Contents: Basics; Is It RFI, Or A Ground Loop?; Sound Systems; Navigation Systems; Persistent Problems; Vehicles From Hades; Don't Give Up!;

Basics

Most solutions to common RFI problems are contained in other articles on this web site. However, there are a few esoteric problems which should be covered. They may effect receive performance (egressed RFI), or effect transmit performance (ingressed RFI). Depending on the type of RFI, the reader should read some of the other articles covering the subject. Those articles include Noise ID, Ignition & Injectors, Digital Electronics, and Ground Loops.

Tech Talk: It is assumed that the antenna system has been properly installed, and that the motor leads are properly choked. If this is not the case, any RFI problems you encounter will be very difficult to cure, if you can cure them at all. Information about the requisite chokes, are in the Antenna Controllers and How To Wind A Choke articles.

As noted above, there are two (main) forms of RFI; those that interfere with you we'll call egressed RFI, and those you interfere with we'll call ingressed RFI. In some cases, both types can be solved with just one solution. For example, proper bonding of the exhaust system will certainly reduce egressed RFI caused by the ignition system, and in some cases will cure engine performance issues caused by ingressed RFI to the engine control computer.

Besides bonding, there are other devices which can reduce, and/or eliminate RFI. Undoubtedly, the biggest gun in the arsenal is the lowly split bead, but not just any split bead. As pointed out in the article, split beads come in different mixes. The proper mix is determined by the application, and in a mobile scenario the correct mix is 31. They're ideal because their usable frequency range is from 1 MHz to well over 500 MHz. At about $3 to $5 each, they're inexpensive as well. DX Engineering is a good source for mix 31 beads.

When you place a split bead over a wire, it looks electrically like a resistor in series with an inductor. Like any inductor, the amount of inductance is to the square of the number of turns. For example, if we're using a split bead to control common mode current, six turns through one bead is equivalent to one turn through 36 beads! From a cost standpoint, it is obvious which is better!

One problem often encountered when using split beads on factory vehicle wiring, is a lack of slack to wind more than one turn. In most cases, factory wiring connectors are available from either the vehicle manufacturer, or through after market auto supply stores. This allows you to make up multi-turn jumpers, which can be removed thus returning the vehicle to factory specs without a trace.

There are also a few things that don't work very well for RFI problems, yet enjoy a large audience. One of those is the brute force filter. They're often advertised as a cure for alternator whine, common mode current, as well as RFI imposed on the power wiring; an exceedingly rare occurrence. For alternator whine, they're a patch at best, and if you wire your installation correctly, you won't need one otherwise. One thing brute force filters will do, is increase voltage drop. For no other reason, they should be shunned. The rule of thumb is, if they cure a problem, then something else in the installation is (was) amiss!

There is one suppression technique you need to be very careful with, and that's capacitive bypassing. If you choose this method, take adequate care to avoid shorts, as a vehicle fire is the most expensive repair you can name. Remember too, some circuits cannot be capacitive bypassed. Sound systems come to mind, as doing so can cause an audio power amplifier to oscillate with predictable results. Use beads instead.

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Is It RFI, Or A Ground Loop?

By definition, a ground loop is a voltage differential between any two points. It can occur between any two power supply points, or between any two grounding points. If you create a voltage differential between two points, and there happens to be a vehicle power or ground connection between these two points, there is a good likelihood you'll create a ground loop.

As pointed out in the article, ground loops manifest themselves as an RFI problem. Almost without exception, ground loops are caused by incorrect wiring practices. Because their severity is affected by the amount of current drawn, they're typically seen (or heard) only when transmitting. However, they can effect receive too, especially when a mag mount antenna is used.

