Ku lnbf on C band feed.
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I wouldn't expect a great result, as Ku LNBFs generally are for offset dishes and therefore are designed for a large f/D. This will seriously underscan almost all prime focus dishes.
What does work is using a Ku LNB with a C120 flange or similar, and putting a Ku feed on it that is designed for lower f/Ds. Invacom makes a number of such LNBs and also a feed that is difficult to find in stock in the US. However it can be shipped from Europe. There are other less expensive alternatives, like Superdish FSS LNBs that also use this flange.
I'm experimenting with various combinations of these on my C-band dishes. At the moment I'm favoring a Frankenstein version that couples a dual ortho feed to a modified C120 Superdish feed section and a universal Invacom quatro LNB. So far it is testing measurably better than decent Norsat Ku LNBs on the same dual ortho feed, which aren't even universal-capable. I've had the best luck on my solid dish. The mesh and perf work ok, but offer no significant advantage over my smaller solid. I think the surface irregularities have some effect, plus the solid has a larger f/D which seems to match the dual ortho better.
What does work is using a Ku LNB with a C120 flange or similar, and putting a Ku feed on it that is designed for lower f/Ds. Invacom makes a number of such LNBs and also a feed that is difficult to find in stock in the US. However it can be shipped from Europe. There are other less expensive alternatives, like Superdish FSS LNBs that also use this flange.
I'm experimenting with various combinations of these on my C-band dishes. At the moment I'm favoring a Frankenstein version that couples a dual ortho feed to a modified C120 Superdish feed section and a universal Invacom quatro LNB. So far it is testing measurably better than decent Norsat Ku LNBs on the same dual ortho feed, which aren't even universal-capable. I've had the best luck on my solid dish. The mesh and perf work ok, but offer no significant advantage over my smaller solid. I think the surface irregularities have some effect, plus the solid has a larger f/D which seems to match the dual ortho better.
I know some folks who do mini buds have the C-Band LNB offset from the KU
tvro...I assume if you mount a KU LNB next to the C-Band it works the same as any other two LNB setup
tvro...I assume if you mount a KU LNB next to the C-Band it works the same as any other two LNB setup
not necessarily... I proved that today as I used both a Prime Focus KU LNB and a offset KU LNB on a project for another member here
http://www.satelliteguys.us/1903812-post13.html
What I was thinking is mount a ku lnbf next to the ADL scaler on my C band feed. After messing with lnbf's and just waving them in front of ku dishes and getting reception it should work. With all the aperture of a 12 foot with ku mesh it has to receive something. Kind of remember the specs on my dish in Ku to be around 57 db. The whole reason Im thinking about this is to see if RTN's signal comes up considerably. 3.8 meters should be in line with what tv stations are using. I know I will have losses because of non prime focus of the feed and mesh vs solid but still should out do a smaller offset.
Next question is where to set the skew on the ku lnbf?
Next question is where to set the skew on the ku lnbf?
skew would be set to 0 as your true south (OK...technically the KU would be off by 5 degrees or so....so set skew for what the skew is for 5 degrees off your true south 
Ku-band LNB on a Mini BUD
If it works, it can be generally recommended for a restricted space like a balcony. :up
Will this method to calculate a prime focal point be accurate for an offset dish:
"The focal length of a (normal prime focus) dish can be calculated from the dia meter and depth of the dish surface.
f (focal length) = D squared divided by 16 times C or D x D / 16 x C, where D is the diameter of the dish surface and C is the depth.
Find the depth by placing a long straight stick or rod across the face of the dish and measuring down to the centre of the dish. Then do the above calculations. The f/d ratio (focal length to diameter ratio) is just that. Divide D into f from the formula and it should come out to 0.3 - 0.4 - 0.5 or whatever. The importance of this figure is in the way the feed apparatus illuminates the dish surface. The larger the f/d ratio, the further away the feed will be and the characteristics of the feed will need to be changed accordingly so as to correctly illuminate the dish with not too much "over-spray" as this can degrade the noise figure of the system as a whole. Similarly, a feed with too narrow a beam will waste some of the dish surface due to under-illumination. A feed with a lobe which is 10db down at the edge of the dish is considered to be about right. You can see how the f/d ratio will affect this figure."
Are you saying, if I mount a C-band LNB on a separate bracket in a calculated prime focal point of a 1.2 m motorized offset dish with a Ku-band LNB mounted in its offset focal point, I can get both C and Ku freq. ranges on a single dish? Will Moteck SG2100 Motor work OK in this setup? Have you seen any pictures of a similar setup? What share of C-band available in NA I can get with 1.2 m (offset) solid dish?
If it works, it can be generally recommended for a restricted space like a balcony. :up
Will this method to calculate a prime focal point be accurate for an offset dish:
"The focal length of a (normal prime focus) dish can be calculated from the dia meter and depth of the dish surface.
f (focal length) = D squared divided by 16 times C or D x D / 16 x C, where D is the diameter of the dish surface and C is the depth.
Find the depth by placing a long straight stick or rod across the face of the dish and measuring down to the centre of the dish. Then do the above calculations. The f/d ratio (focal length to diameter ratio) is just that. Divide D into f from the formula and it should come out to 0.3 - 0.4 - 0.5 or whatever. The importance of this figure is in the way the feed apparatus illuminates the dish surface. The larger the f/d ratio, the further away the feed will be and the characteristics of the feed will need to be changed accordingly so as to correctly illuminate the dish with not too much "over-spray" as this can degrade the noise figure of the system as a whole. Similarly, a feed with too narrow a beam will waste some of the dish surface due to under-illumination. A feed with a lobe which is 10db down at the edge of the dish is considered to be about right. You can see how the f/d ratio will affect this figure."
either anole or qwert15151 (or both) have it setup that way I think
(the offset C-Band LNB on a 1.2m dish)
(the offset C-Band LNB on a 1.2m dish)
I didn't say it wouldn't work, I just pointed out that it might not work 'great'.
