Problem with Prime Focus 2.4M (8 Foot) and Ku Band

[enb141]

Hi I've been playing around with my Prime Focus 2.4M (8 Foot) for Ku band, so first I tested it with the WS International NS741U and I got disapointed to see that my 1.20 meter dish outperformed by far my 2.4 meter solid (petalised) prime focus dish so I got an Invacom SNF-031 thinking that the NS741U was not optimized for Prime Focus on Ku band.

So after testing with the Invacom SNF-031 I got no noticeable benefice using that LNB so I'm wondering if I'm doing something wrong or I will never get any good results with a Prime Focus on Ku band.

I also got a Golden Interstar GI 209P but I haven't tested it yet so first I wanna hear your opinions about what I'm doing wrong?

The distance between LNB and dish should be close or away from the dish?

What else can be.

Thanks.

 

[RimaNTSS]

You did nothing wrong. Combined C/Ku LNBs can not perform good in C and ku, they are pretty bad in Ku-band, that is fact. GI209p will surely perform much better, but then you lose C-band. reception in Ku band also greatly depends on quality and perfect shape of the dish you use.

 

[enb141]

Yeah I read about it and I found that combo Ku/C is junk so that's why I bought the Invacom and the Golden Intestar both for C Band but the Invacom didn't seems to get any better, in fact it was outperformed by my 1.2M offset by far so that's why I'm wondering if I did something wrong or what else can I do.

 

[RimaNTSS]

Show picture of your antenna. Perhaps, it's shape is not perfect and not good for Ku-band. You also could check whether LNB is pointing directly to the center of antenna. And also try to move LNB closer/further from dish.

 

[Titanium]

Factors that affect performance:
Reflector surface accuracy
Proper scalar match to the dish type
Feedhorn placement
Reflector illumination

My guess is that the dish panel accuracy is not very good on that dish and much of the signal is not being focused to the sweet spot at the feedhorn opening. This is part due to poor design and could be exaggerated by improper assembly not creating an accurate parabola.

The Invacom F-series is designed for offset design dishes with a .5 - .6 FD and is a mismatch to this prime focus type reflector with a FD ratio in the .3 - .4 range. This LNBF is not properly illuminating the reflector and may only be "seeing" a portion on the surface.

Is the focal distance properly placing the scalar and feedhorn at the exact reflective convergence point? Don't go by where the arms naturally place the scalar. Calculate the exact distance then make small adjustments to optimize.

Is the feedhorn exactly centered on the reflector? Once again, not placed where the support arms naturally lay.

An offset dish of identical surface area and surface area will have slight performance gains over a prime focus design. The offset design has better performance with rejecting ground noise and the feedhorn placement is not shadowing a portion of the reflector surface.

Hope this helps!



Brian Gohl
Titanium Satellite

 

[Lone Gunman]

Ya know, I hate to rain on anyone's C/Ku LNBF bashing parade here but I keep seeing these posts from time to time where ppl refer to the them as "junk" and stating they don't perform well?? While I do agree that there is likely "some" trade off with them, ie, maybe loose a little performance from each band, I've got two systems that are listed in my signature and I'd be real happy for someone to tell me of some Ku feeds that I want to watch that I can't lock with them? It's my understanding that Montana PBS on 125w is a real bitch and I've got that one on both my systems? Are there others that I can try to see if I can't lock them? And FYI, I'm in central Virginia so don't give me one with a footprint doesn't cover Virginia.

IMHO most of the problems concerning these LNBFs are either systems that aren't up to par (beat up or warped dishes) or users who aren't proficient in dish tuning, or BOTH.

And on a final note, in motor driven systems there is also a trade off as anyone who has a fixed dish already knows. When you sweep the arc from 37W to 139W you're not likely to have the same Q strength on any of these satellites that you could get if you tuned in on just one.

 

[Blue Crystal]

When I was in Florida I used A BSC621-2 C/ku LNBF on my 10 ft prime focus BUD. My motor was bad so I set it up as a fixed dish.

I was new to the hobby and I played with my BUD untill I figured out how to find sats without a signal meter, because I was too cheap to buy one. I had good advice from this site and had a ball. My C band did work better than my KU. But in the absence of a signal meter I most likely had an alignment problem.

We sold that property and moved where I could only use a 36 inch Ku dish.

Now three years later I have my Ku dish in the mountains of McDowell County, West Va where we have a missions site. I get over 600 hundred channels on 97w, however, most of the channels duplicate themselves 2 to 3 times in the channel line up

I drug my 10 ft BUD with me but I don't have room enough here to put it up.

In a few weeks I will be putting my 10 bud up near Va zip code 24502, where I spend 3.5 days a week. I did buy a C band, only, LNBF to experiment with.

