C / Ku reception on AMC1

[jbax]

I have an 8' prime focus mesh dish with dual C / Ku Lnbf I'd like to set up on AMC1. I want to receive the C band stuff as well as the NBC feeds on Ku. Other places I have read the Ku requires a variation on the skew. Is it possible to recieve both with the dual Lnbf? Is this a compromise situation or is there no issue? Thanks in advance.

 

[KE4EST]

No it won't work just straight out of the box, but you could modify it to work. Would require pulling off the Ku side and rotating it in the holder and then rig up a different way to have it stay. Your screws hold would no longer line up. And you would of course only want to do this on a fixed dish, well unless that is the only Ku bird you care about.

 

[jbax]

This will be a fixed dish. The Lnbf is a DMX741. I looked at it closely this morning. Think I can make a thin adaptor ring so the Ku part will screw back on securely. If SES3 ever comes online this may not be necessary. Then I could just remove the ring and put it back like it was.

 

[KE4EST]

Well SES-3 moved on to 108.2 East. It was to replace AMC1 but plans changed. Read more in this thread:

http://www.satelliteguys.us/free-air-fta-discussion/276955-ses-relocates-satellite-meet-growing-customer.html

 

[KE4EST]

This will be a fixed dish. The Lnbf is a DMX741. I looked at it closely this morning. Think I can make a thin adaptor ring so the Ku part will screw back on securely. If SES3 ever comes online this may not be necessary. Then I could just remove the ring and put it back like it was.

If you are going to make this mod I would go a step further. I have found that if I move the lnbf out of the scaler ring more about 1/4" I get Ku signals better. If I push it back in I get I get a better quality on C. I have not done it yet, but have been meaning to. I would do that and see if you have same results and if so, make marks and see how much then, instead of a real thin ring make it 1/4 to 1/2" think. I read a thread somewhere on here where a guy done this with great results. It seems that on the DMX741 the Ku side should have been design to be farther back. I hope all this made sense.

 

[jbax]

Thanks ke4est. Yes it makes perfect sense. I appreciate the info. I'll make it 1/2 inch thick so I can move it in and out to find the sweet spot for both. If necessary, I'll mark and trim to correct thickness for max performance. Will post my findings later so it may be of benefit to others.

 

[KE4EST]

Will post my findings later so it may be of benefit to others.

That would be great!

 

[jbax]

Finally got the chance to do the install yesterday of the dual lnbf for AMC1. First I checked the unmodified unit mounted on the dish with a Sathero meter. Confirmed the NBC Ku is skewed -20 deg. I could get about 70% on the C signal and 40% on the Ku. Moving the feedhorn in and out did not improve the signal. I assumed this had something to do with the relationship of the horn opening and the focal point. I fabricated an adapter ring out of 1/4 inch aluminum plate. Not as sweet of a fix as I had hoped. The -20 deg. rotation meant I had to sacrifice one of the screw tabs as the new position was where one of the coax connected. The modified unit once mounted does receive both the standard C and the skewed Ku polarity. I did not notice any discernible change in signal quality by adding the spacer. These newer dmx741 have a plastic orange cover over the Ku portion. I think the older ones were solid aluminum. Maybe they fixed the the antennae position problem internally. Either way this was a bust for me. The 40% signal quality is not enough to lock the S2 signal on my receiver. Guess I'll just have to get a solid offset dish for the NBC feeds.