On 10/28/2010 12:46 PM, John Brosnahan -- W0UN wrote:
> Again, the noise appears when I press the gas pedal and disappears
> (well, drops 10 dB, which is acceptable) when I let up on the gas
> pedal.
John,
The internal car radio is probably using some sort built-in antenna, and
the coax may have a cheapie shield.
I suggest that you do a temporary install of a ham rig with simple
antennas and try to figure out how it's getting into the radio. Is it
picked up on the antenna, the coax shield coming from the antenna, or
the power cable? Suggestions for how to figure that out.
FWIW, I've always run my rigs sort of laying on the passenger seat, with
coax going to the antenna and bonded to the frame at the antenna, and
with a paired power cable going straight to the battery. That setup
should be fine for this test. My personal philosophy is that /M is
something I do when I'm by myself, not when the XYL is in the car. Happy
marriage 101. :)
1) Remove the antenna at the mounting point, but leave the coax in place
both at the antenna end where it bonds to the body, and at the radio.
2) Trying running the radio from a different battery, with no connection
to the auto body except the antenna.
I suggest multi-turn common mode chokes on both the coax and the DC lead
going to the battery. I think W8JI has done a lot of this too. BTW --
Tom recommends getting power from the alternator rather than the
battery. He's done a lot more with /M than I have, but I don't follow
his reasoning for that.
It's critically important to realize that while the power circuit is
INTENDED as a DC circuit, mother nature will treat it as an antenna.
Another point. It is WELL known that twisted pair has great value in
reducing differential noise coupling, but virtually all DC wiring is
glorified zip cord. IF the coupling is differential on the power cable,
twisted pair would help a LOT.
73, Jim Brown K9YC
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