Focal length and such

[pjcl]

I'm hauled home two 8-footers a few days ago, and I am now trying to make one dish out of the two. The arms supporting the feedhorn on the dish I decided was best are a bit rusty, and I would like to replace them with the arms of the other. Now, I've noticed the news arms are a little bit longer, maybe half an inch, if that. Will that make a difference, considering the size and shape of both dishes is otherwise identical? Can I compensate this by adjusting the lnb closer to the dish? Does the distance of the scalar to the dish matter as much as the lnb itself?

 

[Lak7]

Everything makes a difference, to me at least.
Think of the Scalar Ring and LNB Feedhorn as a single unit.
You first set the Feedhorn to the Scalar Ring, then adjust the whole thing to the proper Focal Point.
Focal Point is measured from the deepest part of the Dish to 1/4" inside the Feedhorn.
It may be better to make new poles from Conduit.

 

[LocutusOfBorg]

I'm hauled home two 8-footers a few days ago, and I am now trying to make one dish out of the two. The arms supporting the feedhorn on the dish I decided was best are a bit rusty, and I would like to replace them with the arms of the other. Now, I've noticed the news arms are a little bit longer, maybe half an inch, if that. Will that make a difference, considering the size and shape of both dishes is otherwise identical? Can I compensate this by adjusting the lnb closer to the dish? Does the distance of the scalar to the dish matter as much as the lnb itself?

If the arms are slotted where the feed mounts it should be easy to set the feedpoint and then re-secure the arms to the feed. It may be possible to center the feed by bending the feed supports without affecting focal distance.

 

[guapoharry]

How? By moving the feed relative to the scalar or what?

If so, what about what Lak said?

 

[LocutusOfBorg]

How? By moving the feed relative to the scalar or what?

My KTI manual said to set focal length with the slotted end of the rods that support the feed, tighten the bolts, and then man-handle the rods at the feed to center the feed. I grabbed ahold of the scalar and adjusted the rods to center the feed. It changed the focal length slightly but not enough to spend a bunch of time getting it dead on. I suppose there's a more elegant way of doing this but it works for me. And I suppose that the feed centering and focal length could be done simultaneously but trying to do that by handling a tape measure (or other method for setting focal length) and boresight tool by oneself while up on a ladder would be very cumbersome and frustrating. I guess each person has to decide what method gets the job done for them.

 

[B.J.]

Focal length is the hardest thing for me to adjust. I have a real hard time with this. Since moving the feed throat inside the scalar ring also affects the F/D performance, it's really best to move the feed and scalar as a unit like Lak said, however with a quad feed support, and not much adjustment possible at the feedhorn, about the only way to do this right is to adjust every quad arm where it meets the edge of the dish, but this is next to impossible when the dish is up on the pole.
What I have done is to twist the whole feedhorn so that the quad arms come in tangentionally (?sp?) instead of axially, and as I twist the feed, the whole assembly tends to move closer to the dish, and I can usually get it pretty close to the fl spec pretty easily. HOWEVER, I'm starting to think that by doing this, that I might actually be distorting the shape of the dish a bit. I'm thinking about going back to the axial configuration and adjust the fl via the throat in the scalar thing, ignoring the F/D setting. I would just adjust it for highest meter reading, except that my twisting operation also changes the polarity, which affects the signal level, plus I have to stand in front of the dish to make this adjustment, which affects the signal, so I haven't had much luck adjusting focal length by signal.
Anyway, I haven't figured out any good way to do it that I'm happy with.

 

[FaT Air]

If the arms are too long, use a longer bolt and a spacer to move the feed towards the dish. If arms are too short, mount the arms to the front face of the lnb.

 

[B.J.]

If the arms are too long, use a longer bolt and a spacer to move the feed towards the dish. If arms are too short, mount the arms to the front face of the lnb.

Good idea. Hadn't thought of that.

 

[phlatwound]

If the arms are too long, use a longer bolt and a spacer to move the feed towards the dish. If arms are too short, mount the arms to the front face of the lnb.

Do you think the ends of the arms, when mounted to the face of the scalar, could be detrimental to the signal?

 

[FaT Air]

I think having the feed in the improper location would be more detrimental. Look at the posts where they've cut a hole in the scaler, to mount a separate Ku feed. Or cut a whole side off to mount 2 C feed/scalers side by side for a dual sat setup.
BTW. the very last "lnb" in my post should have read "scaler"

 

[phlatwound]

I think having the feed in the improper location would be more detrimental. Look at the posts where they've cut a hole in the scaler, to mount a separate Ku feed. Or cut a whole side off to mount 2 C feed/scalers side by side for a dual sat setup...

Thanks, that makes sense.

 

[Anole]

If you add brackets where the quad legs normally connect to the scalar, it spreads them apart.
That in turn, moves the scalar away from the dish.
So, how do we compensate?
Well, see Stogie's work in his thread for some ideas.
How I Learned To Love The Bomb...Or...Paraclipse Modifications......

ACradio had a similar problem, and solved it just about the same way.
By using long bolts to bring the actual scalar closer to the dish, after first moving it away.
[ame="http://s970.photobucket.com/albums/ae184/ACRadio/The%20NEW%20ACRadio%20Dish%20Farm/?action=view&current=IMG_5848r-2.jpg"]The NEW ACRadio Dish Farm :: Moving The Scalars picture by ACRadio - Photobucket[/ame]