FSS stacked LNB 122677 question

[tort]

I can't seem to find this issue addressed, I can see all the posts on this LNB but have a question.

I just acquired one, separate and already removed from a SuperDish, and mine seems to have an adjustable feed horn.

The feed horn can be turned clockwise or counterclockwise and locks into a different position.

Anyone know what the purpose of this is?

See attached photos for the 2 positions the feed horn can be turned to.

 

[Anole]

Yes, one way is for linear, and the other is for circular.
If I can find a picture of it set the way Dish used it, I'll post below.
edit: picture added, showing the default orientation for use on Dish satellites.

Well , I forget which way Dish even used it...
But I believe if the flat sides (inside the feed tube) are vertical, it's linear mode
And if the flat sides are diagonal, it's circular.
Maybe someone will confirm.

 

[HIFI]

Yes, one way is for linear, and the other is for circular.
If I can find a picture of it set the way Dish used it, I'll post below.

Well , I forget which way Dish even used it...
But I believe if the flat sides (inside the feed tube) are vertical, it's linear mode
And if the flat sides are diagonal, it's circular.
Maybe someone will confirm.

We used it like he has in pic2.

 

[Anole]

Also, if you'll flip over to page two of this thread, there is a discussion of improving performance by grafting on a bigger feedhorn.
If you get adequate performance as-is, that's fine, but I wanted to pass along this tip in case you found the signal underwhelming.

 

[richyrich]

This adjustment is called "skew"; and is used to orient an lnbf with dual FSS polarity onto a dish that cannot skew itself, that is, a dish that does not have a skew plate.

 

[Anole]

In this case, it's not about skew.
The LNBF has a clever design to switch between linear and circular polarization.
Those are the two detented positions.
I suspect it was designed prior to confirmation of what satellite they could use, or whether it could be circular or not.
Or maybe for future unknown uses.

See the thread above, where Pendragon discusses the modest performance of the feed (possibly the odd scalar).
Mating a more appropriate feed to the LNB gives a noticeably better signal.
Use of the LNB on Primestar or DirecWay dishes which come with a dish-matched feed, they work pretty well.

In another old thread (which I don't have bookmarked), a member described how to convert the assembly to reverse-circular to use it on a T-90 dish.
You basically have to twist the feed tube the opposite way from vertical to get the reverse circular signal to be received properly.
It just requires dis assembly and reassembly.
(linear reception on a T-90 wouldn't require a reversed LNBF)

 

[richyrich]

Velly interesting; I thought circular is selected/setup by a plastic piece located inside the feed throat itself; such as: taking a directv standard lnbf, removing the plastic plate inside the throat; then you have a linear 12.2-12.7 LNBf;

 

[Anole]

Yes, pulling the diagonal plastic insert will change the feed from circular back to linear.

Some DirecTV feeds are a round smooth tube, but inside there are some molded-in staircase-like steps.
They protrude into the free/hollow space in the tube by maybe 1/8'th of an inch or so.
That apparently makes the LNB sensitive to circular, as well.

I'd never heard of the flat-wall trick that the 105 SuperDish LNBF used, but then most of us don't design feedhorns nor LNBs, either.

Below are pix showing how the feed fits the LNB for first linear, then circular (files are named correspondingly)

 

[tort]

Okay, I have old Primestar LNBs I can use the feedhorn from, as well as 75E dishes.

 

[Anole]

Okay, I have old Primestar LNBs I can use the feedhorn from, as well as 75E dishes.

Excellent!
Use the feedhorn that came with -that- dish.
You will get maximum performance.