Amateur Extra E4E: Noise and interference
These are the 14 official questions in group E4E of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E4E01Question 1
What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals?
- ARemoval of the CW signal as well as the interfering carrier Correct
- BAny nearby signal passing through the DSP system will overwhelm the desired signal
- CExcessive ringing
- DAll these choices are correct
Why: An automatic notch filter can remove the CW signal along with the carrier.
- E4E02Question 2
Which of the following types of noise can often be reduced by a digital noise reduction?
- ABroadband white noise
- BIgnition noise
- CPower line noise
- DAll these choices are correct Correct
Why: Digital noise reduction can help with many types of noise. All the choices are correct.
- E4E03Question 3
Which of the following types of noise are removed by a noise blanker?
- ABroadband white noise
- BImpulse noise Correct
- CHum and buzz
- DAll these choices are correct
Why: A noise blanker removes impulse noise.
- E4E04Question 4
How can conducted noise from an automobile battery charging system be suppressed?
- ABy installing filter capacitors in series with the alternator leads
- BBy installing a noise suppression resistor and a blocking capacitor at the battery
- CBy installing a high-pass filter in series with the radio’s power lead and a low-pass filter in parallel with the antenna feed line
- DBy installing ferrite chokes on the charging system leads Correct
Why: Ferrite chokes on the charging leads suppress conducted noise.
- E4E05Question 5
What is used to suppress radio frequency interference from a line-driven AC motor?
- AA high-pass filter in series with the motor’s power leads
- BA brute-force AC-line filter in series with the motor’s power leads Correct
- CA bypass capacitor in series with the motor’s field winding
- DA bypass choke in parallel with the motor’s field winding
Why: A brute-force AC-line filter in series with the motor's power leads suppresses its RFI.
- E4E06Question 6
What type of electrical interference can be caused by computer network equipment?
- AA loud AC hum in the audio output of your station’s receiver
- BA clicking noise at intervals of a few seconds
- CThe appearance of unstable modulated or unmodulated signals at specific frequencies Correct
- DA whining-type noise that continually pulses off and on
Why: Network equipment can create unstable signals at specific frequencies.
- E4E07Question 7
Which of the following can cause shielded cables to radiate or receive interference?
- ALow inductance ground connections at both ends of the shield
- BCommon-mode currents on the shield and conductors Correct
- CUse of braided shielding material
- DTying all ground connections to a common point resulting in differential-mode currents in the shield
Why: Common-mode currents make shielded cables radiate or pick up interference.
- E4E08Question 8
What current flows equally on all conductors of an unshielded multiconductor cable?
- ADifferential-mode current
- BCommon-mode current Correct
- CReactive current only
- DMagnetically-coupled current only
Why: Common-mode current flows equally on all conductors.
- E4E09Question 9
What undesirable effect can occur when using a noise blanker?
- AReceived audio in the speech range might have an echo effect
- BThe audio frequency bandwidth of the received signal might be compressed
- CStrong signals may be distorted and appear to cause spurious emissions Correct
- DFM signals can no longer be demodulated
Why: A noise blanker can distort strong signals so they seem to cause spurs.
- E4E10Question 10
Which of the following can create intermittent loud roaring or buzzing AC line interference?
- AArcing contacts in a thermostatically controlled device
- BA defective doorbell or doorbell transformer inside a nearby residence
- CA malfunctioning illuminated advertising display
- DAll these choices are correct Correct
Why: Many power line problems can cause roaring or buzzing. All the choices are correct.
- E4E11Question 11
What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?
- AOne or more of the broadcast stations is transmitting an over-modulated signal
- BNearby corroded metal connections are mixing and reradiating the broadcast signals Correct
- CYou are receiving skywave signals from a distant station
- DYour station receiver IF amplifier stage is overloaded
Why: Corroded metal connections can mix broadcast signals and reradiate them.
- E4E12Question 12
What causes interference received as a series of carriers at regular intervals across a wide frequency range?
- ASwitch-mode power supplies Correct
- BRadar transmitters
- CWireless security camera transmitters
- DElectric fences
Why: Switch-mode power supplies make carriers at regular intervals.
- E4E13Question 13
Where should a station AC surge protector be installed?
- AAt the AC service panel
- BAt an AC outlet
- COn the single point ground panel Correct
- DOn a ground rod outside the station
Why: The AC surge protector goes on the single-point ground panel.
- E4E14Question 14
What is the purpose of a single point ground panel?
- ARemove AC power in case of a short-circuit
- BPrevent common-mode transients in multi-wire systems
- CEliminate air gaps between protected and non-protected circuits
- DEnsure all lightning protectors activate at the same time Correct
Why: A single-point ground panel makes all the lightning protectors activate together.