Supressing RF Interferance from NMEA Junction
 

At my nav station I have a bus bar with jumpers between the connectors
to provide NMEA data to a variety of equipment which wants to see info
such as lat/long. It provided a nice way to connect multiple pieces
of equipment to a single NMEA output. The problem seems to be with
the data output from my SR161 AIS receiver. I have the AIS data going
to single set of connectors on the bus bar which are bridged to the
cable which goes to my RayMarine C80 chart plotter. All of the above
cables have large ferrite chokes on both ends.

When I connect the cable to the SR161 AIS receiver, the VHF radio
which is located very near the above equipment receives static on
occasion. By this I mean that I hear random periods of static on the
radio which disappears only when I turn the squelch up quite high. If
I unplug the data cable from the AIS receiver, the static disappears.

How can I shield the bus bar so that I can distribute the data and
bridge the AIS data such that I don't generate RF which interferes
with the VHF?

-- Geoff

Supressing RF Interferance from NMEA Junction
 

GeoffSchultz wrote:
At my nav station I have a bus bar with jumpers...snip

Connecting multiple equipments to a single RS-232 output can drop or
overload the signal levels and affect the quality and performance some
or all of the connections. I think that can generally start happening
with more than two or maybe three three devices connected. That might
be the root cause of your problem here.

In a more professional installation, a signal multiplexer would normally
be used for what you are doing. Using one of those will usually add
some more features too, like allowing more than one talker to share NMEA
data with multiple listeners. And some will allow data exchange with
proprietary flavors of RS-232 (like Raymarine's SeaTalk) and have
Bluetooth and other more modern whizzbang features.

If you are crafty, you can make your own:

http://www.vimms.org/multiplex/docu.html

Or you can buy one of the good ones intended for marine use:

http://www.shipmodul.com/en/index.html

http://www.nolandengineering.com/

http://brookhouseonline.com/nmeamux_ext.htm

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)

Supressing RF Interferance from NMEA Junction
 

Using a bus bar for something like this is generally causing problems
because of the physical layout. A signal wire is supposed to have the return
wire very close, in order to radiate in the least possible way. So if the
TxD signal from the RS-232 port is going to some bus bar contraption while
the ground is going to a different place, you effectively create an magnetic
loop antenna, radiating as hell.

To prevent radiation, keep signal and return in ONE cable at all atimes.
What also helps it to limit the dV/dT of a signal, or in other words: slow
down the flanks of the signal by using a low pass filter directly on the
Sub-D9 connector of the AIS receiver. Such a filter could consist of a 47
ohm series resistor in the TxD signal line, followed by a 10nF capacitor to
the signal ground. This will surpress high frequency signals that could
interfere with an SSB or VHF set.

Meindert

Supressing RF Interferance from NMEA Junction
 

GeoffSchultz wrote in
oups.com:

How can I shield the bus bar so that I can distribute the data and
bridge the AIS data such that I don't generate RF which interferes
with the VHF?

-- Geoff



VHF is much easier to fix than HF......

Where is the VHF ANTENNA, in relation to the AIS data lines? Where is
the VHF radio? If they are near it, move them, instead of trying to fix
the unfixable, namely, trying to shield stupid NMEA open-wire technology-
in-the-plastic-boxes. Move the radio/antenna away from the noisy
nonsense...which is MUCH easier.

The VHF antenna system is a closed system, nicely shielded from the
antenna to the radio. One bad thing happens...loose connectors or
corroded shields open up this coaxial system to the outside noise
intrusion. Check all connections to make sure the SO-239 connectors are
tight. If any of the shields can be moved in the connectors, rebuild the
connectors so the shields are making an ALL AROUND good connection.
Crimped-on coax connectors are fine. So called "solderless" connectors
from RatShack are NOT. They come apart too easily.



Larry
--
While in Mexico, I didn't have to press 1 for Spanish.

It just isn't fair.

Supressing RF Interferance from NMEA Junction
 

On Jul 4, 6:57 am, Jack Erbes wrote:
GeoffSchultz wrote:
At my nav station I have a bus bar with jumpers...snip

Connecting multiple equipments to a single RS-232 output can drop or
overload the signal levels and affect the quality and performance some
or all of the connections. I think that can generally start happening
with more than two or maybe three three devices connected. That might
be the root cause of your problem here.

In a more professional installation, a signal multiplexer would normally
be used for what you are doing. Using one of those will usually add
some more features too, like allowing more than one talker to share NMEA
data with multiple listeners. And some will allow data exchange with
proprietary flavors of RS-232 (like Raymarine's SeaTalk) and have
Bluetooth and other more modern whizzbang features.

