Mobile Radio Install DC Power Myths

Discussion in 'Mobile Radio Systems' started by KF5LJW, Sep 12, 2019.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    This is a real pet peeve of of subject for me because if done incorrectly can not only compromise operation, but very dangerous. I see the question constantly here on the forum, while some advice is good, I see a lot of bad advice again and again. So lets bust some myths and get down how and why to properly terminate your power cables on all mobile radio installs.

    First up is the Black Negative aka Ground wire from your radio. This is the one conductor that can get you into trouble real fast, and most radio manufactures do not help and even give horrible instructions. They make it possible for you to mess up royally. You open up the box and find the DC Power Cables, unroll them, and you have two equal lengths of Red and Black wire with In-line Fuse for both Red Positive and Black Negative wires. You will even see web sites telling you how to improperly use the hardware that goes something like this:

    "Route both positive and negative to engine compartment and terminate both conductors to the respective polarities directly on the battery Term Post".

    DO NOT and I repeat DO NOT terminate the Negative Black Wire to the Battery Term post or use a fuse on the negative wire. Here is why and once you understand might slap yourself for not understanding and doing it wrong. Even worse advising others to do that. You are putting your Radio in parallel with the Bonding Straps in the engine compartment. Pop the hood and look. From the Battery negative term post you will see one, maybe two Bonding Jumper wires. One going to the Chassis, and the second a larger jumper to the engine block for the starter motor. You create a parallel path if you connect the negative directly to the negative battery term post. The path is through your black radio wire to the radio chassis of your radio and completed through the coax shield to where ever you installed the antenna like the roof or bumper.

    Do you know why that is a horrible practice yet? Think about it. It means a portion of everything electrical in your car is now using your radio as a conductor back to battery. Turn on the head lights, some of that current is going through your radio. You name it air conditioning, car computer, electronics, all of it. Even some of the starting engine current is going through your radio. That alone gives you operational headaches. What do you call an unwanted current or voltage? Did you say noise? That is just what you did, added a bunch a noise. You created a bad Ground Loop. There are good loops but that topic is another discussion.

    Even a bigger problem scenario is set up called the Dooms Day Scenario. Your local shade tree mechanic or you worked on your car and disconnected the Bonding Jumper from the battery to chassis. Now he/you goes and tries to start the engine and one of two things happen. Your radio and radio wiring emit magic smoke. Or if you are lucky the fuse you put on the Negative Conductor of your radio to the battery term post blows and saves your radio. Your radio is now the only return path for the vehicle. You have now compounded a problem in the installation making it very unsafe on top of operational issues under normal operating conditions.

    All this can be avoided by following best practice, electrical code, and what all auto manufactures recommend. Bond the Black Negative wire from your radio to the chassis as close as possible to the radio. Keep the wire as short as possible. Now your radio is on its own Dedicated Circuit and cannot share load current with anything else as it is no longer in Parallel. So now when your ignorant mechanic disconnects the Bonding Jumper under the hood and forgets to reconnect it, nothing happens because nothing has power. Your radio is no longer in parallel that can provide an unintended parallel path.That is a bad Ground Loop.

    DO NOT install a fuse on the Negative circuit some radio manufactures supply and many here on the forum advocate using. That is for ignorant people who terminate directly to the battery term post. Your vehicle electrical system is a Grounded System and thus the Grounded Circuit Conductor ( black radio wire), shall be solidly bonded with no disconnect devices inserted. All a fuse and extra wire does is add unnecessary resistance causing higher voltage losses and is DANGEROUS. You never put a switch or fuse on a grounded circuit conductor. You never want to disconnect the ground without disconnecting the battery un-grounded circuit conductor first. Otherwise you loose your protection and turn your Black Radio wire into Red Hot Conductor with full supply voltage available looking for ground.

