"Keith" wrote in message
...
It gets stranger. I did unhook each device from the expander one at a
time,
and nothing made any difference. Of course, turning the GPS off or
disconnecting the power to the NMEA expander made the noise stop. Now get
this: I was hooking up the power to the NMEA expander, and I connected
JUST
the positive lead, and it started transmitting the data, still with the
noise of course. NO ground hooked up at all to the "supply" side. One of
the
four devices hooked up is somehow supplying a ground of it's own.

The four items a
Raymarine Radar
Standard Horizon VHF
ICOM 802 SSB
Computer running Nobeltec VNS

Check the connections on being differential or single ended. I'll explain:
A single ended connection has one signal lead and a ground. A differentional
connection has two signal leads, often marked A and B or + and -.
Oficially, NMEA is differential and with isolated inputs. Many manufacturers
however, abandon the standard and go the cheap way. There are two situations
that require a different wiring:

1. Single ended out to differential in
2. Differential out to single ended in.

1. Single ended out to differential in: this is the easiest to understand.
Connect the output lead of the talker device to "In A" on the expander and
connect the ground of the talker to "In B".

Many GPS's are single ended and are to be wired this way. Some have TX+ or
TX A and TX- or TX B, which can be connected to In A and In B.

The trick part is 2) Differential out to single ended in.
In this case, you connect Out A from the expander to the In lead of the
listener and you leave Out B open!. The ground from the listener has to be
connected to the ground of the expander.

Since you mention that there is a hidden ground path, it might just be that
one of the connected listeners is single ended in and you connected Out A to
the input and Out B to the ground of the listener. This will create a ground
path and a very heavy interference on SSB.

Maybe you could post the labelling of the inputs of the equipment you have
connected to the expander and how they are connected to the expander
outputs.

Regards,
Meindert