Re: Mobile installation question


*** wrote:
(snip)
> Engineers at the automobile manufacturers
> EMC labs recommend direct connection to the battery. If you want to
> read a detailed discussion on the subject, go here.
> http://www.arrl.org/tis/info/rfiauto.html

I have read through this page, and it contains what you say it does.
Unfortunately for my education, the manufacturer's recommendations do
not give any reasons for their recommendations, so you have no way of
knowing if it makes the radio work best, or just protects the
automotive electronics most from interferrance from the radio, or even
if they thought much at all about the question, but just gave the first
answer they thought of. I doubt very much that they are at all
concerned with your radio working, optimally.

Further down in the discussion section, the manufacturer's
recommendations are echoed several times, but the reasons given are
related to avoiding specific problems. So here is whai I found:

(begin excerpt)

Q: When I'm installing the radio, where should I connect the dc power
leads?

A: The positive and negative leads from the transceiver should connect
directly to the battery with fuses in both leads. Route them away from
any
other wiring in the vehicle. This usually means drilling a small hole
in the
firewall between the engine and the passenger compartment and running
the
wires through a rubber grommet (to protect their insulation from sharp
edges). If the wiring or antenna lead must pass near vehicle wiring,
route
it at right angles to the wiring. It is best to run the power and
antenna
leads as close to the vehicle chassis as possible.
(end excerpt)

This is essentially an echo with not much reasoning. Do it this way
because (they, we) I say so.

(begin excerpt)
Q: Why can't I find power inside the ear?

A: It's often difficult, especially in modern cars, to know which power
sources in the passenger compartment can safely carry the high-current
loads
of a transceiver. If you choose a source that also powers one of the
microprocessors, the transmit current could cause problems. In
addition, some
transmitters are not well bypassed and some RF energy can appear on the
power
leads. This is bad news for microprocessors. Even grounding isn't as
simple
as it seems. With all of the plastics used in modern vehicles, not
everything
that looks like ground is really ground. It could be "floating" or even
be
part of the wiring for one of the vehicle's sensors. Even if you do tie
into
the chassis, it is possible that the section you've chosen forms part
of the
return path for one of the control modules Generally, you're better off
running the wires to the battery, especially for high-power
installations.
(end excerpt)

Now, here we have a couple reasons why it is not recommended to just
tap into the existing power distribution wiring.

1. Exceeding ampacity of the wiring.

2. RF leakqage out the radio power wires interferring with the
automotive electronics

3. Chassis as ground return coupling noise between radio and car
electronics.

All good points. None contradict my idea, except, possibly 2. But the
RFI filter I suggest adding to the alternator can bypass this noise.
Beefing up the noise immunity of the power bus seems like an excellent
idea if you are going to add a powerful RF generator to any modern
vehicle, whether the radio requires it, or not.

(begin excerpt)
Q: My VHF installation works perfectly. However, I followed similar
guidelines for HF and found that I had some pretty strong noise on
receive.
Some of this noise goes away when I connect the radio to the test
battery I
used earlier. Any ideas?

A: It sounds like some of the noise is coming down the power leads. To
cure
this, first try ferrite chokes on the positive and negative leads.
Obtain two
FT-140-43 ferrite cores [2] and wrap about 10 turns of each wire onto
the
core. (Fewer turns may not work, and if you substitute a different
ferrite,
make sure it's suitable for HF - unknown materials usually don't work!)
You
can also try installing 0.01-uF capacitors from the positive lead to
the
negative lead, or from the positive lead to chassis ground (or both).
The
capacitors may work best with or without one or both of the ferrites.
(end excerpt

This is a very brief discussion of filters between the power source
and radio. I would like to see a much more scientific expansion of
this topic. There are lots of things to know about making effective,
high current RiI filters, especially ones that must carry large DC
currents. It does imply that a battery is not necessarily an effective
bypass capacitor at RF frequencies.

I still see nothing about why it is so important to connect directly to
the battery, instead of to the power source that maintains battery
voltage, except that people say to do it. It is beginning to look like
common wisdom that may actually be based mostly on a commonly held myth.

.