T4B05 Ignition Interference

T4B05 from the Technician License Course Section 12.0, Avoiding Interference:
Which of the following would reduce ignition interference to a receiver?
A. Change frequency slightly
B. Decrease the squelch setting
C. Turn on the noise blanker
D. Use the RIT control

Receiver interference is sometimes a doggedly stubborn problem with mobile stations. The RF environment of an automobile is often just nasty, with lots of different undesired emitters. Control computers, fan motors, and other devices in a car can be sources of RF noise that gets onto your receive audio. But the ignition system is notorious for the popping, clicking, and whirring sounds it can impose on your receiver. Sparking plugs or coil packs, a faulty alternator or distributor, and other ignition system sources are sometimes difficult to track down and resolve.

There is a brute force method of suppressing the regular, repeating clicks and whines typical of ignition system noise. Let’s examine the response options:

A. Changing frequency slightly will do nothing for you but make it difficult or impossible to understand other operators that are on the desired frequency. Ignition noise is often quite broadband, meaning that it will be comprised of a large range of RF frequencies. So, even if you change frequency a bit it will likely still be present.

B. Decreasing the squelch setting is likely to allow the audition of more noise rather than delete it. The squelch control acts to determine the strength of a signal that is required to unmute the audio and produce audible sound. Even if there happens to be a difference in signal strength that you can leverage to delete the ignition noise most of the time when stronger, desirable signals are not being received, the noise will still manifest when a stronger signal (such as from another calling station) is received. You haven’t achieved much this way.

D. Using the RIT control (receiver incremental tuning) will allow you to adjust your receive frequency without changing your transmit frequency, but it does nothing to mute the ignition noise.

C. Ah… the noise blanker. As the name implies, this control on a receiver activates a special notch filter that will detect and “notch out” regular, repeating spikes of noise such as that from an automobile ignition system. In effect, the filter “activates” on-and-off rapidly, attenuating only during the brief pulses of ignition noise that are imposed on the received signal.

Pulsing noise on a modulated signal may be attenuated with application of the noise blanker.

Pulsing noise on a modulated signal may be attenuated with application of the noise blanker. [Click to enlarge]

While use of the noise blanker is convenient and easy, it can have some (usually) mild deleterious effects on received audio. The noise blanker is a filter, so its notch attenuates the receiver, including attenuation of the desired receive signals during the brief activations. Additionally, due to effectively “mixing” a roughly square wave notch into the signal, some mild harmonic noise can actually be generated by the noise blanker itself, thereby generally decreasing the signal-to-noise ratio and reducing receiver sensitivity.

Various noise blankers use different methods of determining what actually constitutes the noise in a received signal. As such, different noise blankers can be “confused” in different noise conditions and may not always be completely effective in attenuating the annoying sounds.

All that stated, the most efficient policy is usually “just try it.” If the noise blanker does the job, you’ll know it.

The answer to Technician Class question T4B05, “Which of the following would reduce ignition interference to a receiver?” is “C. Turn on the noise blanker.”

Related Questions: T4A10, T4A11, T4A12