So…. this is as good a place as any to discuss this concept. In many professional applications, the power harness for the radio has a long positive lead that is intended to go to the battery from anywhere in the vehicle (radios are often installed in trunks, right?) and a shorter return lead with a ring terminal that might only be 1 – 2 feet long. this lead often gets taken to ground either through a radio mounting screw, a self drilling screw into chassis metal, or a convenient bolt or ground stud.
Many amateurs, on the other hand, insist on running both positive and negative power leads direct to the battery (and appropriately fusing both). Why? Does it matter?
Let’s start with one important concept. What is the difference between GROUND and your RETURN? is your battery negative lead a “ground lead”?
No. it’s not. Strictly speaking, GROUND is a VOLTAGE reference. that being said, YES, your power system ties the cathode (negative terminal) of the battery. This is done for a variety of reasons. One of the biggest reasons is that there is a large amount of equipment (including engine components) that will use the GROUND as a return path to the electrical energy source (battery or alternator). This does NOT mean that your radio’s black power lead is a ground lead. it is not intended for the same purpose. Within your radio, the radio’s CHASSIS is a ground, and should be tied to any RF, analog, and digital grounds within the radio at a single point (“Single Point Ground”… something I might delve into at some other time).
Now, all that said… USUALLY the power return lead is tied to chassis within the radio as part of the single point ground. For that matter, it’s entirely possible to connect the power high lead, and have the radio use chassis to bracket to vehicle chassis as a return path. it’s far from ideal, but it DOES work.
What’s the risk in that, you ask? well, depending on the design of the radio, the risk varies. Undoubtedly, using such an arrangement, you’re sending current across the vehicle chassis in such an arrangement. Since other equipment is likewise putting current on the chassis, and much of it is digital in nature, there is a high likelihood that your radio will “see” some of the noise from other equipment on it’s power input (between chassis and the high lead, remember, our return lead is disconnected). This noise COULD cause unintentional modulation of amplifier circuits or oscillators within the radio (depending on radio design, quality of power filtering, etc.). Moreover, such an installation could very well bypass any powerline filtering on the input that is a design OTHER than a bulk capacitor to chassis on the power high lead, which is not a good design, to say the least.
This is where power return lines become important. Your radio DOES include power filtering (both common mode and differential mode should be considered) at the input to the radio. It might not be much, but even the cheapest CB should have something to try and cut down some alternator whine – though it’s rarely sufficient to completely eliminate it when powering the radio from the fuse panel.
Now… back to my rambling. I might discuss some of that stuff again….
Well, I’ve seen both methods in aviation (local return versus remote return), and I’ve put both into service for radio installs. Realistically, it depends on your application, again.
For WELL DESIGNED FM equipment, like commercial grade equipment (Motorola), a little noise on the power doesn’t matter.
AM equipment (like CBs, or SSB equipment) is often more susceptible to noise on the powerlines. While the noise is not directly imposed on the powerlines themselves, consider the ground path back to the battery (the power source). The body panels and chassis all must be well bonded together. there WILL be resistances in those bonds, and any other equipment using chassis as a return has the potential to inject noise along the path, which will look like noise currents on the chassis. This noise current will become a voltage across any resistances (like a bad bond between a panel and the chassis), and will manifest itself as apparent ripple voltage at the power terminals of your load equipment
Again…. for many types of equipment, these millivolts of ripple won’t make a lick of difference. For some more sensitive AM and SSB equipment, this noise will directly impact the radio, which is using chassis or power return as a reference to measure the received voltages coming in from the antenna. Variations and ripple on the ground will LOOK like ripple on the receive line, and manifest itself as conducted interference.
Now… on to my installation this time….
I opted to connect my power distribution panel power return to a convenient chassis ground connection under the dash. From that point, I ran individual return leads with each power high lead to the various pieces of equipment. I tested out my CB, and just taking the power high lead to the battery eliminated alternator noise on the radio during acceleration. Should I see any other issues, I can easily bring a power return lead from the battery into the cabin, and go to a “remote return” configuration. I think that a newer vehicle is less likely to exhibit floating body panels than some older vehicles (and, I’m using internal structural components, which are far from using a fender). In this case, I employed an older Motorola wiring harness (8 gauge high and return, terminated in a 2 pin polarized connector). The return lead was only ~24″ long and terminated to a ring terminal, which i tied under an existing bolt, and have shown good results thus far, regarding performance.
Keep in mind that before I use anything as a ground or return, I have been measuring with a Fluke meter to see what the resistance is between that component and some known connection to chassis or battery. While I would love to follow the MIL-STD-464 guideline of 2.5 milliohms for each faying surface, my meter doesn’t have that resolution… but I think it has enough resolution to show a GOOD ground connection versus “well, it sortof connects to ground”. All that said, there is often discussion on what constitutes a GOOD ground. 2.5 milliohms goes back to a “good enough” measurement from the 1960s, but doesn’t have a scientific or engineering basis behind it, aside from what was attainable under good conditions. I won’t solve that here except to say <0.1 – 0.5 ohms on my fluke meter is probably good enough….
This is another case where your equipment and purpose defines what you need to use for your methods. Generally, I’m using FM and DMR (digital), so I don’t need to take my return back to the battery, and my noise is low enough that my CB isn’t affected. If i was going to use more SSB, or if i see issues in the future, I might consider running a direct run to the battery on the return line.