On Mon, 10 Nov 2003 07:56:03 +0100, "Meindert Sprang"
wrote:


Any chance of using ferrite ringcores on the NMEA lines to surpress it ?

Meindert


Not as long as the unbalanced lines are all exposed like they are.
Every cable from every instrument uses a Belden foil-shielded pair.
But the radiation is going to happen, anyway, because the output of
some of the instruments is unbalanced, inserting a radiating ground
inside the faraday shield. Most instruments, you have to abandon the
shield (screen connection for you UK readers) at the point where the
instrument's unshielded power cable with its dangling data lead hangs
out. There's no way to complete the shielding to the instrument.

Maybe Icom is right. Make all the NMEA connections via a coax
connector, unbalanced. M802 uses a BNC, in total abandonment of any
NMEA balanced concept. The shield of the coax to that BNC MUST be
connected to NMEA B (-) to get data on the radio's DSC display. So, I
figured RF from the transmitter's case follows this odd ground down
into the network shield and screws it all up. But that didn't pan out
because the network does the same thing with the cable to the Icom
disconnected. Lucky for me that during short SSB transmissions, the
system components just ignore the trashed data, so the users don't see
there's no NMEA data for the 20 seconds the SSB is talking. I think
this is the reason more people don't mention or notice it. The
displays just freeze until you stop talking or take a breath when the
SSB output power drops to a very low level and data stream resumes.

Oddly enough, I don't notice this malady on the Seatalk unbalanced,
unshielded part of the system. This may be because its cables are
much shorter and all the Seatalk instruments are very close together.
(RL70CRC Plus, Smart Heading Sensor, WAAS-GPS). Even the GPS receiver
built into its antenna has a very short cable because I found a great
little unused place right on top of the helm to port of the crank
handle for the main sheet traveler to put both Garmin and Raymarine
GPS antennas. Coverage through the fiberglass hardtop on the cockpit
is excellent and noone uses the antenna for a grabhandle like they did
when the antennas were initially on top of the hardtop. It's cable to
the Seatalk plastic box with European screw terminals is only about
10" long. None of the Seatalk wires are over 24", making them much
too short to fit an 8 Mhz wavelength.

Seatalk radiates, too, even on these short cables. If I shut down the
whole NMEA network and just run the RL70CRC Plus, WAAS-GPS and Smart
Heading Sensor with the NMEA cables unplugged from the RL70, I can
still hear data noise from the Seatalk across the HF bands. Someone
knows about this because none of the Seatalk harmonics is on a marine
HF channel. I only have a few in the ham bands.

It's just too bad there isn't a STANDARD everyone was FORCED to follow
that would completely eliminate this easily-fixed interference. USB
and RS-232C and RS-422 running at home don't tear up my ham station
sitting right next to the computer. I just can't believe marine
electronics cannot be built for these lofty prices that doesn't
interfere, either.

Ferrites won't stop the plastic boxes and unshielded cables with
square waves in them from radiating unless you rounded off the edges,
which would trash the data timing. The computers in the plastic boxes
also radiate. Doesn't take much to trash a submicrovolt receiver and
open its squelch. We used to have an Adler-Barbour 12V
electronic-controlled cold plate that just ATE, of all frequencies,
Channel 16 with its incessant pulsing, opening the squelch of all the
VHF receivers with a maddeningly repetitive pulsing shuusssh of the
closing squelch.....It only trashed 16, the channel you listen to
most. Drove all the helmsmen crazy until we figured out what it was
and shut it down. It's someone else's problem now. This fridge runs
off a 1-cyl engine compressor.



Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"