    As for the Red Positive conductor is pretty simple. Take it to the battery Term Post and use an in-line fuse as close as possible to the battery. In fact they make Fuse Blocks that mount directly to the battery term post leaving no unprotected wire . That completes the Dedicated Circuit and prevents Gremlins from getting inside your radio.
    K8AI, W9WQA, KF5KWO and 10 others like this.
  2. N4MU

    N4MU Ham Member QRZ Page

    LJW: Thanks for taking the time to put everything in layman terms. I'm sure many will appreciate it. I am just now in the process of wiring exactly as you have stated. I will be watching this thread for additional tips and/or disputes (inevitable). I certainly hope that by posting this on Friday the 13th will not cancel the logic!
  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    You are welcome, hope it helps you and others as well.

    Kind of hard dispute, although not a law of physics, like Ohm's you really cannot dispute it. I, or I should say we learned to hard way some 39 years ago. When I came out of college the first time before going back to get my Masters degree, I worked for a large electric utility communication department with a very large fleet of vehicles equipped with Low Band LMR. All the sudden over a period one week we got a call from the garage telling us we had a couple of vehicles with burned up wiring which is not all uncommon and happens once in a while.

    Now what was unusual the wires that burned up was the Negative Battery Return and Coax, not the Positive, thus the fuse never operated on positive. We got a hold of Ford and Motorola and both basically asked us where we terminated the Negative wire. At the time we terminated directly on the battery term post not knowing any better. Well both Ford and Motorola laughed at us and basically said hey stupid you wired the radio in parallel with the battery bonding strap to chassis and told us to terminate to the chassis. What happened if you have not figured it out yet when the mechanic cranked the engine, all that starting current flowed through the radio Negative and Coax Shield and burned them up with a few hundred amps flowing.

    What had happened was the Dooms Day Scenario two idiots working on vehicles. First idiot at bat was the radio technician who wired and terminated Negative to the Battery Term Post. Next moron up was the mechanic who for whatever reason temporarily removed the Bonding Jumper between battery negative term post and chassis and forgot to re-terminate. So when idiot mechanic got in the car to start it, did not notice any electrical problem because radio, lights, and ignition worked normally until he engaged the starter. Smoke and fire filled the cabin inside the truck. Oops!

    Not only did it eliminate the potential of burning up the radio Negative wiring, it also greatly improved Receive Audio and higher TX Wattage due to lower voltage loss. When you use the battery term post negative, you are forcing a portion of all the vehicle load current to flow through the radio because it is a parallel circuit to the Battery Bonding Jumper. Alternator whine, ignition popping and crackling in RX audio all went away. As for TX power, it went up because when you use the chassis as return, you incur less voltage drop because the DC resistance path is much lower using the chassis vs wire to the battery.

    As for the Radio Positive, take it straight to the Battery Positive Term Post. Combined with using the chassis gives you a dedicated circuit that does not share normal load current of the vehicle electrical system. Just like at home in the shack, you want a dedicated circuit for power with nothing down or upstream sharing vehicle load current.

    What really frost my pumpkin is some radio manufactures make it possible to screw up royally, and pretenders that show up on forums pushing their agenda and alien blog sites giving bad dangerous advice. How many of you bought a mobile radio that is supplied with a long black negative lead wire with a in-line fuse for both negative and positive? They do that for ignorant people who terminate directly to the battery negative term post, and like to live dangerously with a lot of noise and power loss.

    If you understand what you are doing and why, shorten the Negative Wire as much as possible (say 12 to 18 inches) and bond it to a chassis hard point or seat rail anchor bolts with NO FUSE in-line. Take Positive directly to Battery Positive Term Post with In-Line Fuse installed directly on Battery Term Post. You will have a Safe and Clean dedicated power circuit for your radio.
  4. AI7PM

    AI7PM Ham Member QRZ Page

    It still amazes me when I open new hammy rig box and see that crap, yet open a commercial rig box from the same manufacture, and it comes with the proper long red-short black cable. Oh, and that 12ga zip cable from the hammy rig has no business being exposed to the under hood environment, yet............
    K8AI likes this.
  5. AF7TS

    AF7TS Ham Member QRZ Page

    Really can't dispute the essence of KF5LJW's post. Really, if you don't want to bother with nuance then just follow his instructions. Thanks for taking the time to write a useful and informative post.