To me, I prefer c band over Ku due the programing I like, such as Gal 16 @ 99w.

I will soon know!

As soon as I get my BUD up again and my C band only LNBF I will let you know my 2 cents worth as relating to C/KuLNBF's

Oh, by the way, now I would not be caught without a signal meter, even a cheap one. LOL

 

[77TA]

Prime focus dishes are not easy to use for ku band. Out of all the people who have tried I’ve only read one person say they were able to get better performance from a prime focus dish than a 1.2M offset and they only received those results after months of working on the dish.



That being said you can get decent ku reception on a prime focus dish. I use a combo c/ku lnbf and agree with lone gunman that they are not “junk” as some would try to make you believe. My 6ft Sadoun with NS741 lnbf get as good a reception on c band as anyone with a c band only lnbf on the same size dish and it gets ku reception better than a 90cm offset and on some feeds better than my 1 meter.

 

[pendragon]

I have a 2.4 m solid Birdview prime-focus with a modified dual ortho feed. Once I had the pole up, the mounting and alignment only took a few hours, and I haven't touched anything since. While there are compromises in the feed, on Ku this beats my 1.8 m offset, and wipes the floor with any of my 1.2 m offsets. After a lot of measurements on this and other dishes, I have found that even when modified the dual ortho loses about 0.5 dB in C-band gain vs. a single ortho, and about 2 dB in Ku-band gain over an adjustable f/D prime-focus feed for C120-flanged LNBs. The dual ortho Ku loss is attributable to a longish wave guide to the Ku LNB, and an erratic beam pattern caused by the inherent design limitations of a dual ortho feed. With an accurate dish and a proper feed, a prime-focus dish does great on Ku.

 

[FaT Air]

Did you use the Ku prime focus feed horn, ADF-120, on the SNF-031?

 

[enb141]

Show picture of your antenna. Perhaps, it's shape is not perfect and not good for Ku-band. You also could check whether LNB is pointing directly to the center of antenna. And also try to move LNB closer/further from dish.

I'll take some pictures and I also need to figure out how close or further from the dish it should be.

Factors that affect performance:
Reflector surface accuracy
Proper scalar match to the dish type
Feedhorn placement
Reflector illumination

My guess is that the dish panel accuracy is not very good on that dish and much of the signal is not being focused to the sweet spot at the feedhorn opening. This is part due to poor design and could be exaggerated by improper assembly not creating an accurate parabola.

The Invacom F-series is designed for offset design dishes with a .5 - .6 FD and is a mismatch to this prime focus type reflector with a FD ratio in the .3 - .4 range. This LNBF is not properly illuminating the reflector and may only be "seeing" a portion on the surface.

Is the focal distance properly placing the scalar and feedhorn at the exact reflective convergence point? Don't go by where the arms naturally place the scalar. Calculate the exact distance then make small adjustments to optimize.

Is the feedhorn exactly centered on the reflector? Once again, not placed where the support arms naturally lay.

An offset dish of identical surface area and surface area will have slight performance gains over a prime focus design. The offset design has better performance with rejecting ground noise and the feedhorn placement is not shadowing a portion of the reflector surface.

Hope this helps!



Brian Gohl
Titanium Satellite

I'll do some measures so I think the LNB Holder could be the problem and I also need to figure out what's the position between the antena and the LNB (f/D) because the Invacom and the GI 209 P doesn't shows where is it as the Ku/C band combo does.

Did you use the Ku prime focus feed horn, ADF-120, on the SNF-031?

Yes I bought the combo, the FeedHorn and the LNB.

 

[enb141]

I have a 2.4 m solid Birdview prime-focus with a modified dual ortho feed. Once I had the pole up, the mounting and alignment only took a few hours, and I haven't touched anything since. While there are compromises in the feed, on Ku this beats my 1.8 m offset, and wipes the floor with any of my 1.2 m offsets. After a lot of measurements on this and other dishes, I have found that even when modified the dual ortho loses about 0.5 dB in C-band gain vs. a single ortho, and about 2 dB in Ku-band gain over an adjustable f/D prime-focus feed for C120-flanged LNBs. The dual ortho Ku loss is attributable to a longish wave guide to the Ku LNB, and an erratic beam pattern caused by the inherent design limitations of a dual ortho feed. With an accurate dish and a proper feed, a prime-focus dish does great on Ku.

When you talk about modified ortho what do you mean?

Is your 2.4M antenna petalised?

 

[RimaNTSS]

I also need to figure out how close or further from the dish it should be.

