Why is the boat not positioned if the VHF is off?
The NMEA output of your GPS is connected to your computer (on a
serial port, a USB-NMEA
adapter) or any other device that allows the computer to
receive NMEA data for positioning the boat on the map using your navigation
software. You also have a VHF DSC which must also receive
the NMEA data to know the position in case of need to trigger a
distress call. Other devices may also be connected to the same
NMEA output of the GPS. You have noticed that if you turn off the
VHF your boat is no longer positioned and your software sends you
an alarm unknown position. In the best case, positioning occurs
from time to time but the position is not refreshed as it should
do every 2 seconds.
This is because the NMEA input of your VHF is not opto-coupled
(isolated) and so it becomes very low impedance when its circuit
is no longer powered (in fact your GPS is trying to power your VHF
with its NMEA output on the entry NMEA of the VHF which of course
can not succeed). The NMEA signal is therefore "overloaded" to
such a level that it becomes unusable by any other device
connected to this output.
The only solution to this problem is the installation of a NMEA
How does a NMEA amplifier work?
A quality NMEA amplifier has a NMEA input that perfectly meets
the specifications of the standard. This input is opto-coupled
(isolated) so that it is completely insensitive to what happens on
the other side of the opto-coupler. It is totally insensitive to a
possible changing of the mass level. The GPS is perfectly
protected against any voltage return that may come from a fault in
a device, wiring, radio-frequency radiation from a wrong tuned
transmitter etc ... On the other side the optocoupled input stage
is a signal calibration stage to rectify the flanks and the level
faults. Finally are the buffers (usually 4, 1 per output). These
buffers will generate the signal that will be sent to the devices.
While respecting the voltage levels of the standard, the buffers
are able to provide much more current than the GPS can provide. It
is now possible to apply this signal to devices whose input
impedance is much lower than the NMEA standard. The buffers being
completely independent, whatever happens on one output can not act
on the others.
The NMEA amplifier AMN010-2:
is a high performance NMEA amplifier. Its powerful, independent
outputs can be used with any device that has an isolated NMEA
input or not, regardless of its input impedance. It has an
auxiliary output and input to couple as many AMN010-2 modules as
needed to achieve the desired number of NMEA outputs without
mobilizing any of the power outputs. Its amplification ratio is
greater than 1000.
the complete information about the NMEA amplifier AMN010-2