[RFI] Radio "Grounding"
w8ji at contesting.com
Thu Aug 9 20:19:31 EDT 2007
> Would a 95%+ shielded DC cable, grounded at both ends, be
> 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
>> AT THE
>> 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.
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