Understanding Grounding Of Radio
Transceivers
There is a serious flaw with
the suggestion mobile radios be wired across battery terminals. The negative
radio lead should actually not have a fuse, and not be wired to the battery
post.
Many mobile radio
manufacturers continue to recommend the above installation practice in
their manuals, the following will help you understand why this is an incorrect
method of wiring your mobile radio power leads across the
battery.
History of
Vehicle Accessory Equipment Grounds
Early vehicles had both positive and
negative grounds. USA passenger vehicles evolved,
standardizing on "12 volt" negative ground systems. The resting voltage was
around 12.6 volts, with ideal running voltage in the low 14 volt
range. Many large commercial vehicles, however, retained
positive grounds. Non-standardization of systems meant two-way radios and other
add on equipment was generally designed to operate with either negative or
positive grounds. This was accomplished by floating the internal negative supply
buss, while grounding all normally accessible external user ports to the case or
cabinet.
These early systems started the trend
of negative fuses, with both power leads directly attached to the battery. This
was not harmful, because the equipment had a completely isolated internal
"ground " that was electrically isolated from all other leads leaving the
device.
Once any other lead common with the
negative bus leaves the case, it is no longer safe to connect the negative to
the battery post. It is also not safe to fuse the negative
lead.
Example System
In most early CB and
commercial two-way radios, the negative buss inside the unit fully floated from
the cabinet and any external ports. This included commercial two-way radios like
the Motorola Motrac, Micor, and other expensive, high-quality, radios. The
floating negative was universal across brands including, but not limited to, GE
and RCA land mobile equipment.
The floating negative allowed use of
radios in either negative or positive ground vehicles. It also solved ground
loop issues, allowing direct connection to battery posts without fire or
equipment damage hazards.
Equipment manufacturers had no way of
knowing if the final installation would be in a negative ground vehicle or a
positive ground vehicle. As a result, systems with a floating negative buss were
shipped with both negative and positive power line fuses. The floating negative
buss system inside the radio allowed safe direct connection to the battery
posts, and safe shutdown if either the positive or negative fuse
opened.
Charging currents to flow through
antenna cables, microphone, or speaker leads. All exiting connections, as well
as the case, were electrically isolated from the negative The floating negative
power buss made it impossible for starter or lead.
Negative
Ground Only Equipment and Radios
Over time, vehicles
with positive grounds disappeared. As this happened, manufacturers stopped using
the more-expensive and more-complicated floating negative power bus
system.
Many vehicle
manufacturers, and most aftermarket equipment manufacturers, never re-thought
the systems others were using. Manufacturers carried over the acceptable
negative fuse idea appearing in ground independent power buss systems, which
could also use a negative battery post connection. Manufacturers misapplied the
allowable fused negative battery post connection to equipment with internally
grounded negative bus systems.
Not realizing the
safety hazard, they continued to fuse both negative and positive radio power
leads and often advised direct battery post negative connections. The battery
post connection actually created ground loop, fire, and equipment safety
hazards.
This terrible idea
has permeated the aftermarket accessory market, including amateur radio, audio,
and performance or race car electronics markets. For example, MSD Ignition's
installation instructions advise a direct battery post negative connection,
putting the vehicle's distributor, the MSD box, and other electronics at
risk.
Although absolutely
incorrect, a popular assumption is fusing the negative lead can protect internal
and external equipment wiring, including gauge, computer, speaker, microphone,
key leads, and antenna connections from open battery ground connection
damage.
The only battery post negative
connection should be to or from another battery negative, the vehicle chassis,
and/or the engine block. There should never be a direct negative post path to
accessory equipment.
Normal vehicle
wiring is shown and discussed in detail.
Example Of Improper Unsafe Radio To Battery
Wiring
Follow the current in this
system....
If the ground or negative wire from
the radio to the battery opens, radio current would flow from the radio out
through the antenna cable, the speaker jack, the key jack, or any other jack or
connector that connects to the radio circuit board grounds to the vehicle
chassis.
If the battery to engine block ground
opens, or the engine block and battery to chassis grounds open, the battery will
ground back through the negative radio connection to the radio, through circuit
boards or other connections, and to the vehicle chassis.
While a radio negative lead fuse will
protect higher current circuits, it will not protect small traces or components
like those connected to insulated jacks or connectors.
For example, when my radio system lost
a negative fuse connection, foil traces for the keyer paddle grounds
inside my ICOM 751A burned open. I could no longer send CW.
The traces opened even though the
negative lead was fused, because the radio's transmitter current of ~20 amperes
flowed through small circuit board traces to my CW key. The CW key was grounded
by touching a metal bracket, and this melted foil traces on a circuit board
inside the radio. With a grounded negative buss and battery post
connection:
1. Anytime the battery negative to
chassis ground opens, vehicle accessory and lighting currents, or a portion of
vehicle accessory and lighting load current, will flow through the
radio.
2. Anytime the negative fuse opens,
the radio will ground itself through the coax or other external
connections.
3. This also sets the radio up for
increased alternator whine, since load currents can superimpose themselves on
traces and connections inside the radio.
Connecting the radio or accessory equipment's grounded negative supply
does nothing good. Grounding to the battery negative post generally increases
alternator whine. It also places the vehicle's computer and electrical system at
higher risk, is unsafe for the radio or accessory, and is unsafe for things
connected to the radio.
I've had vehicles with higher power radios that have opened a negative
fuse, and then melted the coaxial cable shield on RG58/U cables. This is
because, once the fuse connection was lost, the equipment grounded itself to the
vehicle chassis through the grounded antenna cables.
Correcting the Ground - Safe Radio To Battery
Wiring - Use #10 Stranded For VHF/UHF Mobile 40 - 50 Watts, Select Wire
Size According To Amp Draw & Run For HF Mobile.
It is pretty simple to
improve the grounding system.
If we ground the radio
negative lead to a separate but good chassis ground near the battery ground,
ground loops through the vehicle's computer system, through the radio, and
through anything connected to the radio or accessory, are completely
avoided.
Ideally, this ground should not share
the bolt that grounds the battery to chassis ground cable. The negative power
ground should be on its own bolt. The fasteners should be good hardware, with
proper star washers. The ground must provide solid, reliable, mechanical and
electrical connections.
If the radio or accessory device's
chassis power ground lead opens, only radio or accessory current will flow
through the radio circuit boards. While this may still open a trace, it is much
less likely to happen because we have eliminated a needless weak point, the
negative fuse, and we have eliminated the direct connection to the battery where
battery acid slowly eats away at connections.
A vehicle chassis power ground
completely eliminates the current path through the radio or accessory if the
engine block ground battery lead opens, and alternator AC ripple no longer can
drive the negative lead to the radio. Any connection can fail, and in the very
worse case possible we have only radio operating current. In most cases, we will
not even have that.
It also helps to have multiple ground
points as additional insurance against the negative power lead accidentally
floating.
Bob Krueger,
AB7CQ
Web Administrator
RPTR 1: 146.920/146.320 PL 123.0
(WIRES-X Room 28187)
RPTR 2: 444.600 / 449.600 PL 100 (LAN
Linked)
Simplex IRLP Node: 7515 PL114.8
(146.540)
Email: ab7cqradio@ebidpal.com
You are here: Home-Grounding
Previous Topic: Duplexer Tuning Next Topic: Link Your DX-2X
|