LNBs
This text is machine translated.
About LNBs
The basic function of A SATELLITE antenna with boom arm is always the same! The transmitter signals coming from the satellite hit the reflector mirror and are bundled into a point at the front of the mirror. The LNB sits exactly in this "focal point". You can read in the following sections what exactly happens in the LNB, what differences exist in the LNBs and what must be observed when replacing an LNB.
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What is an LNB?
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How does an LNB work?
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What are the different LNBs?
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What must be taken into account when replacing an LNB?
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Frequently Asked Questions
What is an LNB?
The antennas are housed in the LNB.
LNB is the abbreviation for Low Noise Blockconverter . Some LNBs are also referred to as LNC (Low Noise Converter ) or LNF (Low Noise Feed) or LNBF (Low Noise Block Feed).
The task of an LNB is to receive, amplify and implement the signals transmitted by the satellite. These are in the lower frequency band in the range of 10.7 to 11.75 GHz and in the upper frequency band at 11.8 to 12.75 GHz . Unfortunately, these frequencies are far too high for them to be transmitted to the receiver via an antenna cable (coaxial cable). In addition, the transmission power of a TV satellite is limited and therefore the signal strength is not particularly pronounced even in the center of the reception range.
This is why reflector mirrors are used in order to be able to bundle THE SATELLITE signals to the LNB. With the help of low noise amplifiers, the reception signals are then increased to the required level before they can be converted to a lower frequency in the range of 950 – 2150 MHz (converted).
How does an LNB work?
Frequencies and polarization
In order to be able to use as many transmission channels as possible in the frequency range of the TV satellites from 10.7 - 11.75 (low band) or 11.8 - 12.75 GHz (high band), the electromagnetic waves emitted by the satellite are polarized both horizontally and vertically. This means that the electromagnetic waves oscillate horizontally as well as vertically . This allows the number of possible transmission channels to be doubled in the same frequency band . Alternatively, there is circular polarization (clockwise or anticlockwise), which does not play a significant role in the reception of TV satellites in Central Europe.
Construction:
An LNB initially consists of a hollow conductor . This is a metal tube specially tuned to the range of the reception frequency . On the side, which is aligned with the reflection mirror, only a plastic cap prevents the penetration of dirt and moisture. On the other side there are two metal pins which are aligned horizontally and vertically and which protrude into each of the hollow sections. These two short metal pins are the actual SATELLITE antennas , whereby each antenna receives a polarization level.
The attached protective cap was removed on the adjacent figure to obtain a free view into the LNB. From the right, the horizontal antenna protrudes and from the top, the vertical antenna protrudes into the waveguide.
Note:
In older SATELLITE systems, LNBs were used with only one antenna pin. This was either mechanically rotated by 90° by an electric motor or a magnetic polarizer was connected upstream, which "turned" the electromagnetic waves in the hollow conductor. In addition, "old" LNBs could only receive the lower frequency range of 10.7 - 11.75 GHz. LNBs, which were also able to receive the upper frequency range, were also called universal LNBs.
Electrical function
However, only LNBs with two antennas are now used. The function is shown in simplified form in the adjacent diagram in Figure 3:
The signals of the two antennas in the circular waveguide (horizontal and vertical) are first amplified with a low-noise amplifier (AMP) and divided into the two frequency bands (low and high) via frequency filter (splitter).
In the lower frequency band, the signals are then mixed with a 9.75 GHz oscillator. For the upper frequency band, a second oscillator operates at 10.6 GHz . The resulting intermediate frequencies (IF) are amplified again (AMP) and fed into a switching matrix.
The switching matrix connects one of the four possible intermediate frequnez levels Horizontal Low (HL), Horizontal High (HH), Vertical High (VH) or Vertical Low (VL) to the output.
The switch matrix receives the information on which IF level is to be connected to the output via the coaxial connection cable from THE SATELLITE receiver . This is programd accordingly and "knows" on which polarization level H or V and in which frequency band (low or high) certain stations are to be received.
However, in order for the LNB to fulfill the tasks described above, it must be supplied with power. For this reason, THE SATELLITE receiver generates a DC voltage and sends it to the LNB via the antenna cable (coaxial cable).
If vertically polarized transmitters are to be received in the lower frequency band (VL), the SATELLITE receiver regulates the direct voltage on the antenna line to 14 V. For horizontally polarized transmitters in the lower frequency band, THE SATELLITE receiver increases the voltage to 18 V. .
