On 11/21/2011 1:55 PM, Dave wrote:
> Remember, modern vehicles are not just a metal box with seats, 4 wheels
> and an engine. They more resemble a computer network, powered by a local
> engine-gen set, with miriads of controls and systems that all need to
> work well together to stay safe.
Indeed, modern vehicles are NOT metal boxes, they are boxes of various
plastics and metal, much of that metal painted, and often (mostly?) with
no electrical contact between painted metal parts. This renders the
concept of shielding, and of running cables close to a "ground plane" a
bad joke. Because the conductive plane, when it exists, is interrupted
by an insulator (paint) it is completely useless, and the resulting
wiring forms a magnetic loop that couples any RF current into sensitive
equipment and couples noise from that equipment into our radios.
In preparation for moving from Chicago to California six years ago, I
bought a big Toyota Sequoia SUV, and installed a 100W rig feeding a
hamstick mounted to the luggage rack, with the coax shield bonded to the
frame. It was damned hard to find a place to make that bond -- big
screws that appeared to go to the body showed resistance between them in
the tens of ohms, or even no continuity at all. I got the rig installed
and it seemed to be working well, and I headed west on I-80 working 17M
CW and SSB. The only issue was that the fan that circulated air in the
passenger compartment speeded up as I transmited. As I recall, noise
wasn't awful.
Things got much worse when, a day later, I called CQ on 20M SSB in a
rather remote section of the Nevada desert. As I was trying to decide
which of three calls I was going to answer, I noticed that I was slowing
down to 15 mph. As it turned out, RFI had caused the computer to go
into "limp home" mode. After a while troubleshooting, I found that I
could reset the computer and get back on my way by pulling the main lead
from the battery and replacing it. It would have taken a LOT of work to
clean up the body bonding and other wiring issues, and I didn't have
that time in the middle of that move. That was the last time I was
spending enough time in the vehicle on long trips to want to operate HF
mobile, so I've never bothered to clean up Toyota's mess.
For a better understanding of these issues, I STRONGLY recommend a study
of the latest version of Henry Ott's classic text on EMC,
Electromagnetic Compatibility Engineering, published by Wiley a few
years ago. The new version includes all the material of the older
versions (1976 and 1988) that have long been considered the EMC bible,
but adds new chapters on newer issues like printed circuit board layout,
switching power supplies, and the Pin One Problem. Henry is long retired
from Bell Labs, a consultant to a who's who of major companies, and a
ham. Henry is particularly good at breaking complex issues down into
their component parts, and explaining them in a simple, but rigorous
manner, and without "hiding behind the math."
73, Jim Brown K9YC
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