CWS CWS Basic RF Principles 3 — Questions and Answers
Question 1: What is the SNR of a signal received at -65 dBm with a noise floor of -95 dBm?
- 30 dB (Correct answer)
- 65 dB
- 95 dB
- 160 dB
Correct answer: 30 dB
SNR = Signal level − Noise floor = −65 dBm − (−95 dBm) = 30 dB.
Question 2: Which multipath effect causes rapid, short-term fluctuations in received signal strength as a mobile client moves?
- Fast fading (Rayleigh fading) (Correct answer)
- Slow fading (shadowing)
- Absorption
- Reflection
Correct answer: Fast fading (Rayleigh fading)
Fast fading (Rayleigh fading) results from constructive and destructive interference of multipath components as the receiver moves even small distances.
Question 3: A cable with 3 dB of loss cuts transmitted power by approximately:
- Half (Correct answer)
- One-quarter
- One-tenth
- Two-thirds
Correct answer: Half
3 dB of loss corresponds to a power ratio of 2, so 3 dB loss means half the power is lost.
Question 4: Which 802.11 band is generally more susceptible to interference from microwave ovens?
- 2.4 GHz (Correct answer)
- 5 GHz
- 6 GHz
- 60 GHz
Correct answer: 2.4 GHz
Microwave ovens operate near 2.45 GHz and directly overlap the 2.4 GHz Wi-Fi band.
Question 5: What is the primary purpose of antenna diversity in wireless systems?
- To mitigate multipath fading by selecting or combining signals from multiple antennas (Correct answer)
- To increase transmit power above regulatory limits
- To extend the operating frequency range of a radio
- To reduce co-channel interference between APs
Correct answer: To mitigate multipath fading by selecting or combining signals from multiple antennas
Antenna diversity uses multiple antennas to combat multipath fading by selecting the best signal or combining signals.
Question 6: In the Friis transmission equation, doubling the distance between transmitter and receiver causes received power to:
- Decrease by 6 dB (Correct answer)
- Decrease by 3 dB
- Decrease by 12 dB
- Remain unchanged
Correct answer: Decrease by 6 dB
Free-space path loss follows an inverse-square law, so doubling distance quadruples path loss, which is a 6 dB decrease in received power.
Question 7: Which modulation scheme encodes data by shifting the phase of the carrier signal?
- PSK (Phase-Shift Keying) (Correct answer)
- ASK (Amplitude-Shift Keying)
- FSK (Frequency-Shift Keying)
- OOK (On-Off Keying)
Correct answer: PSK (Phase-Shift Keying)
PSK encodes information by varying the phase of the carrier wave rather than its amplitude or frequency.
CWS CWS Basic RF Principles 3
What is the SNR of a signal received at -65 dBm with a noise floor of -95 dBm?