Primestar Feed Arm / LNB mount

[Shawn95GT]

As many people have noticed, on the Primestar feed arm just slapping a universal LNB on top of the saddle positions the LNB noticably higher than the CM feedhorn. In the 40" C-band thread it looks like some of the guys over there are having the same problem with the larger C-Band LNBs. On my dish the universal sits about 3/8" - 1/2" higher. Obviously using a C120 flanged LNB with the P* feedhorn would be the path of least resistance, but it wouldn't necessarily be the cheapest option.

I've started preparing to swap my existing P* for a new dish which is nearly identical to what it's replacing. I'm only interested is swapping them out because the new dish is in like-new condition where-as the one on the motor right now is cracked in several places. I've also managed to find a paint color that better blends with my brick wall.

I believe the new dish was actually a Starband dish. It is identical to the P* dish except for the mount. The mount on the new dish is skewable. I don't need this so I'm going to re-use the mount from the P* dish to save weight.

As luck would have it, the dish I picked up (this was the $1 ebay one) had the optional mounts for the E* LNBs. These mount very securely and look a lot more professional than my existing hose-clamped setup. Obviously 2 DSS LNBs on a motorized dish is kind of silly so I'll probably saw down the bracket so it is just for 1 extra LNB.

What I was thinking was that since the 'add-on' LNB is mounted VERY secure, maybe I could use a multi-LNB mount from it to hold the 'main' Ku LNB in place. I could saw-off the existing 'too high' saddle which would allow me to get the height perfect with a custom mount hanging from the DSS LNB.

Right now the old dish has a less-than high-tech solution of plumber's tape across the top of the LNB which seems to be doing the job just fine. If the new setup doesn't work out, I could fall back on it .

Attached are pics of the new arm and the one with the cross-hatch shows what I'm looking at cutting off.

Any comments / suggestions? Anyone else re-engineer the P* feedarm?

 

[Tron]

I wouldn't saw the DBS LNB support arm... I'd leave it just in case you decide to use the dish as a fixed dish someday. Maybe keep the second LNB off of it.

Great paint job, by the way As for the feedhorn holder, I guess those are readily available if you'd want to someday put a C-120 feedhorn on there. I would imagine the dish would be much more efficient with the separate feedhorn/LNB combo...

Someday I'll find a feedhorn for my Channel Master, get the Invacom C-120 and that'll be a really nice dish

 

[Stefan]

"I would imagine the dish would be much more efficient with the separate feedhorn/LNB combo..."

Indeed! This is especially true of the oval primestar dishes because of their unique shape and the fact that the F/D ratio of the dish is different horizontally than it is vertically. Remember the feedhorn must be optimised for the F/D ratio of the dish otherwise it either won't fully illuminate the entire dish or will "see" around the edges of the dish and pickup terrestrial interference. With an oval dish like these the F/D ratio is different in one plane than it is in the other. Therefore you need a feedhorn designed the same way. Otherwise you'll either only see a smaller circular portion of the dish or you'll pickup background noise from around the edges of the dish on it's minor axis. I think this is why alot people get different results peformance wise with these dishes. Those who try to use improperly designed feedhorns or LNBF are probably only seeing a small portion of their dish and thus they get the performance of a smaller dish as the F/D ratio of most LNBFs is closer to that of the minor axis of these dishes. On my 40 x 30 primestar, I use a C120 LNB and the original feedhorn, and I can pickup most everything on Ku that anyone else can. The only reception problems I've had with it are at the extreme western end of my arc and that's because I have trees blocking the way.

"Someday I'll find a feedhorn for my Channel Master, get the Invacom C-120 and that'll be a really nice dish"

If this is the 1.0 and 1.2M dish, you can find these feedhorns on Ebay every once in a while. Sometimes they are listed as "Andrew" feedhorns as I believe this was the company that purchased channel master. I believe you can also purchase them at Dave's web shop. There are 2 different feedhorns. There's the corrugated feedhorn and the dual mode feedhorn. As for which performs best I can't really say for certain. I do know that the corrugated feedhorn supposedly covers a wider frequency range. Of course that may not matter for FTA as I believe the addition frequency range is mainly used for data services. Anyway, even if you don't eventually pickup a feedhorn made for your dish, you'll have alot less of an issue with your dish as the F/D ratio of these dishes is around .7 (if I'm not mistaken), which is pretty close to that of most other offset dishes. So, other feedhorns or LNBFs will likely be close enough to matching your dish that you won't suffer as much of performance hit someone using one of those feeds on an oval primestar dish would.

One thing to note is that the most wonderful, miraculous LNB in the world will not make signal out of thin air. First you must catch signal with your dish. Then you must efficently transfer that signal, or as much of it as possible, to the LNB. That's what the feedhorn accomplishes. So, tuning the feedhorn to your dish is important.

 

[Shawn95GT]

Well, I've decided to re-concider the C120 flanged LNB.

When I first set up the P* it wasn't really an option since I didn't have the feedhorn for the dish. Since I picked up the nearly identical Starband dish that came complete with the feedhorn etc it is now an option .

If / when I do get a flanged LNB it'll be interesting to compare the results between the current mis-matched 'universal' LNB and a flanged LNB with the optimized feedhorn.

Stephan - can you get the Virgin Islands NBC 11991 H, SR: 2000 on AMC4 (101w)? If I can get that one I'll be impressed :shocked

Tron - I'll post up a pic of the reflector when I get it put together - it turned out PERFECT. It looks like it came out of the factory that color!

 

[Tron]

Great! Looking forward to the pics...

I wish I could find some dishes around the New Orleans area that are unused and need a good home I figured out I have room for one or two more fixed ones on my shed roof, and I'd love to have them aimed at my most watched birds...