To find out focal length, measure exact diameter and depth of your dish and use this site for calculations. http://www.satellite-calculations.com/Satellite/reflector.htm

 

[enb141]

To find out focal length, measure exact diameter and depth of your dish and use this site for calculations. http://www.satellite-calculations.com/Satellite/reflector.htm

Cool thanks for the link but now I need two figure out two values: Antenna Depth and Focal Length.

The Focal Length how is measured?

it is from the top of the LNB or from the LNB holder?


After reading a little bit more about the invacom C120 flange I've found out that the ring has the f/D settings, in other words is not just the distance between the LNB to the antena (LNB itself futher or closer to the antenna), there's also the ring distance that makes the f/D to work.

So now I need to figure out two data, the distance of the LNB and the distance of the RING.

 

[pendragon]

Did you use the Ku prime focus feed horn, ADF-120, on the SNF-031?

I did use an ADF-120 feed, but it was slightly modified to provide a wider range of adjustment of its f/D. I've tried a few quatro LNBs on this, but I've been happiest with an Invacom QTF-031 (universal Ku).

When you talk about modified ortho what do you mean?

I've made several mods to my dual ortho over the years; I documented most of these in the SatGuys forums if you want details. The mods were primarily (1) increase the range of f/D adjustment for C-band (meaningless for the Birdview), (2) switch the Ku side to a single C120 interface to use a QTF-031 LNB, and (3) experiments to improve the Ku beam pattern, most of which didn't help.

Is your 2.4M antenna petalised?

No, the Birdview is solid and appears to have a fairly accurate surface, even for Ku.

 

[FaT Air]

Focal length at C band is the distance from the dead center of dish to ~ 1/4 inch inside the feed throat.
At Ku it's around 1/16 of an inch inside the throat.
Measurements and calculations will only get you close enough to get a signal. There's too many variables for the calculations to be 'spot on'. The performance can be improved upon by 'tweaking' this distance.

 

[enb141]

I did use an ADF-120 feed, but it was slightly modified to provide a wider range of adjustment of its f/D. I've tried a few quatro LNBs on this, but I've been happiest with an Invacom QTF-031 (universal Ku). I've made several mods to my dual ortho over the years; I documented most of these in the SatGuys forums if you want details. The mods were primarily (1) increase the range of f/D adjustment for C-band (meaningless for the Birdview), (2) switch the Ku side to a single C120 interface to use a QTF-031 LNB, and (3) experiments to improve the Ku beam pattern, most of which didn't help. No, the Birdview is solid and appears to have a fairly accurate surface, even for Ku.

Now I see, a solid one piece prime focus will perform very close to an offset dish but since mine is a patelised (more than one piece) I think I'm not going to get a very good results as you did.

By the way what's the f/D of your dish?

After measured my dish I got that has this distances:

Diameter: 240 cm
Antenna Depth: 41.5 cm
Focal Length: (around 100 cm)

I'm not quite sure if that's the correct focal length but that's the distance between the center of the dish to the LNB Holder.

Now I need to figure out the f/D.

 

[pendragon]

Now I see, a solid one piece prime focus will perform very close to an offset dish but since mine is a patelised (more than one piece) I think I'm not going to get a very good results as you did. By the way what's the f/D of your dish?

I had a 1.8 m petal prime-focus for a few years. It was dreadfully flimsy, but it was a decent performer on Ku with the same dual ortho feed I now use on the 2.6 m Birdview (sorry I had those dimensions previously as 2.4 m). Even taking the 2 dB loss caused by the dual ortho, on average the 1.8 m out-performed my 1.2 m offsets. The Birdview f/D is about 0.4.

 

[enb141]

I had a 1.8 m petal prime-focus for a few years. It was dreadfully flimsy, but it was a decent performer on Ku with the same dual ortho feed I now use on the 2.6 m Birdview (sorry I had those dimensions previously as 2.4 m). Even taking the 2 dB loss caused by the dual ortho, on average the 1.8 m out-performed my 1.2 m offsets. The Birdview f/D is about 0.4.

Now I see two diferences, first mine after looking for other similar petaled prime focus I've got the conclusion that mine is f/D = 0.38 so yours is one piece and is 0.40 so I have two problems to solve, first the dish is not one piece second the f/D is too low for my invacom LNB so my only chance is to get an LNB that works on that low f/D.

By the way what LNB were you using with your 1.8 petalised prime focus? and if I'm not wrong your 1.8 is f/D about 0.38 right?

 

[FaT Air]

Diameter: 240 cm - - - Focal Length: (around 100 cm)
Now I need to figure out the f/D.

F/D is Focal length/Diameter
but your focal length measured from the bottom center of the dish to a point just inside the feed throat. (Not to the lnbf holder)
I calculate your Focal length to be 86.75mm. and the F/D at .36