Adjusting the feed horn.

[Greg Mueller]

Got a Co-rotor.
My dish has 4 support arms.

The previous feed horn was mounted on the ends of the arms with bolts through the scalar rings.

This is where I get confused....

If you mount the feed horn via the scalar rings on the arms and set it according to the markings on the feed horn, then how do you adjust the distance from the feed horn to the dish?

It doesn't seem like there is a way to change that distance.

 

[voomvoom]

Mine is so old, I can't remember what exactly it is. But, I know it was made by Chapparel, it's c/ku and it has an allen wrench adjustment that allows you to slide the unit up or down inside the feedhorn, to make it closer or farther away. I don't think it's a Corotor II, but it could be. Maybe Chapparel changed the name of it at some point.
Or, you may have to adjust your leg support arms. Let's hope not.

Al

 

[dfergie]

There are formulas in the FAQ's for calculating focal length...

 

[Greg Mueller]

Yep. I'm going to do that tonight, when the wife gets home.

But since the scalar ring is hard mounted to the poles and the poles are hard mounted to the circumference of the dish, the only thing that is adjustable is the feed horn where it slides in and out of the scalar ring. BUT, you have to set that, according to the FD of the dish. So what f it turns out the front edge of the feed horn is at 46" and the calculations show that it should be at 44". What do you do? Cut the poles?

I guess you could get some threaded rod and make up the difference by mounting the scalar ring on the threaded rods and the threaded rod to the poles, and then adjust it that way. But it doesn't seem like that would be what the manufacturer had in mind.

Otherwise I guess you just hard mount it had hope for the best? Doesn't seem to accurate.....

 

[dfergie]

Yeah, I know what you mean... When I went from an Lna mounted directly to the feed horn to a Chapparel Corotor II ( years ago) I had to engineer it a little as the feed horn was and is mounted to brackets attached to an antenna rotator as my original system did not have the provision to change polarity. It took a lot of shimming with washers etc to get the Chapprel feed horn the same distance as the old one and then adjusting the scaler ring for best PQ after starting with the template provided...

 

[AntAltMike]

There are slots in the feedhorn ends of the feedhorn support arms that allow you to move the feedhorn in and out over maybe an inch and a half. I have serviced dishes that had feedhorn arms that did not allow setting the feedhorn at the right distance, but if you have to, you can use threaded rod to extend the range if necessary. I've done it a couple of times.

 

[spyder]

Got a Co-rotor.
My dish has 4 support arms.

If you mount the feed horn via the scalar rings on the arms and set it according to the markings on the feed horn, then how do you adjust the distance from the feed horn to the dish?

It doesn't seem like there is a way to change that distance.

The scaler ring you would just leave at its 'hard mounted' position and simply slide the feedhorn back/forward within the scaler ring.

If this is the scaler ring and support arms that came with the dish originally, the exact placement of the scaler ring is of little concern. I have my scaler ring notched on each side for additional lnb mounting and notice no diference in signal.

 

[Greg Mueller]

Ahhhh the sloted ends....

The previous owner had them at the scalar ring
I thought they might be for centering the feed horn on the dish.


This makes more sense now. Sure wish I had the assembly manual.....

 

[Greg Mueller]

Wait I'm confusing myself again...

So the slotted ends slide towrads and away from the feed horn changing the distance from the feed horn to the dish?

This ain't the original scalar ring this is a new corotor I just got

 

[spyder]

Wait I'm confusing myself again...

So the slotted ends slide towrads and away from the feed horn changing the distance from the feed horn to the dish?

This ain't the original scalar ring this is a new corotor I just got

It really won't matter. Just simply slide the feedhorn in or out of the scaler ring to get the proper focal distance and/or to fine tune the reception. In all the systems I've ever installed or tinkered with I've never made any adjustment to the scaler ring distance from the dish.

The slots in the support arms are for ensuring that you get the scaler ring centered in the dish. I always take exact measurements from the rim of the dish to the scaler ring edge in several diferent locations to ensure that I have the feed dead centered in the reflector.

 

[caddata]

Proper method, per the Chaparral technical sheet: Assemble the feed horn and scalar ring to the proper F/D setting for your particular dish. Adjust entire feedhorn/scalar ring in or out to allow the focal point to be 1/4" inside the mouth of the feed.

Take great care to be sure that the feed is looking down the axis of the dish by measuring from the edge of the feed to the edge of the dish several places around the perimeter.

Harold

 

[AntAltMike]

It really won't matter. Just simply slide the feedhorn in or out of the scaler ring to get the proper focal distance and/or to fine tune the reception. In all the systems I've ever installed or tinkered with I've never made any adjustment to the scaler ring distance from the dish.

The slots in the support arms are for ensuring that you get the scaler ring centered in the dish.

Ignore the above advice. The people who designed the adjustable scalar ring to match the F/D ratio of the dish knew more about feedhorn reception than the installers do.

The scalar ring is supposed to allow the feedhorn to develop the optimal "edge taper" that actually allows the feedhorn to more efficiently receive the better quality signal from the middle of the dish versus the slightly inferior signals from the edges. It is called "illuminating" the dish.

