March/April 2019 Marine Electronics Journal 35
atellite TV has become widespread on yachts. In the com-
mercial world, some unions require Sat TV for the crew as
part of the contract, making it a mission critical system. In
the US, we use two different providers, DIRECTV and DISH
Network. These two competitors use different methods to
send Sat TV to our boats. Satellite TV is broad-
cast in the Ku- and the Ka-band. Ku-band
broadcasts at about 13 GHz and Ka-band
broadcasts at about 26 GHz. DIRECTV is the
only satellite service provider in the US that utilizes Ka-band, and at the
time of this writing, it broadcasts all of its high-definition content using
the Ka-band frequencies.
The satellites used for television are in geosynchronous orbit around
the earth. This means that they are located above the equator and appear
to be motionless, staying at the same longitude. We name these satellites
based on their location; sat DTV101 is at 101 degrees longitude at the
equator (0 degrees latitude) and 22,000 miles up. Since the satellite stays
still and our boats move around, we use “stabilized” antennas, often
called “in motion” antennas in the marketplace. These dish antennas use
gyros and motors to keep the dish aimed right at the satellite. There are
some great apps for finding satellites in the sky, like SatBeams.com.
Direction matters
Satellites are very directional. Their “footprint” is the area of the
planet where the satellite can be received. DIRECTV has a footprint
that covers the US, as does DISH Network. The actual footprint of the
satellite extends offshore some distance but eventually dies. The area
where you can expect reception to be acceptable is determined by
where the satellite is aimed. When you get near the edge of the coverage territory shown by the satellite footprint, the signal becomes weak
and unusable. Use of a larger, higher-gain antenna dish means a larger
coverage area.
Satellite reception can be a confusing thing. The dish has to see the
satellite for it to work, so there are conditions where Sat TV does not
work, like under a bridge or in a canyon. We need a view of the sky
and we also need to see toward the equator (typically south from these
parts).
To see the spacecraft 22,000 miles above the equator, we must look
up. The angle that we look up is called the elevation angle or the look
angle. This elevation angle is based on latitude or how far north
(south) of the equator our dish is located. Since our boats move
around quite a bit, the elevation angle changes. In far north latitudes,
the elevation angle is very low on the horizon, and this presents problems requiring larger dishes. In southern latitudes, the elevation angle
is very high or straight up, requiring more complex three-axis dishes.
In the lower 48 states, satellite reception is possible with fairly small
dishes, 2 feet or less.
TECH TALK
BY JOHN BARRY
CMET & NMEA Instructor
TALES & TIPS FROM THE BILGE
Satellite TV: installation basics
A satellite is actually a radio which receives and transmits signals
originating from the satellite TV providers. These highly specialized
radios contain patch transponders, which send the satellite signals for
various channels. When we test our satellite TV installation by using
“Check Signal Strengths” in the satellite receiver menu, we see the
strengths of the signals received from each of the transponders. There
are even and odd numbered transponders.
The elliptical shape of the sat antenna dish focuses the RF energy
coming from the satellite on to the LNB (Low Noise Block converter).
The LNB has two jobs. First it is an amplifier to boost the very weak
signal from the satellite to a usable level. This is the Low Noise Amplifier part of the LNB. The other job that the LNB does is to convert the
RF signal from Ku- or Ka-band frequencies down to a lower frequency.
This gives us a frequency that will work through normal coax. This is
the Block Converter part of the LNB.
Installing multi-switches
The LNB is an active device. Once the signal is received by the LNB,
amplified and block converted, the signal goes to a satellite receiver.
The sat receiver supplies 21VDC to the LNB through the coaxial cable
and this is why we need a good shield connection for power as well as
shielding. The LNB outputs 13 and 18 VDC levels with RF superimposed in a composite signal containing the even and odd channels
from the transponders. This allows the receiver to detect all of the
(Continued on page 49)
When testing the satellite TV installation by using Check Signal Strengths in
the satellite receiver menu, you’ll see the strengths of the signals received
from each of the even and odd-numbered transponders. The graphic is
excerpted from the NMEA 0400 Installation Standard training document.
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