The best cure is not to cause them in the first place by wiring directly to the battery as manufacturers recommend. They can still occur even when gear is wired correctly. Quite often, it's the over-zealous use of ground straps. For example, unlike commercial radio equipment, the power cable ground lead in amateur radio equipment is usually connected directly to the chassis. From a purely technical standpoint, no other grounding is necessary. Yet, manufacturers often recommend grounding the chassis to prevent electrical shocks (?), TVI, BCI, and other problems (from Icoms' owners manuals). As pointed out in the ground loop article, if grounding the chassis cured or reduced and RFI problem, then something else in the installation is (was) amiss!

Tech Talk: All mobile installations will have some level of common mode current flowing on the coax. The level is directly proportional to the amount of additional ground loss over and above the vehicle-to-surface (capacitive coupling) losses inherent in every mobile installation. Depending on the level, grounding the chassis might indeed solve the problem. However, it is better to choke the common mode current, at the antenna, with an appropriately placed split bead. This keep the RF outside the vehicle where it belongs. Therefore, the choke must be mounted outside as well, not in the trunk or interior of the vehicle.

There is nothing wrong with grounding the chassis, but as alluded to above, don't do it to cure an RFI problem. In other words, fix the problem, don't patch it!

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Sound Systems

Nowadays, even inexpensive (if you could call them that) vehicles come factory-equipped with premium sound systems. Upgraded models often have Bluetooth^®, Sync^®, MP3^®, and cellphones built in. As a result, most audio power amplifiers are wired hot. That is to say, they're on anytime the ignition is on, and in some cases when it's not. Adding some insult, the wiring for these devices are spread throughout the vehicle to speakers, antennas, and interconnections. Finding the entry point for RF is both tedious and frustrating.

As pointed out above, capacitive bypassing of the various components isn't recommended, which leaves split beads as the only alternative, and a good one at that. However, using them may require adding length to the wiring harnesses. While time consuming, it's usually worth the effort. It's best to apply the beads as close to the offended device as possible. For example, placing beads at the speaker ends of the wiring harness don't work as well as installing them close to the power amplifier. This is also true of auxiliary inputs (Sync^®, MP3^®, etc.). Remember too, it's best to use multiple turns on one bead than one turn on multiple beads.

What was said above about ground loops, is doubly true about sound system RFI. Proper wiring, common mode chokes, and proper antenna mounting are the best prevention measures, and will out-weigh any cure you could ever apply.

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Navigation Systems

Based on current trends, by 2012 or so, every vehicle sold in America will have a navigation system (Navi) as standard equipment. For the most part, the requisite antenna is placed at the rear of the vehicle's roof. This puts them in close proximity to our mobile antennas. Thankfully, very little RFI is encountered if the rest of the installation was done correctly. This said, if you're using a video adapter to display your transceiver's digital readout, you may indeed encounter an RFI problem.

Currently, the only mobile transceiver with video out, is the Icom IC-7000, but this is bound to change. In the case of the IC-7000, the video is composite, and the standard is either NTSC or PAL depending on the model. This output can then be fed directly to the Navi (some models), or to an adapter like the TVandNav2Go^®. Because the video level is just one volt PEP, very little RF is needed to disrupt the display.

If an adapter is used, it's best to power it from the same source the Navi is powered from to prevent a ground loop. Higher end units (≈$300USD) come equipped with the correct harness, but most lower end units (≈$150USD) don't. The latter requires great care in wiring, and proper fusing is a must! For most folks, a RigRunner^® is a better alternative, however the power cable to the adapter should be beaded. Further, the video cable from the transceiver to the Navi or adapter should be beaded at both ends! Four or five turns through a mix 31 split bead is usually enough.

As a side light, the TVandNav2Go has two video inputs. If your Navi isn't equipped with a backup camera, this is a good alternative input for one. Or, you could install a camera to watch your antenna while you retune it. The camera shown right is a Soundstorm^® unit, and is available from a variety of on-line retailers for as little as $25USD.

One last item. Most jurisdictions preclude watching live video while under way, no matter the source, due to the distraction they cause. That's why they're relegated to the back seat area. Obviously, common sense should be applied to any video display scenario.

