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LNB - Low-noise Block Downconverter


LNB

­­­­­­­The box on the front of a satellite TV dish that receives the satellite signal collected by the dish, processes it, and sends it via a coaxial cable indoors to a satellite TV receiver that is connected to the TV set. The box has a scalar horn antenna (the funnel with concentric rings) which collects the microwave beam and directs it into a waveguide, a short closed metal pipe. Two small metal pins project into the waveguide from the circuit board; these serve as antennas which convert the microwaves to radio frequency alternating currents. On the circuit board (center), these are amplified by a low noise amplifier, then a local oscillator and mixer convert the block of frequencies to a lower block of frequencies, which are output to a jack on the outside of the box (black objects, right) to which a coaxial cable is connected to carry the signal inside the house.


Figure: Ku band antenna with LNB (LNB + Feed horn + Wave guide)


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A Feed Horn is basically a horn antenna that lies between the transceiver and the antenna reflector in a satellite dish system. 
When receiving a signal from the satellite, the signal is reflected from the parabolic dish to the feed horn which then passes it on to the Low Noise Block (LNB) where it is down converted and processed further. The feed horn provides some level of gain and is designed to support a particular type of polarization, this way it can filter out unwanted signals by only allowing specific polarity waves depending on the polarization of the horn antenna.


Figure: Ku band LNB

Feed Horn is always located at the focal point of the parabolic dish and is simply attached to the LNB. 

An LNB with an integrated feed horn is called as an LNBF. Normally we use LNBF but called LNB.

On the transmit side, the signal is sent from the satellite modem to the Block Up-Converter for up-conversion and then amplification and then sent out via the Feed Horn towards the dish antenna sub reflector towards the main towards the satellite.

  
Figure: LNB cutoff



Key parameters of a Feed Horn Antenna:
1.     Frequency Range: The frequency band for which the feed horn will be used – For example Ku, Ka-bands etc. The frequency band of interest will determine the waveguide size of the feed horn.
2.     Waveguide Shape: The shape of waveguide defines the sort of polarization the feed horn antenna will support. Ex. a circular waveguide supports both linear and circular polarized waveforms, while a rectangular waveguide supports linear polarization.
3.     Flange Compatibility: The flange type needed to connect the feed horn to the Low Noise Block of the Satellite system.
4.     Gain: The amount of gain the feed horn antenna provides.


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The big round “dishy” part of a satellite dish is called the “reflector” and what it does is focus all those waves like a lens. It looks something like this:

Fig: LNB at the focal point 

It focuses all of them on a point in front, where the LNB goes.


The LNB? What?

An LNB is a complex little computer that does an amazing amount of work. It consists of three parts:
  • a Block down converter, which takes the higher frequency signals which don’t travel well on wires and converts them to lower frequency signals that do;
  • a Low-Noise amplifier, which bumps up the signal power so it can travel over a wire;
  • The feed horn, which is the subject of this article.
LNBs make satellite reception possible. Once all that signal is focused in one place, it’s still not strong enough to go down a wire in its current form. The LNB takes care of that.

Finally, the feed horn

Even though the dish does a pretty good job of focusing the signal, it doesn’t do a perfect job. That’s the job of the feed horn.
A feed horn looks sort of like a funnel. It helps move those signals down the line and concentrates them even further so they land in precisely the right place. The one you see above you is just a typical one I found on the internet.
The feed horn in a satellite system is that part on the LNB (in general) that’s covered in white plastic. The plastic is invisible to satellite signals and serves to protect the electronics inside. The actual feed horn is behind the white plastic cap.


Figure: Feedhorn without LNB