CWS CWS Basic RF Principles 5 — Questions and Answers
Question 1: In the 2.4 GHz band (802.11b/g/n), how many non-overlapping 20 MHz channels are available in the US?
- 3 (Correct answer)
- 11
- 14
- 6
Correct answer: 3
In the US, the 2.4 GHz band has only 3 non-overlapping 20 MHz channels: 1, 6, and 11.
Question 2: What is the primary advantage of OFDM (Orthogonal Frequency Division Multiplexing) over single-carrier modulation?
- Robustness against multipath interference due to long symbol duration (Correct answer)
- Lower peak-to-average power ratio
- Simpler receiver design
- Better performance at very low SNR
Correct answer: Robustness against multipath interference due to long symbol duration
OFDM uses many narrow subcarriers with long symbol durations, making it far less susceptible to multipath-induced intersymbol interference.
Question 3: Which unit is used to express absolute RF power levels referenced to 1 milliwatt?
- dBm (Correct answer)
- dBi
- dBd
- dB
Correct answer: dBm
dBm expresses power relative to 1 mW; 0 dBm = 1 mW, positive values are greater than 1 mW, negative values are less.
Question 4: A signal loses 6 dB passing through a wall. If the original signal is 20 dBm, what is the power after the wall?
- 14 dBm (Correct answer)
- 26 dBm
- 10 dBm
- 3.2 dBm
Correct answer: 14 dBm
Power after the wall = 20 dBm − 6 dB = 14 dBm; dB losses are subtracted directly in the log domain.
Question 5: Co-channel interference (CCI) in a wireless LAN is best reduced by:
- Assigning non-overlapping channels to adjacent access points (Correct answer)
- Increasing transmit power on all access points
- Using a higher-frequency band with more channels
- Reducing antenna gain on all access points
Correct answer: Assigning non-overlapping channels to adjacent access points
Using non-overlapping channels on neighboring APs eliminates the co-channel interference caused by APs sharing the same frequency.
Question 6: What is the effect of increasing antenna cable length on an RF link?
- Increases insertion loss, reducing effective transmit power (Correct answer)
- Increases antenna gain to compensate for the longer run
- Has no effect if the cable is rated for RF use
- Increases EIRP by boosting signal strength
Correct answer: Increases insertion loss, reducing effective transmit power
Longer cables have more resistive and dielectric losses, reducing the power delivered from the radio to the antenna.
Question 7: Which RF phenomenon is primarily responsible for indoor 'dead spots' where signal strength drops sharply despite short distances from the AP?
- Multipath destructive interference (Correct answer)
- Free-space path loss
- Atmospheric absorption
- Antenna polarization mismatch
Correct answer: Multipath destructive interference
Indoor dead spots are typically caused by multipath signals arriving out of phase and canceling each other through destructive interference.
CWS CWS Basic RF Principles 5
In the 2.4 GHz band (802.11b/g/n), how many non-overlapping 20 MHz channels are available in the US?