Help please:Prime focus dish LNBF's distance from dish

Irish-man

Hi everyone,
I have installed a 1.8M Fortec Star dish, and I have a Inverto 0.3 flange LNBF with a universal feedhorn on it. In the dishe's installation guide says the best focal point is at 682mm from the dish, however I believe this distance is for C-band LNBF's. Can anyone help me to find the correct focal distance for my LNBF from the dish please?, I want to use this dish for Ku band only.....I also have a 90cms. offset dish getting more signal and better quality than my 1.8 M dish .
My 90cms dish is getting 96% signal and 76% quality, but the 1.8M dish is getting 88% signal and 65% quality.
By the way, I have checked my dish with the fishing line to ensure the petals are correctly positioned, and the dish seems to be perfect using this method.
At present I have the LNBF face at about 720mm from the dish's center.

I would really apreciate it a lot if you could give a hand. At this point this is driving my mad....some one in this forum always says : "there is a very fine line between "hobby" and "mental illness" ".

Thanks,

Irish-man

Robbo71

Hi,
The focal point of the dish is independent of frequency, it just comes down to the geometry of a parabola. Sorry, can't help further, but I have read a few threads on here where the difficulty of getting the petals aligned is discussed.
Robbo

Irish-man

Thanks Robbo71,
I found this last night, I knew that was the case but it made wonder the fact that in the users guide of the dish the said C-band for the focal distance.
I am not sure where to place the LNBF because the Ku-C band LNBF has a scalar ring of about 18 cms., but my Ku-band LNBF has only only 10cms. scalar ring....and also the throat sizes of both LNBFs are very diferent in lenght......al these has confused me big time....another factor also is that in the case of C-band the f/D ratio means that we need to push forward the throat of the LNBF forward from the scalar ring, but my Ku-band LNBF does not have this movement available.....

Thanks in advance for any further help,

iri-man

Irish-man

Hi Robbo71,
I went to my dish and changed the position of the LNBF to the focal point I calculated, and it WORKED!!....I am now getting 98% signal and 100% quality.
Now the issue is that I need to get the Nordic transponder os Sirius 5.0....I am getting the BSS European beam, but I dont get even a spark of the Nordic beam.
Do you think I will be able to get it with my 1.8 dish??? Do you know of anyone getting this Nordic beam???

Thanks,


George

mhku

Irish-man: don't hit the "Report Post" button without a reason.

__________________

Irish-man

10-4, my mistake.

Robbo71

Well, glad you got that part sorted. As for the Nordic Beam on Sirius, I could only dream of that on my Minidish. My other dish is 90cm, so not much chance with that either. The footprint map on Lyngsat, suggests that a 1.8 could do it possibly, but you're way of the footprint.
Also I don't know if you have done a forum search (Fortec 2.4m) or (Fortec 1.8m), but that dish is known to be very dificult to get the full benefit out of. Maybe do a forum search for 'Sirius Ireland' also.
Robbo