LNBF skew vs. dish skew

[Corrado]

Just thinking out loud a little here and open to others thoughts or ideas.

In my understanding the reason that offset dishes are oblong (tall vs. wide) is for gain. Now with our small Ku dishes we rotate the LNBF angle to match the tilt of the satellite. Is that correct?

Would it make sense to fabricate a bracket for an oblong dish that could rotate the whole dish to set the skew and leave the LNBF at 0°. Could that be more efficient or more open to adjacent satellite interference?

Perhaps a modified Dish500 bracket to fit a Fortec dish reflector for example. I understand why a D500 rotates to allow the reception of 2 orbital positions on 1 dish. Here, I'm thinking about the skew perspective for a single LNBF application.

 

[qwert1515]

I remember reading somewhere that dishes on HH-Motors should have a slightly higher quality because the whole dish is skewed allowing it to capture more signal. (Might have been used to advertise a motor)

However I don't think I have see a higher signal quality with my motorized setup vs. a fixed dish, but I have not done much experimenting with the two setups.


EDIT: Here's an old thread (with a little bit of info): Fixed Dish: LNB Skew versus Dish Skew

 

[turbosat]

yes and yes

I've read someplace that it would probably help with an elliptical dish like the old primestar 84e's. On an oval dish I'm not sure how much more efficient it would be.
That idea (skewing the whole dish) was how I came up with my primestar-on-c-band polar mount project last year. I took the skew bracket off one of those Directv KA/KU dishes, bolted the primestar 1M to it, then bolted the whole thing to a flat piece of steel mounted on the top of a polar mount from a cband dish. Made it easy to go to my true south sat, set the skew at zero and tighten the bolts on the bracket. Now the dish/lnbf skews as the c-band actuator moves the mount east and west.
*Also heard of using a boat-seat bracket, which rotates, to do the same thing.

 

[Anole]

In my understanding the reason that offset dishes are oblong (tall vs. wide) is for gain.

edit: I misread this at first, thinking you said wide vs tall.
Offset dishes are a little taller because from the point of view of the LNB, they are actually round.
But things like the Primestar 84e, StarChoice 60e, and DirecPC dishes are very wide and not so tall.
And I believe that is to suppress interference from adjacent satellites.
As a byproduct, it also supports multiple LNBs pretty well.

Now with our small Ku dishes we rotate the LNBF angle to match the tilt of the satellite. Is that correct?

We do . . . and on round dishes it's probably not much of a trade-off.

Would it make sense to fabricate a bracket for an oblong dish that could rotate the whole dish to set the skew and leave the LNBF at 0°.

Yes, in theory it sure would.
And if you notice, things like DirecPC dishes do that!

Perhaps a modified Dish500 bracket to fit a Fortec dish reflector for example.

The Fortecs are "round" as far as the LNB sees, so I wouldn't bother.
edit: Any advantage gained would be far less on a round dish than on an 84e type.

I understand why a D500 rotates to allow the reception of 2 orbital positions on 1 dish.

Dish Network is circular polarized.
The skew of the dish is to give both LNBs a correct angle to the two birds.
Without the skew, one LNB wouldn't point at its satellite if the other LNB did.

 

[Mr Tony]

skewing the dish is better than just the LNB because when you get into large skew amounts (20 +) there is less dish to hit the LNB...skewing the whole dish is better

Thats why most Chanelmaster dishes can be skewed (and the best for that reason)

I wish more companies would make a dish that could be skewed

 

[Wescopc]

I have found that with the skew of ~20° we have here (Canby, Oregon), 97W (G25) and 101W (AMC 4) are received better with the whole dish skewed (90cm SatPro).
Bob

Edit: I did the skew of the dish with a motor, and compared it to another identical fixed dish with a skewed LNB, I did not exchange the two LNBs or dishes in a true A vs B setup. So Anole could be right, the LNB "sees" a round dish in either case.

 

[SatPhreak]

I personally don't think it matters which is skewed. If you look though the lnb holder the dish looks round. The only reason it is taller than wide is becauseit is an offset. If the dish didn't look round from the lnb holder there would be either overillumination or underillumination going to the lnb and causing ground noise in the former or a waste of metal in the latter.

