DMX741 C/KU LNB mod to fix drifting Ku

Magic Static · Aug 30, 2015

Well guys I am absolutely amazed at this C/Ku LNBF I built. I am going to throw away the Bullseye II with Norsat PLLs. The signal reception is amazing on three of the four polarities. whaattt?? Oh yeah the fly in the ointment

While the Ku horizontal reception blows away the Norsat, the vertical polarity is way weak

If I can get vertical to come in as good as the horizontal channels this thing is golden. And it may have to do with the bar in the c-band LNBF that we were talking about.

KE4EST · Aug 30, 2015

Be interesting to see what happens if you remove that bar, as it is for cross pol. and phasing.

. Raine · Aug 30, 2015

Magic Static said:
Well guys I am absolutely amazed at this C/Ku LNBF I built. I am going to throw away the Bullseye II with Norsat PLLs. The signal reception is amazing on three of the four polarities. whaattt?? Oh yeah the fly in the ointment While the Ku horizontal reception blows away the Norsat, the vertical polarity is way weak If I can get vertical to come in as good as the horizontal channels this thing is golden. And it may have to do with the bar in the c-band LNBF that we were talking about.

Looks like I'll probably be cutting a hole in the back of my C1WPLL. :biggrin

Maybe try skewing the PLL-1KS a bit?

Magic Static · Aug 30, 2015

KE4EST said:
Be interesting to see what happens if you remove that bar, as it is for cross pol. and phasing.

I think I will, but I wonder how much a trade off it will be. It will show up in the Weigel channels on 101W for sure

Magic Static · Aug 30, 2015

A Raine said:
Looks like I'll probably be cutting a hole in the back of my C1WPLL.

Maybe try skewing the PLL-1KS a bit?

That's a thought except the Ku LNB is permanently attached (epoxy) right now.

Magic Static · Aug 30, 2015

Lone Gunman said:
I've got a couple of those DMX741s here and also an NS741 and all of them have a bar up next to the top Cband antenna. Only difference is that on the NS741 that bar doesn't go all the way across and only sticks out about 3/4"?? The Ku ends on'em also have that bar only it's smaller and it's 90* from the Cband one.

I think my next step is to make mine which looks as you described the DMX741 and make it like the NS741, a short bar.

Magic Static · Aug 30, 2015

I removed the bar and I have reception on vertical KU. But this is a tuning nightmare. Finding a happy median is the key. I'm not having the same horizontal performance on Ku at the moment and vertical is not great but ok. I'll have to spend some time trying to find the sweet spot.

Titanium · Aug 30, 2015

The biggest issue is the signal phasing of the KU signal when it enters the KU LNBF. The bar is placed for C-band performance and should be left in place to retain C-band performance. The bar is not the issue with the reduced KU vertical performance.

Magic Static is on the right track to have removed the outer rings of the KU scalar as these are not tuned for the C-band cavity, but the opening of the KU feedhorn needs to be moved fore/aft to optimize KU phasing.

This fore/aft KU opening adjustment will be unique for each dish FD/FL. Set for optimal C-band performance then set the KU-band fore/aft for optimal performance.

This will be a balancing game as the feedhorn is designed for correct phasing of C-band frequencies at the C-band probes, not for correct phasing at a KU band feedhorn opening a few inches behind. The best explanation that I can give is that the KU LNBF will catch whatever falls out the backend of the C-band LNBF cavity....

Trust · Sep 2, 2015

I've been on that same road some years ago , attached a Inverto BlackUltra to a Panorama C lnb .
Struggeled also with that bar at receiving KU , tried to solve it with a turnable bow instead the bar (with little succes)
Even made a construction with a motordriven longer bar to move it in and out .
A greater problem for us here in Europe is the fact that there are a lot of circulair C satellites , therefore the presence of a dilectrum plate is necessary .
Another obstruction for the KU signal wich i tried to solve with a rotating depolarizer with stubs .
I give up for the C/Ku combi ( to much compromises ) and the fact that here in Europe KA band is upcoming so I made a 3 lnb changer ( C-KU-KA ) at my 120 Fibo gregorian offset

. Raine · Sep 2, 2015

Trust said:
I've been on that same road some years ago , attached a Inverto BlackUltra to a Panorama C lnb .
Struggeled also with that bar at receiving KU , tried to solve it with a turnable bow instead the bar (with little succes)
Even made a construction with a motordriven longer bar to move it in and out .
A greater problem for us here in Europe is the fact that there are a lot of circulair C satellites , therefore the presence of a dilectrum plate is necessary .
Another obstruction for the KU signal wich i tried to solve with a rotating depolarizer with stubs .
I give up for the C/Ku combi ( to much compromises ) and the fact that here in Europe KA band is upcoming so I made a 3 lnb changer ( C-KU-KA ) at my 120 Fibo gregorian offset

View attachment 109700 View attachment 109701 View attachment 109702

Wow, I like the 3LNB changer!