The switching threshold between horizontal and vertical is 15 V. This ensures that the switch matrix in the LNB still switches cleanly, even if the LNB voltage is reduced by the use of multi-switches or other switches and is clearly below 18 V.
If transmitters are to be received in the upper frequency band, the direct voltage of 14 or 18 V is added to a 22 kHz control signal.
What are the different LNBs?
Universal single LNB
This LNB is used to connect a single receiver . The LNB receives horizontally and vertically polarized channels in the lower and upper frequency bands . The block diagram shown above corresponds to the design of a universal single LNB.
Individual subscribers SAT.system
Two receivers on ONE SATELLITE line – please not!
Since the voltage or the control signals on the antenna line determine in which frequency band horizontally or vertically polarized programs are received, it does not really make sense to operate two receivers on a single LNB with a distributor.
If an existing system with a single LNB is to be extended to a second subscriber, only the single LNB has to be changed against a twin LNB.
Universal twin LNB:
A universal twin LNB is basically constructed like a universal single LNB. The difference is only in the number of outputs . A twin LNB has two separate outputs in which the connected receivers can access the switching matrix independantly of each other (see figure 3) and receive any desired transmitter.
SAT system for two subscribers
Universal Quad LNB
A universal quad LNB has four outputs that access the switching matrix independantly of each other (see figure 3). As a result, up to four receivers can be connected to the LNB and each subscriber can make their individual program selection independant of the other.
SAT system for up to four subscribers
Universal Octo LNB
With a universal Octo LNB, there are eight outputs that can access the switching matrix (see figure 3). Thus, this LNB offers sufficient connection possibilities to supply even larger houses with many subscribers perfectly.
SAT system for up to eight subscribers
Quattro LNB
The internal switching matrix is missing for a Quattro LNB (see Fig. ..). This means that the four ZF levels Horizontal Low (HL), Horizontal High (HH), Vertical High (VH) or Vertical Low (VL) are directly connected to the outputs. An external multi-switch must then be connected to the outputs of the LNB, which takes over the task of the switching matrix. The subscriber connection lines then only have to be routed to the multi-switch and not to the LNB. This is the ideal solution for large community facilities , with several multi-switches arranged in series and a large number of participants.
Important:
Unnecessary outputs of the multi-switch (MS) must be provided with a terminating resistor. Instead of the resistor, you can also use a coaxial cable with SATELLITE connection box.
SAT system with multi-switch (MS). The number of nodes depends on the number of MS outputs.
Monoblock LNB:
On the geostationary track, the satellites are lined up like pearls on a pearl chain . When aligning the mirror, the reflector is set so that the signals of the desired satellite, e.g. Astra, bundle exactly in the LNB input (see figure 9, sketch A).
The reflector is also illuminated by all other satellites at the same time . Due to the "oblique" angle of incidence, each satellite has a different focal point .
With satellites (groups) standing closely next to each other, such as Astra on 19.2° east and Hotbird on 13.0° east, both satellites can be received with a fixed antenna and a monoblock LNB. For this purpose, there are two separate LNBs in the monoblock LNB casing, which are precisely aligned to the angle difference of 6.2° (see figure 9, sketch B).
Figure 9: With a monoblock LNB, two satellites can be received simultaneously despite a fixed SATELLITE dish.
A monoblock LNB can only be controlled with A SATELLITE receiver with DiSEqC control. The example shown offers the possibility to connect four receivers.
The monoblock LNB s are also available in different versions for connecting one or more subscribers. However, the switching options of 14/18 V and 0/22 kHz are no longer sufficient to cover all reception ranges and to be able to switch between the two LNBs. For this reason Eutelsat, in cooperation with Philips, developed the DiSEqC standard (Digital Satelite Equipment Control) in the 1990er. The DiSEqC switching commands are transmitted as digitally coded data words with the help of the 22 kHz signal from the receiver to the LNB or also to a multi-switch.
If two far apart or even three or four satellites are received with a fixed antenna, multi-feed holders are a good choice. The holders essentially consist of a rail on which the LNBs can be adjusted exactly to the focal points of the respective satellites.
Single cable LNBs
With the LNBs mentioned above, THE distribution of THE SATELLITE signals to the respective subscribers always takes place star-shaped from the LNB or from the multi-switch. But sometimes it is very time-consuming to lay the appropriate cables. Especially when an existing cable connection distribution , in which the coaxial cable was laid from one socket to the next, is to be converted to SATELLITE reception.