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Persistent Problems

No matter how hard you try, there are some RFI issues which defy suppression. One of those is vehicle data buss systems. Honda for example, uses at least three in most of their vehicles. These data busses carry information between the various on-board computer systems. Since they're digital, their signals tend to be rich in harmonics, which often extend well into the HF spectrum. Because they operate on different frequencies, they easily mix resulting in birdies every few kHz. They're usually not much more than a nuisance, as their interference level is typically low. However, the better you RFI-proof your vehicle, and the poorer the band conditions, the worse the nuisance is due mainly to S+N/N ratio issues.

Since these data busses are running throughout the vehicle, split beads are of little help, especially if you don't know which on-board device generates the buss in the first place. This is where a Service Manual is of great help. Even then, it is difficult at best to reduce, much less eliminate, the birdies. The only recourse is to use the built in DSP most late-model amateur transceivers feature.

One often overlooked source of RFI is LED lighting; an almost ubiquitous device. The "D" in LED stands for diode, and diodes do a great job of detecting RF. When they do, the resulting voltage is fed back into the vehicle's wiring. This usually doesn't cause problems, but it can. Remembering that antennas are mounted on the rear, and most CHMSLs (Center High Mounted Stop Light) are LED, enough voltage can be imposed to cause an RFI issue. Here's a suggestion which might give you a hint. Have someone watch the various LED lights (CHMSL especially), for any flickering while you talk. It's best to do this at night, so even a small nuance can be seen. If it is seen, using a split bead installed on the harness as close to the offending device as possible will usually suffice. This won't stop the LEDs from flickering, but any resulting RFI will be squelched.

There's one problem which might be assumed to be caused by RFI, and that's engine hunting (stumbling, uneven idling, misfires, etc.). While RFI can indeed cause it, varying loads on the alternator can also be a cause especially if you run high power. One way to tell is to listen to the alternator when someone else is on the radio. If the mechanical whine from the alternator varies with the load, you're discovered the cause. If it doesn't change, it's probably RFI ingress to the control systems. In any case, it's always best to bead the alternator's control harness. The DC power out cable doesn't need to be beaded due to it's very low impedance.

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Vehicles From Hades

Some vehicles just can't be quieted down, no matter what you do. Even vehicles which are otherwise identical, can have different levels of RFI egress. They're sort of the lemons of the amateur mobile world. If someone ever figures out why this is so, they'll become over-night millionaires!

Hybrids, especially those from Toyota, are some of the RFI noisiest vehicles money can buy (as are plug-in vehicles). There are just too many digital signals to deal with, along with all of those of a standard vehicle when the engine is running. John Pavelich, VE3XKD, has spent untold hours of labor, and a wad of hard-earned cash getting his Prius quiet enough for HF work. Few amateurs possess John's, seemingly unending patients, to take on such a formidable task!

Hybrids and plug-ins aren't the only ones either. Some of the newer common-rail diesel engines emit enough RFI to drown out the best of signals. Listen to this one! Yes, you could use split beads on the offending injectors, if you could get to them! In most cases, they're hidden under the intake manifold and/or under sound insulating covers. This negates the use of split beads, even if you could get to the harness to apply them. About the only recourse is 3M 1181 copper foil tape, and many hours of toil to apply it.

If possible, before buying any vehicle, it's always best to take along a portable AM radio. Tune off a station, and listen to the background noise level. Then start the vehicle. If the background level comes way up, check other similar vehicles on the dealer's lot to see what they do. While subjective, you'll at least have a feel for what you're up against. Fact is, ones as bad as the MP3 example above, can be clearly heard.

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Don't Give Up!

Just about any vehicle can be quieted down if you have enough time, patients, and funds for the beads etc. Here's a few tips about going about the task.

Install your radio gear, antenna, wiring, bonding etc. first, following the recommendations herein. Once you're done, then (and only then) assess what extra work needs to be done. If you just throw the radio in the vehicle, and stick on the antenna like it was a piece of duck tape, you're going to get a false impression of the real level of RFI.

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