    A couple of _small_ points.

    1) the 'coax and black wire forming a parallel path' issue is real if the radio chassis or end of the coax is grounded. This _is_ the most common situation; most mobile installations use grounded antennas of one sort or another. However one can most certainly design a system where the only connection to the electrical system ground is via the single black power wire, in which case it would not form a parallel path.

    2) The general rule is that you _DO NO_ put a disconnect or OCPD in your 'grounded conductor', with one exception: if the grounded conductor is _simultaneously_ disconnected with the ungrounded conductor. So _IF_ for some design reason you want overcurrent protection for the 'black' wire it has to be selected to simultaneously open the connection to the battery positive. Probably not worth the effort but the option is there.

  6. AG6QR

    AG6QR Premium Subscriber QRZ Page

    If the goofball mechanics had used a fuse in the ground lead at the battery, then when the starter motor was engaged, the fuse would have opened virtually instantly, preventing the fire.

    But then what? They would have been forced to diagnose why the starter wasn't working, and would have reconnected the grounding cable to the battery. But would they have noticed the blown fuse? Probably not.

    Everything would have worked fine, it's just that the radio would have been grounded via its coax instead of via the fused wire back to the battery. That would almost certainly work for receiving, but maybe with a bit of noise pickup. But would the coax be capable of carrying the full transmitter current? Maybe so, maybe not. If not, no fuse is protecting it.

    My mobile radios use one big fat short unfused cable to the closest good ground, the way the vehicle manufacture handles power for every other electrical device on the car.
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    You are talking about a Ganged OCPD like every one of you have already, in fact many of them in your home. Open your Main Breaker Box and every 240 circuit you have uses a ganged breaker. However the two conductors are both HOT, not neutral or ground which is forbidden to have any device that might interrupt the continuity.

    Aside from not conforming to any codes or practices because it is not safe to do so, you also shoot yourself in the foot operationally. First priority is always safety. Second priority with an Install is to keep voltage loss to a minimum on power conductors. A fuse, breaker, or any OCPD has resistance. Most of the OCPD' you will likely use is a Thermal Fuse which is a wire resistor that melts when to much current flows. The resistance of the fuse, added connections, and extra wire just add up to unnecessary voltage/power loss that you are trying to avoid. It gains you nothing, only losses.

    Now if you are doing it for Quick Disconnect say to transfer radio between vehicles or in the shackelhouse, time to learn what Anderson Power Pole Connectors are used for. :D
  8. N4MU

    N4MU Ham Member QRZ Page

    It IS amazing the "suggested installation method" in so many publications (not the least of which come from the manufacturer) that show you connections to + and - at the battery and dual fuses! This includes ARRL pubs...where they shouldn't let such stuff out and should clearly know better!
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    Then you did it correctly. The other half, red positive, needs to go directly to the battery to make it a Dedicated Circuit so it does not share any of the vehicles load current or suffer any unnecessary voltage drop.
  10. KF5LJW

    KF5LJW Ham Member QRZ Page

    I am open minded and would like to hear the justification. I can still learn. Perhaps there is something I am over looking along with peers who do this professionally.

    We learned that lesson almost 40 years ago. At the time the radio manufacture was Motorola used in Ford, GM, and Chrysler. When we consulted all four companies told us the exact same thing that we were ignorant and did not realize we were installing the radio in parallel with the battery Bonding Jumper. We even duplicated it in test. Nothing has changed since then and never will with the Battery Bonded to the chassis. You cannot change Series and Parallel Circuit Laws.

    Only vehicles that do not bond battery to the vehicle chassis I am aware of is EV Traction Batteries as that would be extremely dangerous asking for Great Balls of Fire or Electrocution and instant death.
    Last edited: Sep 13, 2019

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