LNB - Low Noise Block

LNB (LOW NOISE BLOCK DOWN CONVERTER) A device mounted on the dish, designed to amplify the satellite signals and convert them from a high frequency to a lower frequency. LNBs can be controlled to receive signals with different polarisation. The television signals can then be carried by a double-shielded aerial cable to the satellite receiver while retaining their high quality.
A universal LNB is the present standard version, which can handle the entire frequency range from 10.7 to 12.75 GHz and receive signals with both vertical and horizontal polarisation. The most commonly used LNB is actually called LNBF, the F standing for Feedhorn (in built).

An LNB can be either single (one output), Twin (two outputs), Quad (four outputs), Quatro (four outputs), or Octo eight outputs). A Twin LNB would be required when more than one receiver is used allowing the viewing of different channels on two independent satellite receivers.
Low noise block converters have a hard life; they operate in extremes of temperature and humidity, and although they generally have a very low failure rate they do not last forever. Some fail as they get older, others suffer a drop in performance, resulting in poor picture quality.

Universal LNB

A universal LNB can receive both polarisations and the full range of frequencies in the satellite Ku or C band. Some models can receive both polarisations simultaneously through two different connectors, and others are switchable or fully adjustable in their polarisation.

Here is an example of Universal LNB specifications:

Local oscillator (LO): 9.75 / 10.6 GHz.
Frequency: 10.7–12.75 GHz.
Noise Figure (NF): 0.5 dB.
Polarization: Linear

LNBF - Low Noise Block Feedhorn

Direct Broadcast Satellite (DBS) dishes use an LNBF (“LNB feedhorn”), which integrates the antenna’s feedhorn with the LNB. In the case of DBS, the voltage supplied by the receiver to the LNB determines the polarisation setting. With multi-TV systems, a Twin LNB allows both to be selected at once by a switch, which acts as a distribution amplifier. The amplifier then passes the proper signal to each receiver according to what voltage each has selected. LNBFs should never be used on Prime Focus dishes, this always results in extremely poor performance.

LNB Noise Figures

Theoretically the lowest noise figure obtainable from any device is limited by any components in the signal chain with the highest thermal noise. The first component in the chain would be the detector circuit and on a Universal LNB this would be a pin diode. At Ku band the detectors are rated at manufacture to about 40K which converts to a figure of 0.5 dB.
Presently the LNB market is active by those selling what appears to be an extraordinarily good device, some use the best of the component batch - and end up with good performance overall (rare). Some modify existing LNBs by the use of fancy smoothing circuits to eliminate any further incoming noise from the power supply - there are definite improvements when used with cheaper receivers, especially those with switch mode supplies (few).

The LNB sets the noise floor for your entire satellite receiving system. Less noise here means that more signal will actually arrive at the receiver. Today's high performance LNBs use Gallium Arsenide (GaAs) semiconductor and High Electron Mobility Transistor (HEMT) technologies to minimize the noise level of the LNB. Note even the best LNB will only marginly improve reception, the only way to significantly improve reception (Gain), is to use a larger dish.

LNB Gain

The lnb gain tells us how much the incoming signal is amplified before being sent off down the coaxial cable to the receiver. The range of gain specified is between 40dB and 70dB (somewhere between 10,000 and 4,000,000 times the incoming signal power). At first sight, the highest gain you can get would be the obvious thing to look for; however that is not the only criterion when it comes to LNBs.
When you have a large dish looking at high power satellites like Astra 2A and Astra 2B, the gain can be so high that the receiver is overloaded with signals. These can 'swamp' the lower powered Astra 2D satellites signals.

Even if the receiver itself can handle a massive amount of signal, there can be problems within the LNB itself when large amounts of amplification are employed. This leads to the generation of spurious signals and distortion. This distortion will interfere with the reception of your signals.
To let the demodulator in the receiver work effectively, the gain at all frequencies, should be the same. This is not a very difficult requirement to meet, except perhaps at the edges of the band, as long as the LNB is constructed properly.

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LNB Skew

Skew refers to the angle of the LNB relative to the rest of the dish, you are maximising the gain of yoy LNB. This could be the difference between a good watchable picture or poor reception.
All geostationary satellites are located above the equator (the Clark Belt).
They are placed here to match the rotation of the earth. Inside a satellites footprint, LNB skew is not all that important, however as you approach the fringes of the signal footprint it becomes ever more crucial to getting good reception.
The LNB is kept in place by either a screw or a nut. Loosen these and it will be possible to rotate the LNB left ot right. The degree of tilt varies depending on your location. Getting the righ LNB for your location could

C120 LNBs

C120 LNBs do not have a feedhorn in the same way as LNBFs, a feedhorn preferably one from the manufacturer of the LNB, has to be attached to the LNB. C120 LNB's should be used on offset dishes larger than 1.2m metres, though with an offset feedhorn. They will almost always produce superior performance than a LNBF, particularly when using Invacom C120 LNB's. Other makes such as MTI C120 perform very poorly in these circumstances.

Feedhorns

The most common type of feedhorn manufactured today is called a scalar feedhorn. This type of feed has a large circular plate with a series of three or four concentric rings attached to its surface. The scalar rings conduct the incoming signal from the outer edges of the focal cloud to the large waveguide opening located at feed center. The scalar feedhorn primarily sees or illuminates the inner portion of the antenna's surface area, while attenuating the signal contribution from the outer portion of the dish by 8 to 22 dB, depending on whether the dish is deep or shallow in its construction.
Molecular motion within the Earth itself generates random noise which permeates the entire electromagnetic spectrum used for the transmission of satellite signals. This random noise is many times stronger than the satellite signals reaching any location.
The attenuation or illumination taper provided by the feed sharply reduces the reception of the Earth noise which lies just beyond the antenna's rim.

The outer area of the antenna's surface therefore acts more as an Earth shield for the feedhorn than as a contributor to the overall signal gain of the receiving antenna.

Inline Amplifiers

Inline amplifiers will not increase your weak Astra 2D signals, infact they will probably reduce your signal. An Inline Amplifier should be used when the cable run from the LNB to the receiver exceeds 75 - 100 feet; install the amplifier between the LNB and the Multiswitch or satellite receiver. Inline amplifiers are powered by the voltage already present on the satellite signal coax. If your signal is nott strong enough in the first place, you are really only amplifying noise. An Inline amplifier is unfortunaly of little or no use when trying to increase a weak signal, it will also 'boost' stronger signals swamping your already weak signal. A better LNB or larger dish will give far better results.

DiSEqC

DiSEqC™ (Digital Satellite Equipment Control) system, which is a communication bus between satellite TV system receivers and peripheral equipment using only the existing coaxial cable. DiSEqC™ can be integrated into consumer satellite installations to replace all conventional analogue switching, providing a standardised digital system with nonproprietary commands and enabling switching in multi-satellite installations.