[Elecraft] Connecting Transceivers in Vehicles - Change

[Tom W8JI]

For HF mobile installation, GM and others recommend that the 
radio NOT be grounded in the cabin. Both battery leads 
should be fused at the battery. The reason for the negitive 
side fuse is in the event that the battery ground lead to 
the engine block should go open or have a high resistance 
there will be very high current trying to find ground when 
you start the engine. Starter motor current can be in the 
hundreds of amps. You do NOT want that amount of current 
flowing thru your rig!>>

The problem actually comes from what is inside the radio, 
not what GM thinks is inside our radios or how GM thinks the 
system works. The main problem is connecting the radio 
negative lead to the battery negative terminal. That's a bad 
idea, and it applies to our station wiring as well as car 
wiring. We can have similar destructive ground loops in 
station wiring.

Our radios and accessories have small thin foil traces and 
small components like RF chokes on the ground leads of many 
accessories connectors. Blow a negative fuse or lose a 
ground connection on the negative supply lead and you can 
damage components inside the radio. The jacks and plugs 
become the negative high current return path.

Our power supplies either need to be ground isolated, which 
they often can't be, or they need to bond the negative into 
a solid ground buss, and all the negative supply leads need 
to connect solidly to that buss. Otherwise we risk damaging 
our equipment from something as simple as a bad connection 
or blown fuse.

In your car, the battery should have a heavy return to the 
engine for alternator or starter current. It should have a 
lighter, but still reasonably heavy, connection at the 
battery to the chassis. The radio should be grounded on the 
same sheet metal near the point, but not on the point, where 
the battery is grounded to the chassis. There should be no 
negative fuse. It should be a solid connection and NOT 
common with the bolt grounding the battery. Then if the 
battery negative comes loose, current will never flow 
through the radio or radio wiring. It can't set the car on 
fire, it can't blow out automotive electronics, and it can't 
damage the radio. Every passenger area accessory connects to 
the chassis as a return, and so do the lights and other 
things. They would never run their stuff, other than the 
starter and alternator back to a negative battery post, so 
why should we do that with our radios?

It is foolish and unsafe to ground radio to the battery post 
upstream of the vehicle chassis ground, just as it is unsafe 
to ground equipment at home directly to the negative rail of 
high current 12 V supplies instead of a ground buss or 
common ground point, no matter who says otherwise. Without a 
common reference point all it takes is a high current load 
negative to come detached, or a positive lead low resistance 
fault to ground, and we have equipment damage. I learned 
this years ago when my radio lost the keying line ground 
trace on a circuit board because the radio negative lead 
fuse developed a bad connection, and the radio primarily 
grounded through the keyer path.

73 Tom