does the focal point on the dmx741 vary from c to k this much? (1 Viewer)

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chaskuchar

Thread Starter
SatelliteGuys Pro
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Apr 16, 2010
807
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saint charles, missouri
http://sadoun.com/Sat/Products/S/DMX7414.jpg

i was trying to get to my focal point of 29.089 inches for my lnb. i figured that the focal point was within the lnb but i did not know there was that much difference. i thought c and k would be the same depth. now if i want to position for k band, my legs are too long for the focal length. i am really confused.

width=90.25 inches
depth= 17 7/16 inches and i used 17.5 in my calculations.

charlie
 

skysurfer

SatelliteGuys Pro
Dec 1, 2006
1,737
42
the way I see it with those inexpensive combo LNBFs, you either peak for C or for Ku (not both) and you notice the pickups are in quite different places within the throat. It's a compromise in performance on both bands to try to do both from my experience. That's why I gave up on that type of combo LNBF and only use Invacoms on Ku-band and ortho feeds on c-band or my ADL or Corotor II+ feed if I want to do both on one feedhorn.
 

Lak7

SatelliteGuys Master
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Feb 28, 2008
5,451
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Near Chicago, Illinois
No need to cut / modify anything.
Make sure you understand how to align the LNBF for proper polarity.
Then set the F/D ratio of the LNBF to the Scalar Ring. The LNBF and Scalar work together as a whole.
Start with C Band, and adjust to Peak Signal, start with Focal Point - adjust LNBF & Scalar as whole unit in / out.
Tweak for Polarity.
Then switch to Ku for more Tweaking.
Keep initial expectations Low.
 

FaT Air

HOA Free Zone
Feb 27, 2010
6,668
913
97W 48N
I wanted a Ku to track along with the BUD, so:
 

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chaskuchar

Thread Starter
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Apr 16, 2010
807
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saint charles, missouri
the horizontal center of my scalar ring is at 29". the legend on the dmx741 is about '6' or real close to where the 'c' focal point is. i am certainly getting educated in physics and i was the one in high school that corrected sister mary beth when she was wrong in physics... maybe she is smiling on me now... i read the disk adjustment procedure again today. i finally see that when i adjust for southmost bird i adjust with elevation and actuator. when adjusting at the end of the travel, adjust the azimuth and move the dish... i had been using the actuator in that position also. too cold to take the tv out this time of year unless we have a heat wave. charlie
 

Pixl

Senior Member
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Feb 27, 2010
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Traverse City, Michigan
I've owned a few lnbf combos and studied the design. The input to the throat is not different than any other C-band feed and is actually a short length of circular waveguide (not to be confused with circular polarization) and delivers signal to the C-band probes. The dimension of the waveguide terribly too big for Ku and has quite a bit of loss, however a dish size suitable for C-band is also way over sized for Ku and a ton of Ku is delivered to the throat. The theory is that some of that signal will find it's way to the Ku lnb at the back end of the C-band waveguide regardless of the high loss.

My experience is that an 8ft dish with a C/Ku combo will perform almost as good as a 1m Ku dish.
My advice, peak for best performance on C-band, take what you can get for Ku. Buy a Ku dish soon.
 

stogie5150

Crazed Cajun Rebel
Jan 7, 2007
3,809
38
Slidell,LA
I did just the opposite, pixl. When I had the 741 on my Birdview, I peaked it on Ku, and took what I could get on C. I do concur about it being near to a 1m Ku dish, though. BV is spun aluminum though, it gets good Ku regardless.

I also agree with you on this: let the big dish do C-band, and get a little dish for Ku. You'll be MUCH happier than the end. :D
 

Lone Gunman

SatelliteGuys Pro
Mar 19, 2010
3,057
749
southeast
One thing that I noticed about the DMX-741S LNBF is that the little antenna wires inside the throat are sometimes NOT 90 degrees to each other. I take a long pair of needle nose pliers and screw driver and make them as close to 90 degrees as my eye can measure and I've had very good luck with performance from those LNBFs on BOTH my systems. In fact, I replaced a Corotor that had both C and Ku LNBs by Norsat with a DMX LNBF and didn't see a nickel's worth of difference between the two.

Your mileage may vary as any setup is usually only as good as the experience and patience of the installer! ;-)
 

chaskuchar

Thread Starter
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Pub Member / Supporter
Apr 16, 2010
807
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saint charles, missouri
hmmmm. so far i haven't experimented with the lnb. i did get the fiberglas pieces for conversion to circular but haven't figured out how to put it in. been real warm this christmas so might be able to play with the lnb this week. thanks for all the input. charlie
 

Lone Gunman

SatelliteGuys Pro
Mar 19, 2010
3,057
749
southeast
hmmmm. so far i haven't experimented with the lnb. i did get the fiberglas pieces for conversion to circular but haven't figured out how to put it in. been real warm this christmas so might be able to play with the lnb this week. thanks for all the input. charlie

On my DMX LNBFs there is two pairs of raised guides to hold that divider in place. Look inside the throat and you'll see what I'm talking about. I've never used mine that way but I did see how it fits inside the feed horn.
 

emuman100

SatelliteGuys Pro
Dec 15, 2007
247
0
Pennsylvania
Lone Gunman said:
One thing that I noticed about the DMX-741S LNBF is that the little antenna wires inside the throat are sometimes NOT 90 degrees to each other. I take a long pair of needle nose pliers and screw driver and make them as close to 90 degrees as my eye can measure and I've had very good luck with performance from those LNBFs on BOTH my systems. In fact, I replaced a Corotor that had both C and Ku LNBs by Norsat with a DMX LNBF and didn't see a nickel's worth of difference between the two.

Your mileage may vary as any setup is usually only as good as the experience and patience of the installer! ;-)

I used this same LNBF and on my 1.8 meter, and just like many on this forum, it was either C or Ku. It worked very well for picking up DW-TV (this was two years ago). I decided I wanted a commercial feed and LNB, so I got a Chaparral dual polarity output feed horn with a Norsat LNB for DW-TV and a cheap filler for the other polarity. I noticed a little bit of performance but not too much. I did notice rock solid stability though, was a bit more stable than the DMX-741. I just wanted C band anyway, but I still go for commercial stuff anytime.
 

stone1150

SatelliteGuys Pro
Dec 2, 2008
353
0
Northern Indiana
I have been running the DMX741 now for a couple years. I get both bands without any trouble. The C and Ku signals are maxed together. That said, I did spend about 4 months tweaking. The pix might help you. The puppy must be centered, properly skewed, and at the correct focal distance. I set mine so the focal distance was correct just inside the throat of the feed. The "0" degree mark is set at the "6" o-clock position with the dish at the highets point in the arc. I did that by simply moving the dish to the top, then marking the dish. Then I move the dish to the lowest position and setting the skew by aligning the "0" degree mark with the mark on the dish. Finally the centering was done in my case by adding guy wires to the scaler allowing me to move ever so slightly the feed into position. Once all that was done, My Pansat 9200 had 90% quality across the arc on both bands! Some of the weaker signals like the RTV mux on the AMC-9 bird only came in at 70% but many can't even get them or if they do they get only about 50% quality. Time, Patience, and Perseverance is the key as the 741 is a finiky puppy. But in my case anyways, well worth it. Have a great day!


NOTE: The skew pic shows the "0" mark at three o'clock. The reason for that is at the time I was still using my 4D receiver and for some reason for the skew to be correct for that, I had to be 90 degrees off. After unhooking the 4D, I set the skew correctly. So don't let the words on the pix throw ya off! LOL!
 

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