Last Modified: August 23, 2014

Contents: Intro; About the Noises; Ignition Noise; Fuel Injector Noise; Fuel Pump Noise; Electric Motor Noise; Alternator Noise; Combinations; Electric Trailer Brakes; Conclusion;


The toughest obstacle a mobile amateur radio operator has to face is attenuating the various undesired noises emanating from our transceiver's speaker. By noise, I mean all of the various forms of RFI, EMP, EPI, and AFI, whether it be induced, inducted, or received. I've collectively called them noise, because that's the manifestation of the interference, no matter the true cause. Remember too, there is no, one, single cure-all for reducing it!

The first place to start is my 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. There are two more points which need to be brought up. First, is common mode current. 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. While a noise blanker might take care of most of the pulse RFI, it also degrades receiver performance. What's more, common mode current is the main cause of RFI ingress to the various on-board electronic devices.

Here are some additional resources: Noise Abatement; Another Look at Noise Abatement, and Beads. If you've read these articles, you know that reducing received noise is a multi-front war which requires a coordinated attack. One of the more important aspects of the war is identifying the enemy, and that is what this article is about.


About the Noises

Olympus Digital RecorderThis article contains several sound files which were recorded on an Olympus® digital recorder. They were transferred into Sound Studio® where the recorder's clicks and beeps were edited out. The files were re-sampled at 11.025 kHz to reduce download time, and saved as AIFF (Audio Interchange File Format) and thus compatible with Macs and PCs. I did my best to record these at a high level, but it may be necessary for you to turn up the output audio on your computer. The sound files open in a new window where you can play them as many times as you wish.

As mentioned above, I'm using the term noise in a very general way. The actual noise we hear can be audio spectrum noise, or a result of RFI, EMI, EPI, and AFI or a combination of any or all. This obviously makes identification difficult. So what follows is a small sample of what you might encounter in any one specific vehicle, but certainly not all. While I'm on the subject of vehicles here's an advisory or two.

First, two otherwise identical vehicles may have vastly different noise levels. If I knew why this was so, I'd be on my way to being rich! Secondly, there are some vehicles which just can't be made quiet, at least not as quiet as you, and I would like. This includes the latest common rail diesel designs, as some of those are far worse than any spark engine. In general, however, if you have to use your noise blanker you haven't done enough.

It should be mentioned that amateur-grade FM transceivers are very susceptible to amplitude modulated RFI (ignition pulses), due in part to their wide-band receive capabilities. Improper wiring and antenna mounting often exacerbate the issue. As with HF installations, common mode chokes, and hood and exhaust bonding, are often necessary to reduce the RFI to an acceptable level.


Ignition Noise

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 mutes ones as well. The point being, don't assume a fix didn't work just because you're still hearing pulsed RFI.

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.

With the exception of the Gen III push rod engines in GM products, and a few older legacy models, all automobile manufacturers 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 left photo shows what they look like. Note the split bead over the control cabling. On all Ford products which use COP technology (and other too I suspect), there is an internal ferrite bead which may (emphasis on may) make the external ones unnecessary.

On the forefront is yet another gasoline engine technology which combines a combustion chamber fuel injector, the spark plug (igniter as it is now called), a coil pack, knock sensor, and electronic controls all in one package. Some models are already equipped with them with more to come. While this new technology answers the mandated pollution requirements, it appears to make our noise abatement ever more difficult. For example, the fuel rail pressure on some models is almost 1,500 psi! This high pressure requires higher current injectors, just like their electronic diesel counterparts.

Here's a bit of trivia. A few years back, a company named Esslinger Engineering designed and built an electronic valve actuation system. It used rather large electromagnetic coils to both open and close the valves rather than use a cam shaft. In theory this would allow minute control over valve lift and duration; an engine design engineer's utopia. They actually had several systems installed on Ford's ubiquitous 2.3L 4 cylinder engine. Although it worked well, one of the stumbling blocks was the huge amount of RFI generated by the system.


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, thus vary precise in metering fuel. 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. The major difference is one of fuel pressure, as some systems operate as high as 8,000 psi! The injector coils are larger, and so is the EMF pulse. This injector sound file was sent to me by Jan Frederik, PE1PQF. You can clearly hear the annoyance factor. By the way, the level changes you hear are during up and down shifts.

The biggest problem associated with injector noise is one of abatement. Except for some 4 cylinder models, the harnesses are typically under or near enough to other hardware, that the use of split beads is a lost cause. Although most of the harnesses have some form of drain wires, they're never adequate. A covering of 3M, 1181 copper foil tape does help, but we're still faced with the difficulty of getting to the wiring in the first place.


