Topband: RFI - and lots of it

[Tom W8JI]

> Way back in the '90s, when I served on the SAE automotive RF immunity and
> emissions (EMI & EMR) committees, questions about why we have 
> negative-lead
> fuses in ham radio gear came up from time to time.  (Many of us on the
> committees were hams.)  The reason we were given is that there was a
> scenario in which the negative battery lead ground failed, and the radio
> negative became the best path to ground for the starter motor, 
> particularly
> if the radio negative was screwed to the negative battery post.  In
> retrospect, this seems far-fetched, but the OEMs don't like to hear 
> stories
> about electrical fires in their vehicles, so somehow, this was 
> communicated
> to the radio manufacturers, and negative-lead fuses became the rule.
>

Any good connection to the chassis anywhere on a unibody vehicle is far 
better than a connection to a battery negative. Motorola is smart enough to 
tell installers to ground to the chassis, not the battery. In the UK the 
directive is to use the chassis or a manufacturer supplied terminal, and it 
specifically prohibits connecting to the battery negative pole.

The negative fuse is just foolishness. If it opens, all the radio current 
goes through small wiring. If the fuse opens, there goes the radio or 
something connected to the radio via a port. All of the radio current will 
go through some small wire.

My shop bench radio has an open foil on the CW key line and the mic because 
of a fuse holder failure, and that isn't the first radio that has that 
happen.   :)

The entire problem centers around use of the battery pole or battery 
connector as a source, and this carries over into our station desks. There 
is a ground loop similar to that in a car created between the power line 
ground, the power supply case, the negative lead, and the radio chassis back 
to ground. As in the car, if this stuff was built or wired correctly, the 
12V bus would only be grounded at one point and there would be no negative 
fuse.

In our houses, many of the problems blamed on RF feedback are actually 
ground loops caused by grounded cabinets common to negatives and voltage 
drops on negative leads.

At least some places in Europe got their together and banned battery 
negative connections because of the hazards.

73 Tom