[RFI] Radio "Grounding"

Tom Rauch w8ji at contesting.com
Thu Aug 9 20:19:31 EDT 2007

> Would a 95%+ shielded DC cable, grounded at both ends, be 
> the
> effective equivalent?

Why would you want to do that and what would you gain?

Isn't the radio dc input properly bypassed?

>> We DO want a good bond of the shield of coax to the body 
>> ANTENNA, we DO want the DC power pair (preferably 
>> TWISTED) to go
>> straight from the radio to the battery terminals, and we 
>> DO want
>> the  body of the vehicle to be bonded together very well 
>> and at as
>> many points as possible.

At great risk of starting a long thread, I'm going to 
disagree with this.

The battery negative terminal was always a good place for 
the radio negative supply return with commercial radios, 
because those radios almost always  had floating negative 
busses in the radios.

Ham radios almost always have the negative lead tied 
directly to the foils INSIDE the radio on PC boards, and 
thus to the case and every connector leaving the case. No 
matter who gives the advice the negative lead should connect 
to the battery, I'm going to strongly disagree with that 
advice. The only exception would be if the radio has a 
floating negative buss.

The negative post of the battery normally has a single small 
link as the primary chassis connection. The heavy lead 
almost always ties to the bock or engine hardware, so there 
is a high current bond for the starter and alternator.

Most accessories outside of the engine compartment ties to 
the chassis and return through the small ground lead to the 
chassis. This means fuel pumps, lights, horns, wipers, seat 
motors, window heating tapes, the stereo, wiper motor, 
heater motor, and many other things return through that 
chassis lead to the battery.

When you connect the radio negative to the battery  negative 
post you now have a second path for all those accessories to 
return through the radio to the battery. This path is either 
through a cabinet ground on the radio, through the antenna 
lead, through an accessory jack like an external speaker, or 
even a key jack.

I had a thin ground foil INSIDE my ICOM radio open from 
fault current through that path when the ground lead from 
the battery to the chassis had a fault current induced 
external to the radio installation. That path was through a 
grounded accessory that was plugged into the radio, and the 
radio negative line fuse could NOT protect a foil that was 
only a few thousands of an inch wide.

I've also found that loop is a source of inducing noise into 
the radio from accessories, since the chassis to battery 
impedance is shunted by a path directly through the radio.

1.) Ground loops, by definition, occur when a device has 
multiple paths to ground points of differing potential.

2.) The battery post is NOT heavily and directly bonded to 
the chassis, it has only a small link. The chassis is the 
return path for many accessories, or at least is in most of 
the vehicles I have seen.

3.) The antenna is bonded to the vehicle chassis, not the 

4.) The minute you screw the PL-259 onto the radio, the coax 
shield forms a dc path to the vehicle chassis to the radio.

5.) You also, if you bond the radio negative to the battery 
post negative, create a ground loop through the coax to the 
radio and out through the power lead to the battery post.

A far better approach is to bond the radio negative lead to 
the vehicle chassis. The best place is generally near the 
battery to chassis ground, or on a primary piece of 
sheetmetal that is part of the main structure of the 
vehicle. This keeps the radio negative power lead at the 
SAME ground potential as the antenna or any accessories 
(like a speaker) that you might plug into the radio. In this 
case there is no ground loop through the radio.

Look at it logically. If there wasn't the potential for a 
damaging ground loop, you wouldn't NEED a negative fuse. You 
need the fuse when you tie to the battery negative post 
because many things can screw up and send fully battery 
current through the radio. If you bond to a solid piece of 
the vehicle chassis and use that as the negative path, you 
don't have that ground loop.

Let's not pretend that connecting across the battery cures 
ground loops. It doesn't.

It creates a ground loop.

At least let's get that right and stop this silly myth that 
somehow putting the radio in a negative lead path from the 
battery to the chassis is a good idea.

Now if you have a real two-way radio like an old Motorola or 
if you have a good quality 2000 watt power inverter, by all 
means run both leads to the battery. That's because MOST of 
those devices do NOT ground the negative lead at the high 
current load, so it never creates a ground path.

But let's not pretend the coax jack on our radios or the 
case is isolated from the power negative. It isn't.

73 Tom

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