 

[voomvoom]

I think as long as you get an acceptable signal, it doesn't matter.... but, that being said, I have two dishes that I have modified mounts to achieve 45 degree's of the skew. I made a bracket to set a 1m P* dish @ a 45 degree angle for G10/G18 ku, it is a skew of 44.3 from my location. It works good, but it also caused the lnbf support arms to be in a slight (almost) wrong position to get the best signal for an additional lnbf for 129.0w. I get a good enough signal, but I believe it was better while the P* was upright...??? I didn't actually record results (I never do), so I'm not 100% sure. Basically, I'm not about to change the P* back, as I put to much work in the modification, and it's up and works good for it's intended use.
Now with the AMC 21 ku in service @ 125.0w, I have put up a 90cm Fortec dish @ a 45 degree angle and I originally pointed @ 127.0w (skew is 46.7) and tried to get 125.0w (skew is 45.6) and 129.0w (skew is 47.8). I could hone in on 129.0w, but couldn't find 125.0w with my make-shift brackets... I could hone in on 125.0w, but couldn't find 129.0w with my make-shift brackets... so, I pointed the dish @ 129.0w, tuned it in, and set the make-shift bracket for 125.0w and bamm... got it. 91% with a Coolsat 5000 using JSC322 lnbf's on 129.0w and 87% on 125.0w, with same type JSC322 lnbf. I'm a happy camper... woo hoo..!!! I'm still scratching my head on why I couldn't nail both sats while pointed at the 127.0w position..???? But now, who cares..!!!!
Also let it be known, that it's harder to find the sweet spot while skewing a dish with home-made brackets. It's just not the same as a fixed dish in a straight-up position, trust me on this...... Anyone wanting to try this, Good Luck..!!! It can be done.

 

[DishDigger]

Wow, Voomvoom, I got lost in all that, but it doesn't take much for me. As I see it, with skewing an offset dish, you now change the azimuth and height settings of the dish when aiming at a desired satellite. Since the signal from the bird doesn't come straight at the dish, rotating the dish causes the angle of entry to be different than intended - much like mounting on a mast that isn't set plumb. The dish should still work, just can be very difficult to calculate the height/azimuth for desired sats........

 

[Anole]

DishDigger -
That's why you have skewable mounts which rotate the dish around it's axis of aim.
To avoid that very problem.
In fact, people who modify such dishes, say by placing the LNB too high or low, will go nuts fighting the aim problem.
(no insult intended to those who've succeeded ) -

skewing the dish is better than just the LNB because when you get into large skew amounts (20 +) there is less dish to hit the LNB...skewing the whole dish is better

Anole could be right, the LNB "sees" a round dish in either case.

These comments got me thinking.
Yes, the LNB sees a round dish. What does the satellite see?
And I see that skewing the (almost) round dish will let the satellite see . . .
. . . I think the satellite will 'see' a bigger dish, and if so, that could mean more signal.
Or, maybe I'm wrong. - - This one makes my brain hurt. -

 

[DishDigger]

I'll buy that (the skewable mounts). I haven't completely figured this blog stuff out yet -- missed the "quote" thingy -- ha.

Thanks Anole....:up

 

[SatPhreak]

The satellite sees a very large portion of the earth. What the lnb sees is what is important as it is capturing the signal that the satellite is spraying on your dish, your car, the neighbours roof and the hospital 200 miles away. The satellite will still see your offset dish as a circle the same size no matter what way it is skewed, just in a slightly different location. Unless you bring it closer to the satellite it will appear the same size.

Try aiming at a sat with a skewable dish then roting the dish 180 degrees. I'll bet you will get the same quality reading. Then rotate it 90 degrees, switch hor and vertical setting in the reciever and you will still get the same quality. I've tried this with 60E eliptical dish and there was no difference except what the offset lnb gets (upside down it picks up a sat 8 degrees west or what it gets rightside up and 90 degrees out it gets nothing).

 

[skewed]

My 2 cents about skew. I have a world direct dish pointed at sbs6, it "aims right at my neighbors fire place stack." When I point my motor dish to sbs 6 which is mounted only a few feet away it poits towards the front of my neighbors house? Several people have verified this at various degrees of drunkeness ranging from sober to sloppy and everyone agreed it is not possible. Even on the roof the dish is not pointed at the same spot in the sky. It is almost as if the 36" is bouncing the signal off the rim. And just for FYI both dishes can get the same channels with almost the same quality.

 

[Wescopc]

A skewed dish always looks as if it is pointing to a different point in the sky than a non-skewed dish. But they are pointing exactly at the same place if the are receiving the same Satellite channels on the same bird. I have had fun showing people two dishes aimed at exactly the same birds, one a GeoSat 90cm with skewed dual LNBs the other a Star Choice with dual LNBs with the whole dish skewed, both receiving 101W and 97W.

This is why it is often harder to aim a skewed dish - it just doesn't look right!
Bob