B. Although there is a completely clear line of sight and transmit power settings remain unchanged on both units, the received signal strength indicator (RSSI) has dropped significantly from -58 dBm to -80 dBm. Inspection reveals that the antenna on Building A remains oriented with vertical polarization, whereas the antenna on Building B was reinstalled turned 90 degrees in horizontal polarization. Which of the following is the primary cause of the link degradation?

  1. Antenna polarization mismatch between the transmitter and receiverAnswer
  2. B
    Co-channel interference from overlapping 2.4 GHz channels on neighboring access points
  3. C
    Subnet gateway misconfiguration on the wireless bridge interface
  4. D
    Automatic Private IP Addressing (APIPA) fallback on the bridge wireless management card

Answer

Antenna polarization mismatch between the transmitter and receiver is the primary cause of the severe RSSI drop.
Wireless signals travel as electromagnetic waves oriented in a specific spatial plane (polarization). For maximum energy transfer, directional bridge antennas must share the same polarization alignment. A 90-degree alignment difference between vertical and horizontal polarization introduces cross-polarization discrimination loss, typically causing a 20 dB to 30 dB drop in received signal power.

Step-by-Step Solution

1
Analyze the physical and RF symptoms reported in the scenario.
The visual line of sight is clear, distance is unchanged, and transmit power settings are identical, yet RSSI dropped by 22 dB after physical reinstallation.
This isolates the problem to a physical radio antenna installation parameter rather than path distance, attenuation obstacles, or radio transmit power.
2
Evaluate the orientation difference between the two directional antennas.
Building A utilizes vertical polarization while Building B's antenna was rotated 90 degrees to horizontal polarization.
Radio waves propagate with electric field vectors aligned to the antenna orientation. Mismatched polarization planes between transmitter and receiver cause significant signal loss (typically 20 dB or more).
3
Select the correct root cause matching the physical findings.
Antenna polarization mismatch correctly accounts for the dramatic drop in RSSI.
Aligning both antennas back to the same spatial polarization plane (both vertical or both horizontal) will restore full RSSI.

Key Concept

Antenna Polarization and Alignment in Wireless Systems
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Topics

Troubleshooting Wireless Connectivity and Signal Issues

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