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is focal point crucial?

ckrys1

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#1
Can anyone tell me how crucial it is that the feedhorn points directly at the centre of my 1.2m prime focus motorised dish?
I have recently had a new dish and lnb professionally installed and although it worked fine, visually the feedhorn was pointing at a point about 15cm from the centre of the dish. Reception was fine on all the satellites I would expect to receive, but I could only get sparkly analogue pictures, and no digital, on 26E and 42E(as mentioned on a previous posting).
Having failed to get the installer back (wasn`t impressed with him), I succeeded myself in refixing the lnb/feedhorn to a better position, pointing to about 6cm from the centre. This definitely improved analogue reception at 26E and 42E, but unfortunately still not enough for digital.
My question is should I get up on the roof again and somehow reinstall the feedhorn so it`s pointing exactly at the centre, or would I be wasting my time? Or should I just try it and see? I`m not that great at heights so wouldn`t want to attempt it unless there was at least some chance of improving things
 

rolfw

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#2
Hello Ckrys1,

Correct setup of the dish is of paramount importance, particularly when attempting to pull in fringe signals. The dish is designed to achieve optimum results with the LNB set not only in the correct position, but also atr the correct focal length, so if the focus is on a different part of the dish it will slightly change the focal point.

Bear in mind though that by moving the focus of the LNB, it will effectively change athe alignment of the dish, which may then require fine tuning.

Rolf
 

Channel Hopper

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#3
There are three steps to getting the feed set up on a prime focus dish.

1) Confirm centre point of the feed is in the middle of the reflector by running around the edge of the dish with a tape measure, and check the lengths to the center of the feedhorn.

If the feed rods are adjustable with nuts on one or both ends, these can be slackened off and the rods moved in or out of the dish surface to help with the centering.

2) Check the boresight if you can by looking through the hole at the back of the reflector an see if the feed entrance is square on. If this is not possible then you can tie a small wooden stake to the feed and see if it points towards the center of the dish (if you have a laser pen then take off the LNB and illuminate the dish surface through the feed - but that would be too easy )!

3) Check the focal distance from the data given with the dish and adjust accordingly. If the installer did not leave the manual with you then the following formula is correct

f = D x D / 16 d

D is the diameter of the dish parabola (NOT to the rim edge)
d is the depth of the dish (back of dish to front rim)

Of course there are other considerations such as the accuracy of the dish itself and whether the feed really is matched to the dish for correct illumination, if it doesnt look right then the chaces are the installer has cobbled together something to get it out of his shed.

If you can take some digital photos and post them then Id be pleased to let you know what youve got (a racehorse or a donkey) A 1.2m should give fine results of at least some analogue channels on all the satellites youve mentioned


Good luck
 

ckrys1

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#4
Thanks Channel Hopper and Rolf.

Will get tape measure ,calculator, and brain into gear and hopefully have a go this weekend. Will get back to you.
 

ckrys1

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#5
Now- don`t know if I`ve made a total balls up or not - think I need some reassuring counselling!

Decided to upgrade my lnb to a 0.3db (genesis 3). Previous lnb had metal circular `thing` which screwed into lnb and into bracket at junction of 3 metal bars from dish (is this called a flange type or something?). Genesis 3 was different - no screw holes,moulded plastic and no way would it attach to the existing bracket (is this a 40mm type? and would it be for offset dish rather than prime focus?).

Anyway, decided bulls should be taken by horns, so managed to bend the existing bracket, slip in the genesis, bend the bracket back over it tightly,making sure it was pointing at exact centre of dish, and secure whole dog`s breakfast with tape and, I must say, it feels totally rigid and secure. Also, it has greatly improved reception, with perfct analogue pictures on 42 E, and digital too, which is really what I was after. No improvement on 26E though - ?footprint problem, living in NE Scotland.

Problem was I didn`t know about `flange` and `40 mm` lnb`s till I looked ot another website later, then realising what I had done.

So, is the lnb I`ve used only for an offset dish, or can you get another bracket to fit it to a prime focus? And is the way I`ve cobbled it together a disaster waiting to happen, which is what my wife thinks, or should I be proud of my ingenuity? Thanks for any advice.

PS very windy up here today, and it`s still working!

Ckrys1
 

rolfw

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#6
I think that if it works then it's OK, there are a lot of DIY rigged dishes around that give excellent performance. The 40mm colar is normally used for offset dishes, but this doesn't mean that it won't work with prime focus equipment, I though that the Genesis was also available in C120 format as well though.

Rolf
 

ckrys1

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#7
Thanks Rolf - feel better now!

Havn`t seen the genesis in C120 format, but will have a look.

As a matter of interest, how do the 40mm lnb`s normally fit onto a bracket, as there are no screw holes that I can see. Is there a specialised fitting for them?

Ckrys1
 

rolfw

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#8
They are fitted with a two piece circular clamp

Rolf