LNBF is made of horn antenna, waveguide, half/quarter wave dipole and some circuitry on PCB. I know Horn antenna is used to match impedance between waveguide and free space but my doubt is, why we dont use half/quarter dipole antenna directly instead of using horn antenna, waveguide and all?
Electronic – why we use LNBF instead of simple dipole antenna, at the focal point of reflector of satellite dish antenna
antennareceiver
Related Solutions
Dipoles are excellent for receiving. It would appear as a 75 ohm source, as you guessed. Impedance matching is usually more important for transmitters because of the levels of power involved. Mismatched impedance with receivers doesn't tend to melt anything, because there are only micro-watts of power involved, but you may get more signal into your receiver's front end if you match the antenna.
From what I can tell it's a conical scanning antenna. From my limited understanding, it allows precise targeting with a wider beam.
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Best Answer
Dipoles are linearly polarized. Most satellite signals are circularly polarized, this allows satellites to rotate, eliminates alignment issues on the ground (instead of horizontal and vertical linear polarization you would end up with n/s and e/w linear polarization, or whatever axis was used). Also, signals can get rotated when traversing the ionosphere which does not affect circularly polarized signals (they still get rotated but the hand-ness of the circular polarization does not change).
Also, many satellites transmit separate signals with right hand and left hand polarization. A dipole would mix both signals which is not desirable.
There are some satellites with linear polarization (probably mostly older satellites in geosynchronous orbits).