If you are crafty, you can make your own:

http://www.vimms.org/multiplex/docu.html

Or you can buy one of the good ones intended for marine use:

http://www.shipmodul.com/en/index.html

http://www.nolandengineering.com/

http://brookhouseonline.com/nmeamux_ext.htm

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)

Thanks for the pointers to multiplexors. I have 3 devices connected
to the 4800 bps NMEA output and I don't seem to have any problems with
data reliability or noise. It's when I connect the 38400 AIS data
that I get the problem. I don't have time to analyze my options right
now as I'm in the process of storing the boat for hurricane season,
but when I get home I'll re-think my solution and perhaps get
something that bridges to the SeaTalk bus so that I can get full
access to that data.

-- Geoff

Supressing RF Interferance from NMEA Junction
 

On Jul 4, 7:06 am, "Meindert Sprang"
wrote:
Using a bus bar for something like this is generally causing problems
because of the physical layout. A signal wire is supposed to have the return
wire very close, in order to radiate in the least possible way. So if the
TxD signal from the RS-232 port is going to some bus bar contraption while
the ground is going to a different place, you effectively create an magnetic
loop antenna, radiating as hell.

To prevent radiation, keep signal and return in ONE cable at all atimes.
What also helps it to limit the dV/dT of a signal, or in other words: slow
down the flanks of the signal by using a low pass filter directly on the
Sub-D9 connector of the AIS receiver. Such a filter could consist of a 47
ohm series resistor in the TxD signal line, followed by a 10nF capacitor to
the signal ground. This will surpress high frequency signals that could
interfere with an SSB or VHF set.

Meindert

I understand. I guess that I should simply solder the two cables
together and bypass my patch panel arrangement. Thanks for the
suggestions.

-- Geoff

Supressing RF Interferance from NMEA Junction
 

On Jul 4, 6:25 pm, Larry wrote:
GeoffSchultz wrote groups.com:

How can I shield the bus bar so that I can distribute the data and
bridge the AIS data such that I don't generate RF which interferes
with the VHF?

-- Geoff

VHF is much easier to fix than HF......

Where is the VHF ANTENNA, in relation to the AIS data lines? Where is
the VHF radio? If they are near it, move them, instead of trying to fix
the unfixable, namely, trying to shield stupid NMEA open-wire technology-
in-the-plastic-boxes. Move the radio/antenna away from the noisy
nonsense...which is MUCH easier.

The VHF antenna system is a closed system, nicely shielded from the
antenna to the radio. One bad thing happens...loose connectors or
corroded shields open up this coaxial system to the outside noise
intrusion. Check all connections to make sure the SO-239 connectors are
tight. If any of the shields can be moved in the connectors, rebuild the
connectors so the shields are making an ALL AROUND good connection.
Crimped-on coax connectors are fine. So called "solderless" connectors
from RatShack are NOT. They come apart too easily.

Larry

The VHF antenna is forward, on top of the mast; the VHF radio is at
the nav station; the patch panel is mounted behind the VHF radio; the
AIS cable run from the patch panel to the C80 which is at the wheel;
the two cables are only in the same proximity for a few inches.

Moving the VHF really isn't an option. I think that I'll follow up on
Meindert's suggestions.

-- Geoff

Supressing RF Interferance from NMEA Junction
 

"GeoffSchultz" wrote in message
ups.com...
AIS cable run from the patch panel to the C80 which is at the wheel;

Ah, I don't know how your wiring is setup, but the C80 had an gavanically
isolated NMEA input. To maximize surpression of interference, make sure you
have one single cable with a twisted pair between the RS-232 plug of the AIS
receiver and the NMEA input of the C80. Do not combine the signal return
with the power ground of the AIS receiver or any other device. This ensures
the currents in both wires are equal and thus maximum surpression of
radiation is achieved. Eventually you could use screened cable, use a
twisted pair for the data and return and connect the screen to ground at the
C80 side only. The C80 is an IEC945 approved device so it is safe to assume
that it's signal ground is clean.

Meindert

Supressing RF Interferance from NMEA Junction
 

GeoffSchultz wrote:
snip
Thanks for the pointers to multiplexors. I have 3 devices connected
to the 4800 bps NMEA output and I don't seem to have any problems with
data reliability or noise. It's when I connect the 38400 AIS data
that I get the problem. I don't have time to analyze my options right
now as I'm in the process of storing the boat for hurricane season,
but when I get home I'll re-think my solution and perhaps get
something that bridges to the SeaTalk bus so that I can get full
access to that data.


I don't have a multiplexer or a boat that needs one but If I did I'd
probably give them serious consideration. Being able to mix baud rates
and different flavors of RS-232 and NMEA would have to come in handy
sooner or later.

The AIS thing is new to me, I've not been around it or used it at all
yet but it sure sounds like a good thing and and something I'd like to
have working as well as possible. I think having to diddle with the old
hardware when you add new hardware to boats, is just one of the prices
paid for

Meindert is too modest to have said it here, but he is the designer and
proprietor of the Shipmodul line I linked in my post. He knoweth of
what he speaks and posts here regularly. He can always be relied on to
straighten out and better explain my layman's mumblings about what is
happening on stuff like this.

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)