f/d on my 10'

[truckracer]

I recently installed a corotor II on my 10' sami as many of you know.
It is supposed to have an f/d of .38 which according to sami spec sheet the focal distance is 45 5/8" from the center.

If I set my scalar on the .38 line with the feedhorn, I am not at the correct focal distance. I get decent signals.

I can move the feed inward toward the center of the dish where the signals come up significantly. But.......The f/d marking on the corotor is now between .32 and .34

Shouldn't this be at exactly .38 with the feed supports placing it at 45 5/8" to 1/4" inside the feed opening?

Should I set the scalar to .38 and then shim the whole thing in to the proper setting.

I know on the non adjustable ADL feeds that is what one member here said to do.

I would think the sidelobe rejection would be affected by the scalar not being the correct distance from t the feed opening for the specified f/d ratio.

What do you guys think?

 

[starman345]

I can't remember how the sami feed struts are, can you shim the feed with washers on the bolts to get the scaler closer to the dish? You can adjust with washers until you get the right measurement on both the scaler and focal distance.

 

[Hermitman]

Set your f/d to .38 then adjust the entire feedhorn/scalar ring unit to the proper distance. If your ring is attached to the support struts with just the four bolts, you may be able to twist the entire unit (like screwing it downward) to obtain a smaller distance. Nothing says you have to have the support struts at perfect right angles. The amount of signal decrease due to strut blockage doesn't change. Good luck.

 

[truckracer]

I always thought that the scalar and feed horn works as one piece. I would think it would block sidelobes much better set at the proper .38 and at the proper distance.

 

[starman345]

I always thought that the scalar and feed horn works as one piece. I would think it would block sidelobes much better set at the proper .38 and at the proper distance.

You are correct, set the feed in the scaler ring at .38, then set the whole assembly so the front lip of the feed is the proper focal length from the dish, you may have to shim it on the struts. I think that is what Hermitman is saying and I know that is what I meant, maybe didn't say it clearly.

 

[Lynskyn]

The quad feed support legs should be slotted where you can leave the f/d at .38 and adjust it in using the slots. If it's still not close enough to the reflector after adjusting it all the way in on the slots, then you would need to shim it.

 

[Anole]

thinking outside the box

If your ring is attached to the support struts with just the four bolts, you may be able to twist the entire unit (like screwing it downward) to obtain a smaller distance.
Nothing says you have to have the support struts at perfect right angles.

Pure genius, Hermitman! - :up

 

[truckracer]

Thanks guys.

I want to tune this thing to peak performance and as it stands now I cannot lock the G3-C equity tp on c-band. This was after going to the corotor. My old astrotel lnbf at .38 fell right on at the focal distance. However the corotor is shorter. When the rain stops I am getting some stainless steel bolts and washers (longer bolts).