If you take a really deep dish, like a Winegard Pinnacle, with an F/D ration of .28, you have to have the waveguide tube protruding about an inch beyond the plane of the scalar ring to properly "illuminate" the dish. I have seen C-band feedhorns perform poorly on these Pinnacle dishes when the feedhorn's focal length was set by simply sliding the waveguide tube back with respect to the scalar ring.

Chaparral actually used to make a feedhorn tube extension it called a "gold ring" to adapt its adjustable feedhorns that could only be set as low as .36 to extend their F/D range down to .30. They sold for about $5. I imagine Skyvision can still furnish them.

 

[Greg Mueller]

We measured out the dish and I got a depth of 20" and a diameter of 120" if I did my math right I have an "F" distance of 45" so 1/4" off would give me 44-3/4" to the edge of the corotor. With an "f" ratio of .375

Right?

 

[spyder]

Ignore the above advice. The people who designed the adjustable scalar ring to match the F/D ratio of the dish knew more about feedhorn reception than the installers do.

The scalar ring is supposed to allow the feedhorn to develop the optimal "edge taper" that actually allows the feedhorn to more efficiently receive the better quality signal from the middle of the dish versus the slightly inferior signals from the edges. It is called "illuminating" the dish.

If you take a really deep dish, like a Winegard Pinnacle, with an F/D ration of .28, you have to have the waveguide tube protruding about an inch beyond the plane of the scalar ring to properly "illuminate" the dish. I have seen C-band feedhorns perform poorly on these Pinnacle dishes when the feedhorn's focal length was set by simply sliding the waveguide tube back with respect to the scalar ring.

Chaparral actually used to make a feedhorn tube extension it called a "gold ring" to adapt its adjustable feedhorns that could only be set as low as .36 to extend their F/D range down to .30. They sold for about $5. I imagine Skyvision can still furnish them.

Take it any way you wish. I'm just speaking from years of experience and first hand know how. What we are dealing with is not rocket science. It's simple mathematics and physics. The numbers don't lie and niether do the installation results.

Actually the scaler ring was designed to cut down on terestrial interference from sources ouside the dish reception plane.

So you're on your own.

 

[spyder]

We measured out the dish and I got a depth of 20" and a diameter of 120" if I did my math right I have an "F" distance of 45" so 1/4" off would give me 44-3/4" to the edge of the corotor. With an "f" ratio of .375

Right?

those numbers look correct to me


Focal length= diameter squared / depth x 16

120 x120 =14400

20 x 16 = 320

so focal length would be = 45

F/D is focal length deided by diameter

so F/D ratio would then be .375

 

[spyder]

If you take a really deep dish, like a Winegard Pinnacle, with an F/D ration of .28, you have to have the waveguide tube protruding about an inch beyond the plane of the scalar ring to properly "illuminate" the dish. I have seen C-band feedhorns perform poorly on these Pinnacle dishes when the feedhorn's focal length was set by simply sliding the waveguide tube back with respect to the scalar ring.

Chaparral actually used to make a feedhorn tube extension it called a "gold ring" to adapt its adjustable feedhorns that could only be set as low as .36 to extend their F/D range down to .30. They sold for about $5. I imagine Skyvision can still furnish them.

Actually I will backtrack and admit that your post was correct in this part of the information. If you are dealing with a very deep design dish then it may become necessary to allign the scaler ring so that it properly eliminates any extra noise which may be picked up by the side lobes being projected at such a steep angle. Thus more properly 'illuminating'.

But in my opinion it's only necessary in this type of application. On the shallower / high gain dishes that the majority of consumers have in use currently it really doesn't apply to that extent, since the bounce angle of the surface illuminates in more 'shallow' angle than that of the deep dish where the signal is coming in from an almost sideways angle.

I actually once set up a dish without a standard scaler ring. What I did was take a piece of flat 1/8 metal plate, had it cut to the correct diameter, machined the mounting holes and fabricated the center feedhorn collar which allowed feedhorn depth adjustment etc. This was being used on a high gain dish with a depth of 19 inches and diameter of 124 inches, so the interference bands of a normal scaler ring would have never come into play due to the illumination angle of that dish.

 

[AntAltMike]

I don't bother with scaler rings on some of the commercial dishes that I service. The difference between using a scaler ring and not using one at all on the more common, relatively shallow dishes, is less than one dB. On my commercial dishes, since the feedhorn is fixed for one satellite and the receiver tuned to one transponder, I know definitively whether I have enough signal surplus that I don't care about tweaking out a tiny fraction of a dB. But for people with motorozed dishes trying to get as many signals as they can, they might care about the fraction of a dB because it might help them in the "worst case" situations.

 

[Greg Mueller]

Thanks for the tip on the slotted ends AntAltMike. I put on the poles and the scalar ring and slid in the feed horn for a test, this morning. The slots do provide enough adjustment to get the feed horn the correct distance from the dish. Happy days!

When I got the dish from the farmer's field, I also got about 120' of "ribbon cable" and although the coax has a couple of nicks in it down to the braid, the stranded wire(s) all look to be in good shape. So I'll just have to pick up a couple hundred feet of decent coax for the C and KU feeds.

Thanks everyone for the help. More questions later......