A network technician is setting up a new point-to-point wireless bridge between two buildings. After configuring the access points (APs) at both ends, the link establishes but the signal strength is much lower than expected, and throughput is poor despite a clear line of sight. Both APs are using directional antennas. Which of the following is the MOST likely cause of the issue?
- AIncorrect channel selection, leading to co-channel interference.
- BDynamic Frequency Selection (DFS) interference from radar signals.
- CAntenna polarization mismatch between the two APs.
- DExcessive cable loss between the APs and their respective antennas.
Show answer & explanationAnswer & explanation
Correct answer: C. Antenna polarization mismatch between the two APs.
With a clear line of sight and directional antennas, a significantly lower-than-expected signal strength and poor throughput strongly suggest a fundamental issue with how the antennas are aligned or configured. Polarization mismatch, where one antenna is oriented vertically and the other horizontally, can lead to a significant signal loss (typically 20dB or more) even with perfect aiming, as the radio waves are not aligned to be received efficiently.
Why the other options are wrong
- A. While incorrect channel selection can cause interference and lower throughput, the primary symptom of 'much lower than expected signal strength' for a point-to-point link with clear line of sight points to a more fundamental physical layer issue.
- B. DFS interference would cause temporary link drops or channel changes, not a consistently low signal strength and poor throughput once the link is established on a channel.
- D. Excessive cable loss would certainly reduce signal strength, but typically a technician would check cable lengths and types during installation. While possible, polarization mismatch is a more specific and potent cause for a 'much lower than expected' signal despite otherwise good setup.
Antenna Polarization Mismatch
Occurs when two communicating antennas have different polarization orientations (e.g., one vertical, one horizontal), causing significant signal loss and poor link performance.
- Can cause 20dB or more signal loss.
- Critical for point-to-point links with directional antennas.
- Requires physical adjustment of one or both antennas.
Memory trick: When the signal's weak, and the path is clear, check antenna alignment, far and near, especially the polarization, my dear!