"Larry" wrote in message
...
"Roger Long" wrote in
:
[.. major snippage..]

BTW I have a ferrite core on the GPS lead. When I install the
autopilot, I'm going to avoid tying the power supply to in in the same
convienient cable run as the GPS and lead it separately with its own
ferrite core.

All for naught. You'll never be able to stop the NMEA from radiating.
Don't waste your time on cores and tin foil.

The cores *might* help. They will knock the high-frequency edges off the
NMEA signals, and will even turn some of the signal/ground common-mode
signal into a balanced signal by acting as a balun (balanced to unbalanced
transformer). The cores will only do this for the high-frequency
components, but that's where the radio interference comes from. Even if the
box radiates, choking off the connecting wires with the ferrites will reduce
the signal that gets into your antenna.

I'm not guaranteeing that this will help enough to make a big difference,
but it might be enough to be useful. If they will fit, try running the
wires through the ferrites a couple of times (through the hole, around the
outside, and back through the hole in the same direction). This will
significantly improve the ferrite's effect on HF signals. Put on two or
three ferrites. It all adds up, until it doesn't.

I've found that it is much easier to clean up this type of interference by
using ferrites than by trying to shield small devices. For a critical
application, a combination of shielding, bypassing (with capacitors), and
external ferrite chokes may be required, but for the typical boat situation,
you can get 80% of the way there by just using the ferrites. I used to do
this for a living, bringing computer and telecom products into compliance
with FCC and international radio interference regulations.

But, as Larry points out, sometimes there just isn't a reasonable fix.
Shutting stuff down will definitely do the trick in that case.

-Paul