"Larry" wrote in message
...
"Keith" wrote in
news:1148989790.905246.127580
@u72g2000cwu.googlegroups.com:

any ideas?


Welcome to NMEA, the big interference transmitter aboard!

Nonsense. It's the sometimes crappy implementations that do the harm.

I've given up
trying to cure it. Damned unbalanced connections, grounding the - NMEA
terminal causes a lot of it. Stupid NMEA, itself, trying to save the
manufacturers a few bucks with no shielding, no shielded connectors, screw
terminals all over in plastic boxes....we did it to ourselves. How
stupid.

Again, nonsense. Example: our multiplexers are tested and being certified
for IEC 945/EN60945 which imposes very low limits of allowed radiated RF.
The tests are done with cables attached to the multiplexer, transmitting
data over the cables. These cables are not shielded and the emissions stay
below the legal limits, which means you can hardly hear them above the noise
floor on HF and VHF.

Best of luck trying to cure it. None of the plastic boxed computers
running all over the boat on the network are properly shielded, either,
making the problems even more acute. They all radiate like hell to the HF
antenna.

Again nonsense. The very same multiplexers are housed in plastic. It just
comes down to proper filtering on all terminals and a decent EMC-proof
printed circuit design.

I just turn it all off when I'm on HF. The big transmitter scrambles the
data, anyways radiating into all the unbalanced, unshielded crapware.

Oh and the same rules apply the other way around. They have to withstand a
fieldstrength of10V/m, 1kV induced spikes on the terminals, contact
discharge of 6kV on the terminals and 8kV near the housing. All tests
passed. What's that again about stupid NMEA?

NMEA is nothing more than a RS-422 connection, something that is used all
over the world without problems. It's just that some manufacturers screw up
big time...

Meindert