For this reason, the Unicable-Unicable-System was developed according to DIN EN 50494. A Unicable LNB (or Unicable multi-switch) provides a certain number of ports, whereby one port is assigned per SATELLITE receiver. The port is nothing more than a fixed frequency in the range of 950 - 2400 MHz, on which the transponder data (desired program) requested by THE SATELLITE receiver is transmitted. However, this only works if the SATELLITE receiver also supports the protocol according to DIN EN 50494 . Theoretically, up to 32 ports can be assigned, but this is not used to the full extent in practice.
On the receivers with the shortest path to the LNB/multi-switch, the ports with the highest frequencies should be set. This means that the losses caused by the cable attenuation can be kept as low as possible.
What must be taken into account when replacing an LNB?
Before a new LNB can be replaced with the existing LNB, there are a few points that should be clarified in advance. werden sollten. Otherwise, there may be unpleasant surprises when installing the new LNB.
Mechanical fit
LNBs are usually attached with a circular bracket, which is placed around the feed pipe. The current LNBs have a feed diameter of 40 mm . Occasionally you will find LNBs with 23 or 60 mm. In this case, adapter rings help. The new LNB must be so tight that it cannot be rotated by hand.
Select the correct LNB
When selecting the new LNB, it is necessary to clarify exactly which LNB is required . The number of outputs is not the sole decision feature. If a Quattro LNB is installed instead of a Quad LNB and connected directly to the receivers, a malfunction is pre-programd.
For many LNBs, a noise factor is specified in decibels (dB). This value indicates the quality of the internal amplifier. The lower the noise factor, the better the signal quality . An LNB with low noise factor is particularly noticeable in difficult reception conditions (rain or snow barriers).
Safety at work
Many SATELLITE antennas are mounted on roofs or on walls and can only be accessed via roof hatches or ladders. In this case, make sure that they are safe and use safety harnesses and safety cables. Check the required ladders for stability and also observe any other safety regulations, such as non-slip footwear.
Electric connection
LNBs are connected via screw-able F-plugs. Here, the plugs are screwed onto the stripped coaxial cable . It is important to be careful that no wire of the shielding braid has contact with the center conductor . . In this case, this would be a short circuit of the supply voltage and the LNB cannot function.
The connectors must be protected against moisture by appropriate measures . Some of the LNBs have sliding covers that protect the connection plugs.
Alignment of the antenna
When replacing the LNB, it is recommended to check the alignment of THE SATELLITE antenna with the same. Simply connect A SATELLITE finder to the LNB output. The antenna cable coming from the receiver or multi-switch is connected to THE SAT Finder.
Important!
When adjusting, do not touch the reflector mirror but the reflector holder. This prevents the measuring result from being distorted by twisting the reflector surface.
Frequently Asked Questions
My SAT-system didn't work anymore. What can I do to check the LNB?
First, the LNB can be checked for visible damage. Especially with older LNBs, the cover caps like to become loose or cracked and water enters the hollow conductor. Then the connections of the LNB (F-plug) should be unscrewed and checked whether a thin wire of the shield might have contact with the center conductor and thus the supply voltage for the LNB is short-circuited. The supply voltage of 14/18 V/DC can be measured with a simple multimeter on the screwed-off F-plug. For LNBs with several outputs, it is easy to check whether an LNB output is defective or whether the cable is faulty by swapping the connection cable.
There was a short circuit on the F-plug which was removed. But the SATELLITE system still doesn't work.
Many SATELLITE receivers have electronic fuses that switch off the LNB voltage in the event of a short-circuit. In order for the LNB voltage to be switched on again, the receiver may have to be disconnected from the 230 V mains supply for a short period of time.
My SAT-system has a bad picture, although the mirror holder is really firm and the mirror didn't turn in the storm. What to do?
Trees or bushes may have grown so large that they partially or completely shade THE SATELLITE mirror. In older systems, it can happen that the boom arm is lowered due to weather influences or material fatigue and that the LNB is no longer at the focal point of the reflector surface. This can occur especially in multi-feed systems or when using large and heavy LNBs with many connections.
Why is the LNB at the bottom of THE SATELLITE dish and not in the middle?
The reflector surface with an offset mirror is specially shaped so that the focal point lies at the bottom and not in the middle. This makes the reflector quite steep, allowing snow, leaves or dirt to slip down slightly or wash out of the rain. In addition, the reflector surface is neither shaded by the boom nor by the LNB.
An LNB with 23 mm feed diameter was installed on my SATELLITE system and the new LNBs have every 40 mm.
If the LNB holder cannot be converted without great effort, it is often cheaper to replace the complete outdoor unit so that 40er LNBs can be mounted.