Contents: Wiring Caveat; Basics; Most Prevalent Cause; Common Connections; Consensus of Opinion;

Vehicle wiring fires are the costliest repairs on the books! Even a brand-new luxury vehicle can end up on the scrap heap as a result of a wiring fire; they're that costly! One major cause is overloaded accessory sockets, or what used to be called cigarette lighter sockets. They were designed originally to be used intermittently, but nowadays just about every mobile devices made is plugged into one. While properly (?) fused, the wiring is undersized (typically #16) for full-time loads. Adding some insult, most plugs made to be used with them, use spring pressure to hold them in. The resulting poor connection can cause overheating, with predictable results.

Yet, MFJ has come up with a device which they claim will safely power a 100 watt transceiver while plugged into an accessory socket. In my personal opinion, this is an incredulous advertising ploy with no merit!

If you use one, or plan to use one of these devices, please keep the aforementioned in mind.

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Basics

Proper installation of antennas, radio gear, and wiring are the key issues for preventing ground loops.

If there were but one bane in amateur radio, it would be the ground loop. Although they may appear in any type of installation, they're more often found in mobile ones. In any case, too many amateurs don't know what a ground loop is, or what causes them to occur. Even rarer, is the ability to recognize that a specific problem is caused by one.

The term Ground Loop in itself isn't very descriptive, but here's a stab at it with credit going to Mr. Webster: An unwanted electric current path in a circuit resulting in stray signals or interference, occurring, e.g., when two earthed points in the same circuit have different potentials. Please pay particular attention the last few words: ...when two earthed points in the same circuit have different potentials. Another way to look at this is, one is a better ground than the other. Or, one has a lower resistance path back to the power source; the battery in this case.

Most vehicle manufacturers, Ford, and GM among them, recommend that both leads (positive and negative) go directly to the battery, and properly fused (both leads). At a minimum, the negative power cable lead should be tied to the same point as the main battery's negative connection to the chassis of the vehicle in question. Here, the negative lead of the power cable needn't to be fused.

However, it should be noted that vehicle battery cables have two standard negative leads. The main one (bigger of the two) goes directly to the starter motor housing, or very near it. The other lead (always smaller) is connected to the chassis of the vehicle. The former is to lessen the losses to the starter motor, the latter to provide a return for the various on-board electrical loads (headlights, accessories, etc.).

Further, I don't want to get caught up in the argument about whether a mobile transceiver's ground lead should or should not be directly connected to the battery terminals, as there are pros and cons to each method. Suffice to say, the negative lead should be collocated close to the battery. However, there is a case to be made about overloading the accessory ground, especially if you're running high power (amplifier use). In this case, direct connection to the battery is your only alternative.

Remember this; If you decide to connect the negative lead to the nearest chassis ground point (seat support, trunk brace, etc.), there will be a difference in resistance (typically three to five ohms) between any of these points, and the battery's chassis ground. This will cause a large voltage drop to occur under load, which is reason enough to follow vehicle manufacturers' recommendations, regardless of the potential for ground loops.

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If I had to choose just one example of a ground loop, it would be the presents of alternator whine. Especially so when it only occurs, or predominantly occurs, in the transmitted signal. In some cases you can mask the whine by using a brute force filter in the positive lead. Besides the additional I²R losses incurred, it's a band-aid rather than a cure. While a bad or leaky diode may cause alternator whine to occur, the truth is, it's a very rare occurrence.

In the whine cases I've been involved with, there are usually three common themes, alone or in combination. First, power is taken from the vehicle's wiring rather than directly from the battery. Two, the negative lead was connected to the nearest chassis ground point (seat frame for example) rather than directly to a battery ground. Three (and most common), the use of a mag mount antenna.

It should be apparent that direct battery connections should be made, at least the positive connection, as using existing vehicle wiring is fraught with problems. This includes the use of Accessory Power sockets as neither the wire size nor the fusing are adequate for any HF radio.

If you're using a mag mount antenna, and you have whine on your transmitted signal, try this. Replace the antenna with a dummy load. If you can still hear the whine on a near-by receiver, the problem is in the DC wiring, or the alternator (albeit doubtful). If you can no longer hear the whine, then ditch the mag mount and drill the necessary hole.