Fuel Pump Noise

Another common noise is caused by electronic fuel pumps. Most are mounted inside the fuel tank, but a few are mounted on the frame rail. Some high performance vehicles have both kinds, and some trucks have two or more. The one you're about to hear is a tank mounted one. You'll notice a slight delay after the ignition is turned on, then you hear the pump charging, and then a rhythmic pulse as the pump maintains fuel rail pressure. Fuel pump Noise In this case the engine was not running. Had it been, 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. In this case, it was a 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.

Just for the record; the fuel pump noise issue in Ford products has long since (circa 2003) been addressed. 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.

Typically, 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.

By the way, some ULE (Ultra Low Emission) vehicles with GDI (Gasoline Direct Injection), have fuel rail pressures exceeding 2,000 pounds! Diesels may be as high as 6,000 pounds. Working on these systems is not for the faint of heart!

Word Of CautionIn some cases, RFI from electronic devices, including fuel pumps, can be best cured by capacitively bypassing the leads to ground, and across one another, rather than use ferrite beads. However, caution should be exercised! This technique should never be used to bypass speaker systems. Good wiring practices must be followed, including the use of heat shrink tubing, and the avoidance of any type of tape material, not matter what it is! Remember! Any capacitor, ceramic ones in this case, can fail short. If the circuitry is not properly fused, the results could be catastrophic as evidenced by the photo above right! Use bypass caps as a last resort, and only if you're technically competent to install them correctly.


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. I did this because the hash you hear is very similar to atmospheric noise leading some folks to believe it is. Electric Motor 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 I listed in this paragraph, 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.

Alternator whine is not as prevalent as it once was due in part to better designs, and much better diode arrays. Getting dealers to repair units under warranty is difficult at best. If the alternator puts out the correct voltage and current, their obligation is over. Replacing the diode pack or the complete alternator is not a guaranteed cure as some replacements, new and rebuilt, are just as bad if not worse than the original.

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.

QuicktifierHybrid vehicle present a new level of sophistication especially with respect to alternators. A lot of hybrid units replace the alternator and the starter motor with just one big unit. Although my brother did well operating mobile in his Toyota Prius, the jury is still out on the majority of these new systems. We'd better get used to them, however, as they are the vehicles of the future even for trucks and SUVs of all types.

If you just have-to-have the best in external diode arrays, visit Alternator Parts. They sell a device called Quicktifier®, as shown in the photo. While not adaptable to every alternator, it allows your alternator to achieve full output, under extreme conditions.



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. I fought one noise problem in my Acura for nearly a year before I found and corrected the problem. I was still plagued with an RFI ingress problem with the traction control system on 17 meters, up until I traded the vehicle for a new one. In short, it often 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!


Electric Trailer Brakes

One source that resurfaces from time to time is the pulse noise generated by electric trailer brakes. Dexter, Brandon Fuller, Hayes, and others offer electric trailer brakes. The complexity varies with the model. Some are simply all manual, but the better units are computer controlled. They use a variety of sensors, and/or interconnections to the towing vehicle's braking system, both mechanical and electrical.

Top of the line units use a pulse width modulation to keep the trailer brakes from locking up. These system, work a lot like a normal automatic braking system (ABS). Because they utilize an electric solenoid, the rapid on-off sequencing generates hash high into the RF spectrum.

If there is no trailer attached, some systems continue to operate. When you step on the brake, the hash lasts for about 2 seconds until the system applies full continuous power to the wiring (not pulsed in other words). While this shouldn't occur when there is no trailer attached, the systems are often wired or designed incorrectly and operate regardless.

When there is a trailer attached, the hash is continuous anytime the brakes are applied until you come to a full stop. Split beads do help, but they must be attached as close to the brake solenoid as possible and this can present mounting problems. Well placed capacitors (.01 uF are adequate) may be a better choice, but this requires splicing and may void some warranties.

I've been told that most late model units have built in RFI suppression, however, data from my e-mail in-box would indicate the factory systems are lacking in the suppression department, regardless of brand.

I don't have a sample of the hash, but it sounds a lot like background noise we all put up with, only much louder. If you have a recorded sample, I'd be delighted to have it with due credit applied.



If you have a similar or unique problem with RFI noise, I'd like to know about it so I can post it. If we all combine our efforts, perhaps we can all benefit from it.

I mentioned contacting your dealer when you're plagued with RFI egress from on-board devices, an I highly encourage you to do so. If enough owners complain, perhaps the manufacturer will remedy the problem. While vehicles are not specifically designed with amateur radio operators in mind, RFI egress effects every facet of the communications industry, a fact manufacturers can't ignore.

Lastly, transportation vehicles, whatever they are, are exempt under Part 15. So don't try to use it as a means of defence.