When mounting multiple fixed LNBs for close satellites, say 4 degrees apart, does a larger dish make it so that the LNBs are mounted farther apart from each other? Also does the F/D of the dish make a difference in the spacing of the LNBs?
Spacing of LNBs vs Dish size and F/D
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Let me clarify my question. If I was to set up a new dish of any size, and I needed to hit two satellites 4° apart. Would I have more room between the centerlines of the two LNBs with a 120cm dish than with a 76cm dish?
Thanks Iceberg,
To follow up, with a fixed two LNB setup, such as SatelliteAV is coming out with, one would need to have the dish that that set-up was designed for.
Both diameter and F/D?
To follow up, with a fixed two LNB setup, such as SatelliteAV is coming out with, one would need to have the dish that that set-up was designed for.
Both diameter and F/D?
There seems to be NO WAY, to adust the spacing on the new SatelliteAV double LNB holder (designed for 2º linear Ku spacing).
If you want to adjust this pre-set spacing you would need one like below.......
http://www.dmsiusa.com/2_lnbf_bracket.htm
If you want to adjust this pre-set spacing you would need one like below.......
http://www.dmsiusa.com/2_lnbf_bracket.htm
I know 2+2=4º
So if I were to purchase a fixed LNB holder, would I need to install it on ONLY the dish size that the holder was designed for?
OR if I had a dish that was the right size but had a different F/D than the "holder" was designed for (but not the LNBs F/D) would that create a problem?
OR if I had a dish that was the right size but had a different F/D than the "holder" was designed for (but not the LNBs F/D) would that create a problem?
wescopc said:
The F/D also affects separation of the lnb's. A higher F/D (focal point further out) will increase the separation of multi lnb's. The LNB F/D must allways match the dish F/D.
Offset dishes and lnb's are almost all in the 0.6 - 0.65 F/D range.
Size of the dish (greater) will increase separation for a given F/D.
A fully adjustable lnb holder will provide preatest flexibility & success with accurate positioning (see pic with lnb's ..... well not really aligned for anything)
And lastly a larger (standard single lnb offset) dish will provide better performance for non focal point lnb's. This is a bit of a black art.
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Thanks, Wahroonga, Iceberg, and PSB.
So if I purchase a 120cm dish would I be able to pick up two satellites 4 degrees apart by using standard LNBs and an adjustable holder. Without using a special holder with the small LNBs that are required for it to get 4 degree spacing on a 90cm dish.
So if I purchase a 120cm dish would I be able to pick up two satellites 4 degrees apart by using standard LNBs and an adjustable holder. Without using a special holder with the small LNBs that are required for it to get 4 degree spacing on a 90cm dish.
Take a look at the Multifeeds section of the Global Communications website.
I wrote a paper several years ago, with a revision last summer, with tips on what works in the real world. Detailed pictures of adjustable holders are shown, primarily for use with a Primestar elliptical dish, but with a little imagination,
most offset antennas at least 90 cm in diameter have some multifeed potential.
It's just that until you get larger than one meter in size, the average LNBF physically gets in the way of anything 4 degrees apart or closer. Solution for 2 degree spacing is two dishes, with LNBFs alternated 4 degrees apart. There are severe limitations how far off center you can go on medium powered Ku-band satellites, but I have also played with 1.8 meter (6 foot offsets) and got not only 2 degree spacing, but solid signals between 91 West and 123 West. I mean extremely huge signals within 15 degrees of center. A typical STAR CHOICE Ku-band signal is often 12-13 dB carrier-to-noise here in the upper Midwest on such an antenna (2 dB needed above threshold to lock in a signal). It takes a lot of heavy rain to shut down reception. Does happen in the summer during thunderstorms, but not for long. Go to www.global-cm.net and look around.
I wrote a paper several years ago, with a revision last summer, with tips on what works in the real world. Detailed pictures of adjustable holders are shown, primarily for use with a Primestar elliptical dish, but with a little imagination,
most offset antennas at least 90 cm in diameter have some multifeed potential.
It's just that until you get larger than one meter in size, the average LNBF physically gets in the way of anything 4 degrees apart or closer. Solution for 2 degree spacing is two dishes, with LNBFs alternated 4 degrees apart. There are severe limitations how far off center you can go on medium powered Ku-band satellites, but I have also played with 1.8 meter (6 foot offsets) and got not only 2 degree spacing, but solid signals between 91 West and 123 West. I mean extremely huge signals within 15 degrees of center. A typical STAR CHOICE Ku-band signal is often 12-13 dB carrier-to-noise here in the upper Midwest on such an antenna (2 dB needed above threshold to lock in a signal). It takes a lot of heavy rain to shut down reception. Does happen in the summer during thunderstorms, but not for long. Go to www.global-cm.net and look around.
Hi Mike,
The attached pic is my interpretation of how multi lnb works on a conventional offset dish (ignore the sats ... they're overhead for us).
I notice on your web site that many of the lnb's are quite sharply angled into the dish, whereas experience here is that lnb's are more or less parallel, but mounted in a shallow 'arc' inverse to the Clarke belt.
The attached pic is my interpretation of how multi lnb works on a conventional offset dish (ignore the sats ... they're overhead for us).
I notice on your web site that many of the lnb's are quite sharply angled into the dish, whereas experience here is that lnb's are more or less parallel, but mounted in a shallow 'arc' inverse to the Clarke belt.
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I try to find the hottest focal point for each satellite, despite the laws of physics on occasion. Experience has been that you do have to follow a slight curve when using a round offset dish (such as the Paraclipse Millennium 0.6 f/d 90 cm antenna). However, many elliptical antennas, such as the 84 cm Primestar (70 x 100 cm), seem to allow a more or less straight mounting point up to about 15 inches left or right of the center of the antenna. For stability, I use 1/2 inch EMT electrical rounded and attached to the outer side of the antenna about 1 foot below center. Most LNBFs seem to allow in and out adjustment of up to 2 inches depending on how you mount the device, so placement is tweaked to wherever it works best. Rocket science may have got it up there, but bending conduit certainly it is not!
Tks again Mike,
Just applying a bit of science again, theory would say that the lnb should always be looking to 'fully illuminate' the dish, whatever the degree of offset from the centre position. This should be the case with any standard offset dish ie the lnb should be aligned to roughly look into the middle of the dish.
Again it is all about only picking up signal from space and not picking up ground noise from beyond the rim of the dish.
So I'd better re-engineer my actual set-up and my model.
The wide beam offsets have a wider focal point. So probably less critical.
And as you say. Suck it and see!
Just applying a bit of science again, theory would say that the lnb should always be looking to 'fully illuminate' the dish, whatever the degree of offset from the centre position. This should be the case with any standard offset dish ie the lnb should be aligned to roughly look into the middle of the dish.
Again it is all about only picking up signal from space and not picking up ground noise from beyond the rim of the dish.
So I'd better re-engineer my actual set-up and my model.
The wide beam offsets have a wider focal point. So probably less critical.
And as you say. Suck it and see!
I finally got some free time this evening. With Wahroonga's diagram & Mike's website and little trig - I got it! Thanks guys. This is great fun just looking at the possibilities of assembling a new fixed dish to look at two satellites 4° apart.
BTW Wahroonga the f/d of the dishes I have been looking at are .56 (90cm) & .5 (120cm).
BTW Wahroonga the f/d of the dishes I have been looking at are .56 (90cm) & .5 (120cm).
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