[RFI] HF Mobile Installation in 2001 Mazda

[Tom Rauch]

> Unlikely if you follow good engineering practice.
>
> 1) Get your power directly from the battery ( both
positive and
> negative).

So far as "good enginering" goes, number one above causes a
conflict with number 2 below.

 > 2) Run coax with good copper braid to the antenna, bond
the braid
> to the car body at the antenna, AND -- VERY IMPORTANT -- 
MAKE SURE
> THE CAR BODY AT THE ANTENNA IS BONDED TO THE REST OF THE
CAR BODY.
> This will also maximize the radiation efficiency of the
antenna
> and minimize the receive noise.

Think about it a while, maybe while drawing the entire
system on paper.

Inside the radio, the negative leads of any HF radio and
almost all VHF/UHF amateur radios are virtually always
connected together via the chassis or PC board grounds.
There isn't any sound engineering logic that suggests
connecting the negative radio lead to the battery negative
terminal is a good idea or that it reduces potential
problems in a system like this.

As a matter of fact, the exact opposite is true!

Connecting the radio's negative lead  to the battery
negative post when the radio has an internal power lead bond
to the antenna shield or any other ground opens the system
up to all sorts of problems.

That very bad idea is probably firmly rooted in mythology of
radio installations because years ago many two-way radios
had floating negative power leads. Those leads were totally
isolated from the radio chassis and any external ground
paths from the chassis. This was primarily to allow use in
positive ground vehicles. In very rare cases where the radio
has a fully DC isolated negative lead, connecting the
negative to the battery is a GOOD idea. It has no ill
effects. It actually has very clear electrical advantages.

On the other hand when the radio internally connects the
negative lead to the case or to any cable or conductors
(like antenna leads) exiting the radio case, connecting
negative radio power to the negative battery post is at best
useless and at worse harmful.

The proper place to ground the negative power lead of a
radio without an isolated negative DC feed (to minimize
disturbing the electrical system and to avoid creating
potential hazards to equipment or the vehicle) is to ground
directly to sheet metal that is firmly connected to the
vehicle chassis. If you are unsure of vehicle chassis
electrical integrity the ground should be somewhere near
where the negative battery post has been grounded but NOT
sharing the same ground fastener. It can safely be to the
chassis anywhere on the vehicle as long as the radio ground
point has low resistance continuity to the battery negative
terminal ground point. That's almost anywhere in a unibody
chassis.

73 Tom