LNB Questions

[fudda]

Here's a couple of questions for all the experts here. I know someone will know!

1. Why does an LNB have to be angled at about 20 minutes to the hour or you don't get any signal quality? Why isn't it 45 degrees, halfway between vertical and horizontal? Or 20 minutes after the hour?

2. Why is the Lo frequency on modern LNBs 10,700GHz and not 11,300Ghz like it used to be?

I don't want to cross over wondering.

[trash]

For circular polarity it does not matter.

For linear, orientaion does matter.
Just like TV antennas, satellite LNC's are also polarised.
TV antennas are usually horizontal in the cities, and sometimes vertical in the country.

In space, there is no up and down. So it's not possible to have a vertical plane. What does happen is that satellites align their axis so that vertical polarisation is in line with the Earth's north and south poles, while horizontal polarisation is in line with the Earth's equator.

If you look at the satellite as a point in the sky and draw an imaginary line from that satellite to North on the horizon. This like represents vertical polarity.

So the lower your satellite appears to the east or west horizon, the more skew your LNC will have. At the horizons, vertical polarisation will appear to be horizontal.

All satellites follow this rule... unless for some strange reason, the satellite owners decide otherwise. The Optus series satellites are slightly offset from what is expected.

[fudda]

Thanks Trash, for your great answer! That takes care of question 1.

[Farmsky]

Get your grey matter round this little lot.



Basically a 10700 has a wider bandwidth than an 11300 and a universal (9750/10600 lo's) has the widest.

If u can work out the chart you'll be able to give me the answer

[Chieflets]

lol farmsky - you make the hobby sound like a maths class.

Chieflets

[fudda]

Get your grey matter round this little lot.

I'm sorry I asked now! But I sort of get it. The incoming RF reflected off the dish is mixed with the Lo frequency of the LNB to produce an IF, presumably with some amplification as well.

[Farmsky]

Spot on. It's a direct subtraction (lower sideband). In the case of C band it's the upper sideband

[bulbous]

Spot on. It's a direct subtraction (lower sideband). In the case of C band it's the upper sideband

Using "sideband" like that is going to confuse people as sideband is generally used in relation to suppressed carrier radio transmission for voice.

What you maybe should use as a description is high (or low) side LO injection.

[Farmsky]

Same principle