Last Modified: January 23, 2015

Contents: Intro; Ignition Noise; Fuel Injector Noise; Fuel Pump Noise; Electric Motor Noise; Alternator Noise; Combinations;


The toughest obstacle a mobile amateur radio operator has to face is attenuating the various undesired noises emanating from our speakers. Collectively, the noises may be caused by any or all of the various forms of RFI, EMP, EPI, and AFI, whether it be induced, inducted, or received. While a noise blanker might take care of most of the pulse RFI, it also degrades receiver performance. And, no matter the cause or source, there is no single cure-all for reducing any of them.

The first place to start is the Wiring article. If you don't take the necessary time to wire your installation correctly, finding and curing a noise problem becomes much more difficult. For example, ground loops are the primary cause of alternator whine, not the alternator itself!

Bonding is the second most important item in minimizing any form of noise. For example, the primary cause of excessive ignition noise is failure to properly bond the exhaust system. And one strap is never enough, and sometimes multiple and excessive length grounds are worse than no ground at all!

Another major consideration is proper Antenna Mounting. Excessive ground losses reduce the strength of the received signal, which in turn reduces the signal to noise ratio. Although more commonly associated with FM radios, SINAD (20 log [signal + noise + distortion/noise + distortion]) is the measure of how well you're receiving an incoming signal. As you can see from the formula, too much noise and/or the resulting distortion, and you can't hear well enough to carry on a conversation.

Common mode current can also be a major bane. Improper mounting, and other causes of excessive ground loss, increase the likelihood of common mode current. If the RF can get out, it can get in as well. In other words, the coax acts like part of the antenna! Since coax is routed inside the vehicle, which is almost as noisy as it is under the hood, steps must be taken to assure a low-level of common mode.

Common mode current is also the main cause of RFI ingress to the various on-board electronic devices. Flashing dash lights; hums, squawks, and squeaks from the AM/FM radio; and engine misfires are all manifestations of common mode current.


Ignition Noise

The most prevalent RFI we have to deal with is Ignition Noise. The biggest problem is, it can sound similar to injector noise or shuttle noise in a diesel. In some cases, the noise pulses from both the injectors and ignition systems interact and the noise appears more of a hash than what you hear in the sound file. Nonetheless, if you listen carefully you'll actually hear the two pulses which are very close together. The lighter pulses are from the injectors, and the heavy pulses are from the ignition. This is a very good example of a compound noise which makes identification and abatement difficult at best.

It is important to note, that ignition RFI can be so intrusive, that it can cover up a multitude of other, very similar sounding, pulse noise sources. About the time you think you've got the problem solved, you start hearing the muted ones as well. The point being, don't assume a fix didn't work just because you're still hearing pulsed RFI.

Most all automobile manufacturers (except GM) have abandoned the use of plug wires all together, and switched to COP (Coil Over Plug) technology. This technology combines the coil, plug, and sometimes the switching circuitry all into one neat package. The right photo shows what they look like. It should be noted, that shielding COP units, or the wiring to them, may cause more issues than it solves.

Lastly, high-intensity, discharge lighting can cause RFI which can mimic ignition RFI, especially after-market add-on ones.


Fuel Injector Noise

njectorsAll modern gasoline engines use electronically-controlled fuel Injectors. These injectors are all tied to what's known as a common (fuel) rail. The injectors themselves, are very close in design to the pressure balanced systems used in sprinkler value units, but much faster in operation. Like COP units, they contain a coil. When the field around the coil collapses, a back EMF pulse is generated. It is this high rise time pulse, rich in harmonics, which we hear as RFI.

Modern common rail diesels use similar systems to meter their fuel. This injector sound file is typical of the RFI generated by a diesel. The level changes you hear are during up and down shifts. The level of the RFI is somewhat more evasive than most gasoline engines, but sounds very similar.


Fuel Pump Noise

Another common noise is caused by electronic fuel pumps. Typically, there is a slight delay after the ignition is turned on, before you hear the pump charging the fuel rail, and then a rhythmic pulse as the pump maintains fuel rail pressure. In this recording the engine was not running. Had it been running the pulses would be much closer together, and may in fact sound like a whine.

