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OK, I've been playing with the mathematics of extending my 8 foot dish to a 10 footer. Think I may have a way figured to be able to carry the proper curve out. I like to have a decent plan laid out before starting off on an adventure and as such have discovered that if I expand to 10 feet, my F/D goes from .40 down to .29, but the focal length remains the same, as it should.

That would in essence place the throat of the LNBF about 1.2 inches +/- (an estimate) lower than it is now. Can someone tell me just how important the FD setting is. Just wondering how detrimental it would be to leave it at .40? Or, should one plan on extending the legs in order to place both the FD and focal point correctly?

Thanks in advance.

bpalone said:

but the focal length remains the same, as it should.

Click to expand...

bpalone said:

That would in essence place the throat of the LNBF about 1.2 inches +/- (an estimate) lower than it is now

Click to expand...

Conflicting statements - If the focal length remains the same, the feed placement does not change. Only the scalar placement changes if you readjust the F/D setting.
Think the illustrations here http://adlfeed.com/fp_feed_pat.htm should answer your question about not changing the scalar placement. Leaving the F/D setting at .4. Think the 3rd one down on the left is what you'd end up with -
F/D setting higher than the dish calculation.
One way to lower the F/D, without extending the legs, would mount to the front of the scalar.

The beauty of the written word. Absolutely clear to the author, but somewhat like mud to the reader.

Yes, I absolutely understand that if the LNBF is adjusted to the calculated FD point, that the entire assembly would need to be raised in order to place the calculated focal point in the proper place in the feed horn of the LNBF. I made the assumption that it went without saying at this level. Then again, could very well lead someone new to things down the wrong path, not by intent though.

Will review the link you posted. Thanks.

Setting the F/D ratio higher than what's calculated, Has on a few occasions, Been beneficial. Has made reception work on a couple of under sized dishes. (think one, or maybe both, were on this site)
One rememberable one was a fixed 5ft. All TP's on the satellite were good quality, except one. Yup, the desired one. After quite a bit of investigating, a possible adjacent satellite TP that could cause problems was found. (can't remember if it was at 2 or 4 degrees)
Suggested they set the F/D higher than calculated, up to .05 (or so IIRR) higher, Next post was a 'thank you, got it'.
Anyway, Higher than calculated F/D setting under illuminates the dish, reducing gain some, but at the same time it shifts the sidelobes away from the interfering satellite. It will also reduce T.I. coming in from around the perimeter of the dish.

BITD, we set quite few .05(?) higher than calculated to combat T.I. from those 4 Ghz telephone relaysI. (which are now all gone around here, elsewhere? IDK.)
So as a rule, treat the calculated F/D as a 'nominal' and incorporate the above as necessary. YMMV.
(Wouldn't it be nice to have a spectrum analyser for this?)
If this technique is used by some 6ft BUD operators, IDK.

NEVER set the F/D lower than calculated.
The feed then sees too much around the edge of the dish so the T.I. problem is exaggerated and gain is lowered substantially.
Happy 'bird/feed' hunting

just an update. I gave up on my enlargement project since I followed the f/d ratio exactly with my new sections it really didn't enlarge my dish enough. so the small amount of gain increase say 1db wasn't big enough for all the added drilling and shaping of panels. I found that following the f/d ratio exactly (0.35 to 0.35) only increased my 10 foot dish to 11 feet at the most. So I just figured it would be simpler to find a ready made 12 foot dish. it also could be I couldn't get my head fully wrapped around what exactly needs to be done to increase the size of dish after I just simply bolted on my existing panels and followed the original shape it turned out like the pic below called "incorrect". then once I used the parabola calculator program I see it should have turned out like the pic called "correct". You can see in the pic called "correct" following the parabolic formula really doesn't add much diameter to the dish so it wasn't worth the extra effort for me. I hope the pics illustrate things better.

Fat Air, thanks for the feed back.

Beavs2112, I haven't decided to do it. Did set up a spreadsheet to calculate the curve distances (y) from zero on the x line and have it also calculating for the curve section to be added. Kind of thinking like you, just watch for a ten footer. Thinking right now of possibly building a 4 or 5 footer prime focus for Ku. Don't know how well it would do. but think it would be fun and give some training/insight for an extension project later.

If anyone wants the spreadsheet, I will gladly upload it.