Noise is often a real problem for radio amateurs. Fortunately, by understanding how noise is generated and how to reduce or eliminate it, noise can be tamed.
Atmospheric noise is naturally-occurring noise that appears across a wide bandwidth. To help you copy signals when there’s a lot of atmospheric noise, or any wide bandwidth noise, you can often use a receiver’s noise blanker. One undesirable effect that can occur when using a receiver’s IF noise blanker is that nearby signals may appear to be excessively wide even if they meet emission standards.
QUESTION: Which of the following signals might a receiver noise blanker be able to remove from desired signals? (E4E03)
ANSWER: Signals that appear across a wide bandwidth
QUESTION: What undesirable effect can occur when using an IF noise blanker? (E4E09)
ANSWER: Nearby signals may appear to be excessively wide even if they meet emission standards
Many modern receivers now use digital signal processing (DSP) filters to eliminate noise. The types of receiver noise that can often be reduced with a DSP noise filter, include broadband white noise, ignition noise, and power line noise.
QUESTION: Which of the following types of noise can often be reduced with a digital signal processing noise filter? (E4E02)
ANSWER: All these choices are correct
- Broadband white noise
- Ignition noise
- Power line noise
Some receivers with DSP filter have an automatic notch filter, or ANF. An automatic notch filter recognizes interfering signals in the passband and attempts to set the notch to filter out that signal. When receiving CW signals, however, the filter may remove the CW signal as well as the interfering carrier.
QUESTION: What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals? (E4E01)
ANSWER: Removal of the CW signal as well as the interfering carrier
While filters can be very effective at reducing noise, it is often better to figure out what is generating the noise and take steps to reduce or eliminate the amount of noise generated in the first place. Loud roaring or buzzing AC line interference that comes and goes at intervals could be caused by arcing contacts in a thermostatically controlled device, a defective doorbell or doorbell transformer inside a nearby residence, or a malfunctioning illuminated advertising display. To determine if the AC line noise interference is being generated within your home turn off the AC power line main circuit breaker and listening on a battery operated radio to see if the noise goes away. If by doing this you determine that an electric motor is a problem, noise from an electric motor can be suppressed by installing a bypass capacitor across the motor leads.
QUESTION: What might be the cause of a loud roaring or buzzing AC line interference that comes and goes at intervals? (E4E10)
ANSWER: All these choices are correct
- Arcing contacts in a thermostatically controlled device
- A defective doorbell or doorbell transformer inside a nearby residence
- A malfunctioning illuminated advertising display
QUESTION: How can radio frequency interference from an AC motor be suppressed? (E4E05)
ANSWER: By installing a bypass capacitor in series with the motor leads
Sometimes your own equipment may be the cause of received noise. Cables in an amateur radio station, for example, can radiate or pick up interference. Common mode currents are the culprits. Common mode current is current that flows equally on all conductors of an unshielded multi-conductor cable. Common mode current on the shield and conductors can can also cause shielded cables to radiate or receive interference. To eliminate this interference, make sure to ground the shield at one end of the cable.
QUESTION: Which of the following can cause shielded cables to radiate or receive interference? (E4E07)
ANSWER: Common-mode currents on the shield and conductors
QUESTION: What current flows equally on all conductors of an unshielded multi-conductor cable? (E4E08)
ANSWER: Common-mode current
The main source of noise in an automobile is the alternator. Conducted and radiated noise caused by an automobile alternator can be suppressed by connecting the radio’s power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads. The capacitors help filter out the noise, and short, direct leads to the battery help prevent noise pickup.
QUESTION: How can conducted and radiated noise caused by an automobile alternator be suppressed? (E4E04)
ANSWER: By connecting the radio’s power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads
Personal computers and other digital devices can also generate noise. One type of electrical interference that might be caused by the operation of a nearby personal computer is the appearance of unstable modulated or unmodulated signals at specific frequencies.
QUESTION: What is one type of electrical interference that might be caused by a nearby personal computer? (E4E06)
ANSWER: The appearance of unstable modulated or unmodulated signals at specific frequencies
Noise can even be generated by the most unlikely things. For example, if you are hearing local AM broadcast band signals on one or more of the MF or HF ham bands it’s possible that nearby corroded metal joints are mixing and re-radiating the broadcast signals.
QUESTION: What could cause local AM broadcast band signals to combine to generate spurious signals in the MF or HF bands? (E4E11)
ANSWER: Nearby corroded metal joints are mixing and re-radiating the broadcast signals
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