What is an LNB – and what is it for?

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What is an LNB – and what is it for?
The low noise block is the actual heart of the satellite antenna. Basically, it’s a cavity resonator which receives at one end the focused satellite signals that are reflected from the antenna and then processes these signals. Similar to an organ pipe it oscillates and triggers internal dipoles which convert the transmission energy into electrical signals. An additional electronic switch amplifiesthesesignals before they are sent to the coax cable and converts them into a lower frequency in order to minimise signal loss in the cables.
Even though the descriptions may sound like there is a big difference between individual
models, most currently used LNB types use the same technology, the major distinguishing
factor being the noise figure which has been reduced to the theoretically lowest possible value of 0.3 dB in the most recent models. A universal LNB is used to divide the Ku band – which is predominantly used in Europe – into two partial frequency ranges.
Each LNB can only be used for a single frequency
band, because the S, C and Ku bands each require different cavity resonators. There are also individual types for linear and circular signals, which mainly differ in the way the internal dipoles are arranged.
The power supply for the electronic switch is of particular interest. The power is provided
by the receiver and transmitted over the coax cable. The coax cable therefore not only transmits the reception signals from the antenna to the receiver, but also the required operating power from the receiver to the LNB (together with additional control signals).
Switching features
when changing
Transponders have one of two different polarisations (horizontal/vertical and left/right circular, respectively). That’s why the receiver has to tell the LNB the polarisation for any given signal, so that the appropriate dipole can be activated. The voltage of the power supply takes care of this: 14 V acti Satellite
signals are very weak. That’s why we need a parabolic antenna to focus them and a low noise block, also known as LNB or sometimes LNBF, universal LNB or feedhorn mounted in its focal point to collect them. But what exactly happens inside this small component?the LNB with an individual coax cable, thus allowing signals to be received independently for each of these receivers.
A quattro LNB with a switched output delivers
all four possible signal configurations(horizontal/vertical and low/high band) simultaneously
and is not suitable to be connected directly to a receiver. Its output signals are connected to a switching matrix. With the help of matrix cascades and intermediate amplifiers
it is then possible to connect any desired number of receivers to this system.
Multi feed for
Multi feed means receiving signals from more than one satellite simultaneously with a fixed satellite antenna.The advantage of sucha solution is that switching between satellites takes place very quickly. However, several disadvantages
or restriction are associated with multifeed reception:
Due to the reduced reception efficiencyitisnecessary to go for a larger dish.
Not more than four satellites can be selected.
The possible orbital range comprises not more than +/- 10 degrees (less rather than more). Satellites must be spaced at least
What is an LNB – and what is it for?
Heinz Koppitz
Basic Functions vate the vertical polarisation, while 18 V activate
the horizontal polarisation. Even though DiSEqC has developed into a very powerful control tool with more than 256 commands, it is still not used for switching between the polarisation levels.
A universal LNB features a second switching
mode for the extended Ku band. Since the frequency range of satellite receivers is not wide enough the actual frequency range has to be split up into two partial ranges. Switching
between these ranges is controlled by a 22 kHz signal which the receiver also sends to the LNB when selecting a certain channel. This 22 kHz signal is also used as carrier frequency
for DiSEqC control commands in more complex system configurations.TheseDiSEqCcommands serve for controlling multiswitches and antenna motors (see issue 189).
Various designs
There are several design types for different purposes. The table lists the most common LNB types for the extended Ku band and indicated
how they are used:
Motorised dish
Single LNB
One receiver
One satellite
2 – 4
Twin LNB
Two receivers
One satellite
2 – 4
Quad LNB
Four receivers
One satellite
2 – 4
Quattro LNB
Multiple users
One satellite
2 – 4
Octo LNB
Eight receivers
One satellite
2 – 4
Monoblock 2
Two receivers
Two satellites
2, fixed
Monoblock 4
Four receivers
Two satellites
2, fixed
Monoblock 8
Eight receivers
Two satellites
2, fixed
Single LNBs are suitable for individual reception.
The reception principle of a single LNB is also included in flat antennas.If the receivercomes with DiSEqC 1.2 and features the commands
required to control a motorised dish, a single LNB in combination with a dish motor allow you to receive signals from any number of satellites. This makes for a very elegant configuration,except for the time you have to wait until the antenna has moved to the right position when selecting a channel from a different
(i.e. not currently tuned into) satellite.
All other designs are only suitable for fixed antennas. Twin, quad and octo LNBs are intended to support two, four or eight receivers.
Each of these receivers is connected to three degrees apart from each other
A DiSEqC command is required for switching between signals.
If more than one receiver is to be connected a signal matrix is required.
It can be difficult to properly
adjust the antenna.
monoblock LNB
This dual LNB is the simplest solution to achieve multifeed reception for two satellites.
This design consists of two independent LNBs in a single case. The two LNBs can be automatically addressed with any DiSEqC 1.1 receiver. However, they are only available for satellites with a fixed3-degreeor6-degree spacing. In Europe, for example, there are monoblock single, twin and quad LNBs for the Ku band, which have a pre-defined spacingof 6 degrees (for Astra1/Hotbird or Astra2/Astra3A, for example).


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Low Noise Block-downconvertor (so called because it converts a whole band or "block" of frequencies to a lower band).

Is there actually a different LNB for prime focus dishes + offset dishes? Surely an LNB's innards are the same and the feedhorn or the C120 flange is the only difference?

In the old days, LNB noise figures were high, the gain (amplification) was low and satellite transponder power was typically 20 Watts. Imagine trying to see a 20 Watt light bulb 24,000 miles away! (You'd have trouble seeing a 20W bulb at the end of a 24 yard corridor).

So, an LNB and feedhorn had to be matched to the dish. The internal antenna of the LNB had to be at the exact focal point of the dish and the horn had to be flared in such a way that, with the LNB at the focal point, the horn could "see" the exact circular area of the dish - no more and no less. If it was less then it wasn't collecting signal from the full area of the dish. If it was more, it was also collecting unwanted "noise" from any warm object (wall) or from the sky behind the dish.

A good compromise was to take just part of a much larger paraboloid dish and mount the LNB in an "offset" position. The curvature of this partial dish is such that the focal point is now much lower so the LNB and feedhorn no longer obscure the signal path as they would with a "prime focus" dish.

Nowadays, satellite transponders can produce typically 50 or 60 Watts and LNBs have higher gain and lower noise figures. With these strong transmissions, you can get away with murder. People stick any old thing on the end of the boom arm - which rather explains why one man's 0.6dB LNB is another man's nightmare when the signal strength is not optimum! The Sky minidish, for example, is a compromise between size and performance. It's very important that the LNB matches the dish exactly. This is one good reason why the dish comes with its own LNB.

The manufacturers might "fudge" the issue if asked. After all, if they admit that their LNB works best with, say, an 80cm Lenson Heath dish and you just bought an 1 metre dish made by someone else, you might not be too happy.

If you "mix 'n' match" by picking a 60cm dish and a Universal LNB at random, the chances are that the performance could be no better than that of the Sky minidish.

As a general rule, any standard LNB will work with a circular (prime focus) dish or an offset focus dish which is taller than it is wide (which "looks" circular when viewed by the LNB).

However, a dish which is wider than it is tall will need a special LNB.
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