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I have always been in favor of common grounds, and common power supply points. Doing so, lessens the possibilities for ground loops. One way to do this is to use a RigRunner^® or similar common connection device. Just remember, the RigRunner's power cable needs to be fused very close to the battery connection points.

Some amateurs tend to go crazy installing DC grounds on every single piece of installed gear as if God commanded it so. One common practice is to ground the transceiver's chassis to the nearest hard point. If this cured a problem (ground loop or RFI related), then something else in your installation was amiss, and it's usually a poor coax connection or ground plane issue.

It's assumed that most vehicle manufacturers scatter ground connections hither and yon throughout the vehicle. That is not the case. While there are multiple ground connections made, the same ground points are used for any given sub-assembly. For example, there are no separate grounding points for each headlight. Indeed, they're common. Another way to look at this is, if power for more than one device comes from the same supply point (relay, switch, battery, etc.), then the grounds for those devices should also be common. In case you missed the point, this is to lessen any potential for a ground loop (pun intended).

If you cause a potential to appear between two factory ground points (e.g., ground lead to a seat frame, positive lead to the battery), the resulting ground loop may manifest itself as an RFI problem. This is especially important when high currents are involved (amplifier use for example). Split beads, brute force filters, and ground straps ad nauseam, won't cure the apparent RFI simply because RFI is not the cause!

The most important point that can be made in preventing ground loops is to use correct wiring practices. Not only are direct battery connections necessary, adequate sized conductors and fuses are also required. The truth is, most factory supplied mobile transceiver power cables are barely adequate for the load. Extending them just adds insult to injury, unless you're using a RigRunner^® or other device equipped with over-sized cabling.

Tech Talk: If the wiring is inadequate (albeit directly connected to the battery), allowing excessive voltage drop under load, a ground loop can be the result. The reason why this is so, is rather complex, but has to do with factors discussed in the third paragraph of this section. Thus, as mentioned in the wiring article, the voltage drop, under full load, should be kept below .5 volts.

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There is no consensus of opinion whether the negative power lead of a mobile transceiver should be connected directly to the battery negative lead, or to the battery's negative lead chassis connection (or close to it).

In a recent post on eham.net, Tom Rauch, W8JI, answered an Elmers inquiry about this issue, in which he brought up some very important points which should be reviewed before continuing.

One of the issues was the fact that most amateur transceivers do not have floating grounds. A little closer look at the schematics for several Icoms bolsters this fact. It also shows that the grounding is through circuit board traces, albeit rather large ones. As Tom eluded to, placing a power source between the transceiver's negative lead and its chassis could cause internal damage before any protecting fuse opened.

Let's assume we're running an amplifier, and we connected the ground to a chassis point, rather than to the battery's negative post. Whatever current draw there is, has two paths back to the battery. One is through the small lead, the other is through the large lead via the drive train, and whatever grounding straps it may or may not have. This is not an ideal scenario. What's more, the additional current could cause the smaller lead to fail! And, it doesn't address any ground loop problems which may occur as a result.

Ford, and GM both advise direct battery connections, while Chrysler recommends that the ground lead be connected to the same point as the battery's chassis ground. These facts quite obviously cause further confusion among mobile operators.

Personally, I've never had a problem connecting directly to the battery posts in any of my installations. Or at least one that could be traced to doing so. Further, in the majority of the cases, I've used a second, trunk mounted, battery to help handle the peak loads imposed by an amplifier, all but mandating direct connections.

Collocating power and grounds is a good method to minimize ground loops. What's hidden in this statement is the possibility of creating a positive voltage differential, a Power Loop as it were, if you don't collocate them. Here's one example. Most late model vehicles use some sort of MDL (multiple data link) system which allows the various on-board computers to talk to one another. Creating a voltage differential between interconnected systems, whether it be by Ground or Power loop, can cause data corruption. This can cause error codes to be written to the OBDII. Whether this causes the Check Engine light to go on, is moot. There is more information on this, in the Digital Electronics article.

I can't speak for Tom, but if there is a consensus of opinion, it is that adequate wiring be installed, including proper fusing; that intermediate grounding (seat rails, etc.) be avoided; collocating power and ground wherever possible; and that a neat and orderly installation is a necessity.

Remember the old adage: If you didn't find time to do the job right the first time, where are you going to find the time to do it over?

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