Titanium · Sep 2, 2015

Trust said:
I made a 3 lnb changer ( C-KU-KA ) at my 120 Fibo gregorian offset

Very Cool!

What type of motor positions the 3 LNB changer? How is is controlled and signaled?

Trust · Sep 2, 2015

Its a SG2100 with a 360º nylon gear , modified the end switches to allow to turn 180º and running at 24 V=
Controlled by a Omron PLC , a 4/1 diseqc switch is connected to the inlet ports to select the proper position .
At the moment I do have 4 positions because I use from a Directv SL3k lnb the outer KA lnb's
One port for circular signals ( Turksat 42ºE ) and the other for Lineair ( Astra 19.2º E ) with a dielectric plate in the extra feedhorn .
Because the former changer motor was controlled by that PLC , I did not tried to use the diseqc 1.2 option .

Magic Static · Sep 2, 2015

The LNBF changer is very cool idea. I am going to go down a different road with the combo C/Ku LNBF I put together and borrow some ideas from Chaparral's corotor design.

Lone Gunman · Sep 21, 2015

Here's a pic or two of that NS741U I have. Notice that the Ku part has no scaler. What I'm wondering is if Robby used something that was already in production to use with these NS models trying to avoid the additional engineering costs to make a completely new one? Anyone seen a Ku one like this before somewhere else?

And isn't that Cband part the old ESX241 that he sold? I gave one of those away with that 7.5ft SAMI dish the guy screwed me on a couple of years back.

Titanium · Sep 21, 2015

This design was in production for several years before brought into North America by WSI. It was an OEM unit.

Magic Static · May 18, 2016

Back to an old project. Old but not forgotten. It's been laying around in my den garnering my attention at times. The C/Ku LNBF that I assembled. I've been thinking about what Brian said about phasing the Ku feed horn by moving it fore and aft. I had just glued the Ku LNBF on the back of the C-Band LNBF so adjusting it wasn't possible. Well I picked it up and drilled and tapped some 3mm holes and made some shims. I can at least try the "aft" adjustment. I probably wouldn't have perused this except for the results I had from my initial try. I was getting a Ku channel from a sat that is out of footprint for me at 80% quality and I have never had that better than 36%. That has really bugged me

. Raine · May 18, 2016

Shimming made a big difference when I was messing with the one I have here, being able to skew the C and the Ku sections separately was another thing that made a really big difference, if you can work that in there somehow. Yours looks a lot neater made than mine did!

FaT Air · May 18, 2016

I had read that the fore and, really just, the aft more placement of the Ku LNBF improved the Ku performance drastically. But I was sceptical thinking it may be frequency sensitive. Peaking performance towards the lower band edge in one position, and towards the upper part of the band at a distance just a little further from the first. IIRR the conversation was quite prolific, then all of a sudden, nothing and thread went cold. Made me wonder, what's up with this? Again, IIRR, it was one of the European forums.
Many times it only takes one or two technicians to tell 20, or so, engineers why it doesn't work.
Anyway, whatever works, damn the theory, it's what works, that works. But beware: YMMV.

Magic Static · May 19, 2016

FaT Air said:
I had read that the fore and, really just, the aft more placement of the Ku LNBF improved the Ku performance drastically. But I was sceptical thinking it may be frequency sensitive. Peaking performance towards the lower band edge in one position, and towards the upper part of the band at a distance just a little further from the first. IIRR the conversation was quite prolific, then all of a sudden, nothing and thread went cold. Made me wonder, what's up with this? Again, IIRR, it was one of the European forums.
Many times it only takes one or two technicians to tell 20, or so, engineers why it doesn't work.
Anyway, whatever works, damn the theory, it's what works, that works. But beware: YMMV.

For me this ended up being sort of a polarity issue on Ku. I had tremendous signal on Horizontal transponders only and almost no Vertical signals. I fiddled a bit with the F/D ratio and depth settings but made no gains so I shelved it until now. I figured a way to make my shims so I could stack the unused ones like washers under the screws. This way the screw length doesn't change through the range of adjustment and it won't interfere with my results by protruding into the C-Band feed horn cavity. I had not considered this effecting frequency sensitivity from one end of the band to the other. So now that I'm all set to do some testing the spring rains have started. It may be a week or two before the weather is good for testing.

Lone Gunman · May 19, 2016

Magic I too am still looking into this. I finally tore the plastic housing off that Titanium PLL-1KS LNBF and cut the outer scaler ring off so it would fit into the hole on a DMX741. I too had wondered about being able to "tune" the Ku end without moving the Cband part but haven't come up with a way to mount it yet. Stay tuned though as I'm not done yet!! :oldwink

DMX741 C/KU LNB mod to fix drifting Ku

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