Relation between FEC and Dish Size

[al_madhi]

Hi guys I read in a satellite magazine that there is a direct relation between FEC and the dish size - If you receive a channel and its FEC = 1/2 using 90 cm dish , but if you receive another channel and is FEC = 2/3 then the size of the dish has to be increased by a little bit by 90 cm X 1.3 = 117 cm , and by the same token if the FEC = 3/4 the size of the dish has to be icreased by 90 X 1.4 = 126 cm And if the FEC = 5/6 the size of the dish has to be increased by 90X 1.6 = 144 cm And if the FEC = 7/8 the size of the dish has to be increased by 90 X 1.8 = 162 cm .

Please advise where do they devise these numbers 1.3 , 1.4 , 1.6 and 1.8 so that

1.3 corresponds to FEC = 2/3

1.4 corresponds to FEC = 3/4

1.6 corresponds to FEC = 5/6

1.8 corresponds to FEC = 7/8



Best Regards


A.M.AL-MADHI

 

[Hermitman]

I'll give this a shot. As you may be aware, this dish size vs. FEC is based on the number of error correction bits in the data stream. A 7/8 FEC has 1 correction bit for every 7 data bits while a 3/4 FEC has 1 correction bit for every 3 data bits, giving the data stream using a 3/4 FEC more opportunity to provide an error-free picture. This is important with the tightly crowded bandwidth on a satellite's transmitters. And the easiest way to deal with less error correction (a higher FEC) is to increase the clarity of the received signal (a larger dish). If you graph this out, with the FECs on one line and the dish ratios on the other line, you'll see that there is a direct linear relationship between the two. I sure there's some big, thick book that will detail this out with formulas but the bottom line is - a smaller dish receives less signal which may result is more data errors. And if you're reducing the number of error correction bits by using a 7/8 FEC, you need to receive a stronger signal by using a larger dish to get an acceptable picture.