For example, let's take a 10' prime focus and assume it has a f/D of 0.36. Now center a Ku LNBF on this that is designed for a f/D of 0.6. Because the Ku LNBF has a fairly narrow 'look' angle, it will only 'see' a 3.5' dish (1.06m). It will work, but you will have lost 4.5 dB of SNR compared to using the whole 10'. If it's a little off axis because it's sidecared to a C-band feed, you might lose another dB or so. Of course if you happen to have a very shallow BUD, your losses will be less.
Nevertheless if you want to have your full dish gain, I would recommend putting on a Ku feed with a better matched f/D.
It's one of many threads I'm waiting to write when I find some time. At the moment Frankenstein is sitting on top of the roof pointed in an inaccessible location, because the motor is disconnected while I install the rest of my switch matrices.
That project is lower priority than getting my 3.2m up because I have a building permit and want to get it closed before the city has any second thoughts. Seems as though one of the neighbors isn't pleased about my dish farm. I have to have the hole inspected tomorrow and hopefully the concrete pour will be immediately thereafter.
Not really, the formula is for a prime focus dish. An offset dish is a section from a prime focus dish, so you would need to get the dimensions of the 'virtual dish' from which the offset was cut. There are other ways to do this, but they tend to be more complicated. However if you have a Ku holder, the focal length is normally the distance from the feed to the bottom of the dish.
To illustrate Pendragon's post I have a 2.7 m D&H solid prime focus at my Cyprus (Eastern Mediteranean) house on Astra 28 using an Invacom quad and an LNBF offset for BADR6 just 2 degrees apart. Badr6 has a phenominal output but the signal quality is worse that on an adjacent 1m offset dish with cheap .3 LNB all due to the F/D of prime focus dishes and the virtual dish size used by the offset LNB.
The C-120 flange is a standard flange used for interfacing a Ku-Band (FSS or DBS) LNB to a Ku-Band feedhorn, when the LNB and feedhorn are separate parts (versus a one-piece LNBF such as the QPH-031). This is found on the DirecPC and DirecWay dishes, the Dish Network Superdishes, the DirecTV International dish, and the Primestars (Channel Master dishes), among many others.
Here is a link to the page on Invacom's site where the flanged products are pictured. Notice the prime focus feedhorn at the bottom of the page. Also notice that they sell a flanged version (C-120) of each of their LNBFs. These flanged LNBs will bolt onto the feedhorn of a Channel Master, DirecWay/DirecPC, Superdish (although I wouldn't waste such a nice LNB on such a small dish), etc.
Thanks to all, guys. Very useful info.
Can you add a close range picture of your setup?
Are you talking about expected by the LNB offset dish size, or a virtual prime focus dish size, that offset dish is a section of? Can someone give in-depth technical details and drawings on an Offset Dish LNB illumination design approach?
I don't have time for details, but I've attached a modified diagram of an analysis I did for linuxman. The magenta line is a parabola representing the cross-section of an offset dish. If you imagine the parabola extended to the right, you would have a cross-section of a prime focus dish.
Normally the top of an offset dish would be at the rim of the 'virtual' prime focus dish from which it was cut. In this case I have shown the bottom of the offset dish at the origin (0,0). As in a prime focus, the focal length is the distance from the feed to the origin. However if you pointed an offset feed at the bottom of the offset dish, most of the feed's view would not intercept the dish. Instead the feed is pointed to the center of the offset dish, as SEEN by the feed and as shown in the diagram. As is noted by the dimensions, this is not the center as measured on a string stretched from the top to the bottom of the dish. Many people seem to get confused about this. The idea is to have about the same angle to the top of the dish as to the bottom when referenced to the direction the feed points.
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Thanks pendragon!
Based on your explanation, I have a couple of questions:
Ku-Band LNBs and offset dishes are made by a number of manufacturers. Different makes have somewhat varying offset angle. Am I right that all LNB makers maintain the same f/D range for various offset dishes and Ku-LNBs? What is that range, and how dish offset angle variation affects a Ku LNB's dish illumination zone?
Also, many offset dishes are cut off on both sides, looking from Front View like a Vertical Oval. What's the physics behind it? Please answer, when you have more time.
Based on your explanation, I have a couple of questions:
Ku-Band LNBs and offset dishes are made by a number of manufacturers. Different makes have somewhat varying offset angle. Am I right that all LNB makers maintain the same f/D range for various offset dishes and Ku-LNBs? What is that range, and how dish offset angle variation affects a Ku LNB's dish illumination zone?
Also, many offset dishes are cut off on both sides, looking from Front View like a Vertical Oval. What's the physics behind it? Please answer, when you have more time.
The nominal f/D for offset dishes is around 0.6. Almost all fall within the range from 0.5 to 0.7. Ku feeds are rarely specified for their intended f/D, but I presume they work within nominal limits. Unlike prime focus dishes where the dish normally shields the earth from the feed, an offset dish primarily shields the sky. This makes overscan on offset dishes a little less critical.
Also, many offset dishes are cut off on both sides, looking from Front View like a Vertical Oval. What's the physics behind it? Please answer, when you have more time.
Typical FTA offset dishes should appear as a circle when viewed from the feed's perspective. Because there are varying distances to the edges of the dish, this results in the actual shape. Offset dishes designed for multiple satellites are wider to improve gain for off-axis feeds.
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