Pagina 146 - TELE-satellite-1207

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TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 06-07-08/2012
tion is situated, and reaches zero at 81
degrees northern latitude. The maxi-
mum reception range can be achieved at
the equator at 81 degrees. Of course this
is only true for satellites with a geo-sta-
tionary orbit.
The angle ω is calculated starting from
a southern direction towards east or
west. In order to find the largest possible
number of available satellites (azimuth)
the following formula applies:
Towards east: λ + ω
(The correct algebraic sign has to be
used for the geographic longitude λ of
the location of the antenna)
Towards west: λ - ω
(λ is positive for all positions east of
the zero meridian.)
Figure 4b illustrates that the eleva-
tion changes automatically when the
antenna is rotated. This at the same
time implies that the antenna’s orien-
tation towards the satellites remains
unaffected and the polarisation levels
are the same for all receivable satellites
and therefore need not be changed.
Nevertheless, most receivers with inte-
grated motor control come equipped
with a device that corrects any possible
tilt of satellites.
An LNB with built-in positioner can
achieve this with either a coil with elec-
tric current flowing through it or an
impulse-controlled tongue. Both options
require a dedicated separate circuit
from the receiver to the LNB. However,
such measures are mostly obsolete
these days as satellites can be control-
led much more precisely than what used
to be the case.
Figure 9 compares two basic antenna
types for wavelengths of a few centime-
tres (approx. 10 GHz). For professional
applications only parabolic antennas
with a central reception unit (LNB) are
in use, while for private reception offset
antennas as shown in figure 9a are used
up to a size of approximately 1.2 metres.
With its more upright position an offset
antenna is less exposed to the elements
(rain, snow) and thus more convenient to
operate in the private sphere.
Fig. 9
Motor control
Traditionally, motorised antennas are
powered by an actuator which consists
of a shaft whose length is varied with
the help of a DC motor. Such a motor
has a uniform supply voltage of 36 V and
changes its direction through switching
the poles. A photocell is integrated in the
motor for exact positioning. It creates
the impulses for positioning through a
perforated disc.
When this technology first surfaced
external control devices had to be used
to control the antenna’s movements.
Soon, however, new receiver models
were introduced which were able to per-
form all required tasks and offered on-
screen displays to set up the system in
the first place.
36 V power packs usually supply cur-
rent with at least 2 ampere, while the
motors that are used generally require
less than 1 A. Unfortunately, however,
these devices are disappearing very fast
and in most instances are replaced by
so-called H-H motors.
Plus: sturdy design, hardly any
repairs needed, suitable also for larger
Minus:separate circuit (5 wires) must
lead to antenna, (+ and -36 Volt, ground,
+5 V for electronics, impulse circuit)
As more and more control functions
had to be provided for one or more
antennas and/or LNBs new and intelli-
gent ways had to be found for provid-
ing control impulses via existing antenna
cables. It then was a logical step to also
integrate motor control into this system
and using the regular antenna cable for
impulse transmission (DiSEqC). As LNBs
are supplied with 13 or 18 V depending
on the polarisation level it made sense to
use the same voltage for the motors as
well. A current of between 0.2 and 0.3
A is required for these motors and can
easily be generated by suitable receiv-
Plus: No separate line to the antenna
required; DiSEqC via existing antenna
cable is used instead. If motor is
detached for repair there is still recep-
tion of one satellite, for example Astra.
Minus: As the current for the motor is
13 V or 18 V depending on the polarisa-
tion level of the received signal, both the
speed and force of the rotation move-
ment vary. Not suitable for very large
So-called H-H motor controls have
been promoted by various companies
for some time now. However, they do
not fall into the polar mount category as
their movement rather follows the circu-
lar line-up of satellites above the horizon
that is shown in figure 4a. The antenna
cable transmits all control signals. Due to
their design H-H motor controls have to
be physically attached to the antennas,
which means that any repair requires the
complete antenna to come down from
the roof.
Plus: Rather easy installation, because
all you need to know is the geographic
latitude and the southern direction.
Minus: If repairs are necessary there is
nothing left for reception. Only suitable
for small dishes.
Practical example
We chose Munich is the location for our
practical example. First, we collected all
relevant data referring to our location
and then performed all required calcula-
tions we might ever need for installing
our antenna:
Geographic longitude:
λ = 12,1 degrees east
Geographic latitude:
φ = 48,1 degrees north
Verification of the reception range:
This means, in general the range to
the east λ + ω = 12 +77 = 89 degrees
and to the west λ - ω = 12 – 77 = -65
degrees for the reception of satellites –
azimuth +89 to -65 degrees. In actual
fact, however, we will probably only be