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RF Grounding for Marine Vessels
By
Dr. John Gregory
CEO & CTO cruiseEmail
Addressing the RF grounding problems that are necessary to perform
good and clean signal transmission while sending e-mail and or
transmitting other data and or receiving clean clear signals.
Problems:
The grounding of a vessel either sail or power is basically address
the same. This paper will address the basic sail vessel either
mono-hull or cat configuration.
Many of the problems appear when transmitting with the single
side band high frequency radio. The indicator lights on the DC
panel start to light up, air conditioner shut down, the propane
alarm goes off, noise in the SSB radio in intolerable, and good
gathering of weather faxing is noisy, and or e-mail get cut off,
or extremely difficult to send e-mails.
Causes:
Let first take an understanding as to what is causing these problems
into the RF system, of the vessel, and to why these situations
happen.
By using the old concept of grounding to everything that is metal,
has been the concept since installation of radio and electronics
on board private vessels.
For many years grounding to all metal objects has been the standard,
and that manufactures of radio and electronic equipment have just
accepted, and not reasoned with or challenged the old concept.
What happens is that the ground side of the antenna also carries
the RF signal as it's leaving the ATU [automatic antenna tuner]
it couples with the DC system of the vessel. The DC system is
also connected to the bonding system of the vessel, which means
that when ½ half of the sine wave is transmitted, that
portion of the transmission is coupled into the electrical DC
side of the vessel directly.
Another miss understanding is that many vessel owners believe
that the copper strap that goes to the ground show is the ground
and counterpoise to the ATU.
This is the major causes of interference to not only SSB radios,
but to autopilots, and noisy GPS'', electronics, laptops, and
many other electronics devices.
Grounding:
Let's not mistake this grounding as radial/counterpoise of the
RF radio system.
For proper grounding of a RF radio system ground shoe needs to
be mounted on the outside hull of the vessel. This ground shoe
should be located within 5-7 feet from the antenna tuner. This
ground is now the ground rod to earth ground as many visualize,
"the stake into the ground" It's also the electrical
current return from the antenna via the tuner.
As an average mounting place, many ATU's are mounted in the aft
lazerret of the vessel.
A copper strap with the width of 1-2 inches, anything larger is
a waste of time any money, and have no electrical advantage. This
copper strap should be no thinner than 5 mils in thickness. The
copper foil that is usually available will devolve within month
in the salt-water environment.
The ground shoe that is to be mounted in the water, we recommend
a bronze or copper plate with dimensions of 6-8 inches wide and
a length of 16-18 inches. The theory of the 100 square feet is
a myth, and will be address later.
This plate should be mounted deep under the waterline, as when
the vessel heels, it does not come out of the water.
The 2-4 bolts that come through the hull for mounting should be
all strapped together giving as much transfer of RF grounding
as possible.
Another fallacy from the marine yards when mounting the plate
is that the yards will take 5200 sealant and slap the bronze up
against the hull and then bolt it to the hull.
This is a poor solution for a few reasons.
By placing ¼ to 3/8 spacers on the outside of the hull.
The bronze now receives water from both sides, thus doubling the
capacity to water and lowering the resistance to 4-12 ohms to
water, which is ideal for grounding.
Some vessel owners feel that raising the ground plate will slow
their vessel by a few hundredth of a knot.
A deep conversation with a few renown marine architects state
there is more drag from the growth on the hull of the vessel then
from the extended ground shoe.
Once the shoe is mounted then from the inside the placing of 2-4
copper straps to each terminal of the ground shoe, and with both
sides of the ground plate exposed, will be more then sufficient
grounding for your electrical needs.
RF connections:
The RF connections to and from the ATU to the main radio are also
of extreme importance.
The coax connection from the radio to the tuner can be either
RG-8 or mini-8 coax 52-ohm coax. It should be marine grade, which
means that the copper shield is silver tinned, and not the typical
copper color. This type coax will not rot after 6 months of use.
This rotting is identified by an off color green around the connector.
The PL-259 connect should be stainless steel or the new silver-tin
PL-259 which makes the soldering much easier. Proper solder of
this connection is covered in another paper.
The coax is now connected to the RF output of the radio to the
tuner. There is another control line, which will send DC and a
key/start command to the ATU to pre-sense the ATU processor for
setting up the frequencies in the ATU.
The radio is also grounded to the ground shoe via a 1-2 inch ground
strap with the same quality as previously mentioned.
At the ATU a ground strap is then also attached to the same ground
shoe to the ground wing nut on the tuner. Again this copper strap
to the ground shoe, from the ATU should not exceed 5-7 feet.
With the ATU installed, along with the radio, the counterpoise/radials
need to be installed.
Counterpoise/radials:
These counterpoise or radials will attach to the same grounding
lug on the ATU as the copper/bronze strap going to the ground
shoe.
The twin lead ladder lines with be laid in the hull on both sides
of the vessel.
Each counterpoise will have certain lengths, with the opposite
side being shorter than the other. This is to maintain the different
frequencies of the marine bands, and or ham bands. The line should
be approximately 3-4 feet apart understanding that many vessels
starting from the rear lazerette, that these counterpoise, line
will run together either via the engine compartment or prior under
the berths. This is acceptable as long as you do not run then
on top of each other for distances greater than 10 feet.
This will now conclude the installation of a good RF grounding
and counterpoise system for your vessel.
Additional Filtering:
Additional filtering is deemed necessary to perfect your ground
and RF system. RF inline filters placed both at the input of your
radio and another inline filter placed at the input of the ATU.
These filter minimizes the RF feed back the can cause intermod,
and interference to your radio system, and prevents RF feedback
to your TNC modem and radio. Sometime this is sensed with RF biting
while your talking into your microphone.
These devices are also called RF balun, and are made up of RF
clamshell iron filters.
These RF core chokes are3 also installed in the computer cables
from the laptop to the TNC/modem. Each located at the ends of
each cable.
Another set of claim shell filter chokes are installed at the
beginning and end on the cable connecting.
DC Blocking:
The final set is resolving the interference problems is that stopping
the DC loop that accurse when connecting all these straps and
cables to a common ground source.
The copper ground strap that connects from the radio to the ground
shoe carries a DC component which is also referred to as a ground
loop. This DC loop also causes interference and can be eliminated
by placing a DC block inline with the strap. This stops the DC
from passing from the antenna tuner back to the radio. The DC
block contains a special capacitor that stops the DC, but allows
the RF to pass to the ground shoe.
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