Just recently, Jim Hilt, N8NSN, send me a movie of his fuel pump noise generated by his 2003 Jeep, and the noise is clearly a continuous whine. There are so many different kinds of pumps being used these days, it's difficult to access if indeed the fuel pump is the culprit.

The fuel pump noise issue in Ford and Chrysler products have long since been addressed (circa 2003). If you own a late-model vehicle, Ford or otherwise, and you're experiencing an RFI problem you think might be the fuel pump, here's some enlightenment. Almost without exception, all late-model vehicles use some sort of data busing between the various on-board CPUs. The usual RFI is a series of evenly-spaced birdies, some of which may be pulsing. These buses operate whenever there is engine oil pressure, as does the fuel pump. In the case of the fuel system, during the brief time it takes the oil pressure to drop, the fuel pump continues to operate to facilitate purging the vapor canister. Thus, the RFI from the mixing of the data bus' frequencies (pulsing or otherwise), coincides with the operation of the fuel pump. It is therefore easy to make an incorrect assumption about where the RFI is coming from.


Electric Motor Noise

Every modern vehicle on the road has at least five or six electric motors, and some with as many as 50! Engine cooling fans, AC fans, IAC (Idle Air Control) motors, power seats and window motors, some cruise controls, remote controlled mirrors, power door locks, and the list is almost endless. When any of them operate they create hash, spikes, and/or pops in our received audio. Most of them sound similar albeit some are steady like AC fans, and some are intermittent like IAC motors. The noise you're about to hear is an AC fan which is being turned on and off to make it easier to identify, because the hash you hear is very similar to atmospheric noise leading some folks to believe it is. Electric Motor sound file. You can imagine the difficulty of identifying just one motor when several are running at the same time. The ones you really have to worry about are the first three listed above, and the throttle position motor if your vehicle is drive-by-wire equipped.

There is a trend in the industry to switch over to brushless DC motors. Believe it or not, this is being done to reduce RFI (it is about time something is being done at the manufacturing level!), and increase mechanical life. In some applications, linear actuators are replacing rotating motors for the same reasons. Let's hope our perceived level of vehicle noise goes down, not up!

Many vehicles control the speed of the HVAC blower (and sometimes the cooling fans too) by using PWM (pulse width modulation). The devices themselves are usually RF quiet, but may cause RFI when defective. The sound they emit is broad-spectrum, and siren-like, varying with the fan's speed. They can also cause an AF noise to be generated in the fan motor itself. The fix is to replace the fan control module.


Alternator Noise

The following information assumes there is indeed a bad diode array, and that the noise is not caused by a mag mount-induced ground-loop. It also assumes that proper maintenance of all battery connections have been performed.

This Alternator Noise was recorded on a second receiver as the whine was impressed on the transmit signal, as it was not evident on the received signal. This is a common occurrence as most transceiver finals are hooked directly to the incoming power, where as the receiver usually has some DC supply filtering. This is not always the case, however. If you listen carefully you'll hear the IAC motor pulsing in the background.

Some alternators pulse the stator current similar to a switching power supply, while the others (usually older models) use an analog method like a linear power supply. Similar to a switched bench supply, the frequency and/or the pulse width may be varied (independently or together). The RFI from these models sounds like the old cartoon rat-a-tat-tat used for machine gun sounds. Although it may sweep across the bandpass, that is not always the case. If you're plagued with this particular noise, the alternator is a good place to start looking. Unfortunately, I don't have a recorded example of this form of RFI. If you do, I'd like to hear from you.



The above examples are by no means the only noises present in modern vehicles, nor are they isolated as they are here. Fact is, most of the time you can hear several at once. Combination This is why it is so important to do the obvious bonding and beading first before trying to fix the remaining maladies.

There is one key word every reader should take to heart - perseverance. After all, it always takes time to run down an RFI source. Sometimes it takes a lot of work to abate it, and once in a while you can't abate it no matter how hard you try!