NorthStar 961 Install Manual page 34

SECTION TWO - Installation
Installing an 8410
ACU (for use with
the AN150 only)
Page 24
If you want the 961/962 to receive differential corrections, but you're
using the AN150 antenna, you must use the Northstar 8410 Antenna
Coupling Unit (ACU). Although similar in appearance to Northstar loran
ACUs, only the unit labeled "8410" will work with the internal DGPS
receiver. The ACU also serves as a sturdy mounting base for the whip
antenna.
Mounting the 8410
The Northstar 8410 differential receiver antenna should be mounted as
high as conveniently possible (but not at the highest point), and as far
away as possible from other antennas. If you have several possible
antenna locations, you can evaluate each one by operating the system
with the DGPS antenna mounted temporarily in each location.
The best location is one providing the lowest "noise" count and the
highest Signal-to-Noise Ratio (SNR). "Noise" refers to atmospheric
impulse noise generated by thunderstorms and other conditions,
including vessel-generated noise and any on-board interference. You can
view noise and SNR values after turning the 961/962's power on; see
"Turning the system on" beginning on page 28.
Another major concern, precipitation static (P-static), must be addressed
when installing the differential antenna. The 8410 receives weak AM
signals and can be sensitive to nearby noise sources and P-static. P-static
generally appears only during rain or snow; consequently, it can easily be
overlooked during installation. Much like a loran antenna, the effects of
P-static can be minimized if the differential antenna is mounted so that it
isn't the highest metallic object on the vessel. The highest metal object
acts much like a lightning rod, attracting tiny static discharges in the
atmosphere. If the differential receiver antenna is the highest object,
these discharges can totally override the differential signals and cause
poor operation in bad weather. The optimum antenna location is high
and clear for fair-weather operation, but below the top of a metal mast or
other antenna for best foul-weather operation. On sailing vessels, the
ACU might be mounted on top of a low mast, or on the stern rail if no
other suitable location is available.
The 8410 differential receiver ACU can be mounted on a standard marine
antenna mount (1" - 14 threads per inch).
The ACU consists of two pieces, the body and the base. The circuitry is
sealed in a rugged, waterproof, fiberglass polycarbonate body, the upper
end of which is threaded to accept a standard 4-foot whip antenna. Use a
48-inch fiberglass whip, such as Shakespeare 4-foot #173 loaded or Radio
Shack #21-934. A large female thread is molded into the lower portion, or
base, of the ACU body, into which the upper half is secured (see
Figure 15, "ACU Assembly," below). A gasket and rubber O-ring on the
upper half provide weathertight sealing when the two halves are secured.
961/962 INSTALLATION MANUAL Revision A