OTHER Multiple LNBs? There can be only 1!

[jessica6]

During various discussions I've had, it has been emphasized several times how important it is to get that LNB in the right spot (distance from the dish, location, rotation, skew, offset, etc.). How can anyone have multiple LNBs? I've seen some crazy photos where people have a whole row of them! How can this work? Isn't there theoretically only a single point where the dish focuses where the LNB must be?

 

[comfortably_numb]

I've had 3-5 on my dish at one time.

 

[comfortably_numb]

 

[jessica6]

Case in point!
This is the one I saw: OTHER - Adding Multiple LNBs
Looks like a MacGyver meets Red Green job!

 

[mikekohl]

I wrote a paper on this about 20 years ago. Click for a review of techniques and designs.

global-cm.net

www.global-cm.net

 

[arlo]

"The angle of incidence is equal to the angle of reflection."
On a properly designed parabolic antenna a spot beam will be formed at the focal point. At precisely the focal point.
Offsetting the incident angle combined with an offset focal point makes for an oblong "image" at the focus.
Try it. I dare you. Take a magnifying bathroom mirror out in the sun with a piece of paper.
Aim it straight at the sun. Take a strip of paper and find where it is focused the sharpest.
You have a nice, round image of the sun. If the mirror is big enough you could set the paper on fire.
Now. Point the mirror off axis a little bit. Point it away from the sun a touch.
Find the focus again. The image will look oblong.

Same thing on a dish. A prime focus dish with an offset LNB like I have for Ku....still gets good signal.
The C-Band lnbf at prime focus is at its optimum placement. But it would still work if the throat of it weren't pointed dead-nuts at the dish center. To optimize for the best signal strength. It needs to be pointed right at the center so it gets signal equally from the entire dish diameter. Think of filling a dinner plate with water and tilting it to one side.
Lost H2O equals lost satellite signal.

Hence "orthomode" or combo C/Ku lnbf's. Like the Chaparral Bullseye feed.
But......
They get signal from one satellite. The dish has to be moved (obviously) to receive another sat. ¿Si?
Multiple offset lnb's can be aimed to receive several satellites. At slight loss in signal strength.
Signals bounce in at different angles. And focus on the lnb's, adhering to rule #1.

Angle of incidence = angle of reflection works with pool & billiards too. I suck at pool. Let's play for a beer.

Happy New Year!!!

 

[cyberham]

I tried offset LNBs on a fixed dish and they can work OK. But in my location, I need every bit of signal since I am at the edge of some of the satellite footprints. With an offset LNB, you have less fade margin when weather is bad. For these reasons, I prefer a motor which always places the LNB at the best location for maximum signal.

 

[a33]

"The angle of incidence is equal to the angle of reflection."

That, of course, is true for a flat mirror, or for the 'dot' mirror that you construct a (one) ray on.

For the whole of all the offset rays to a parabolic mirror, you'd have to take the 'beam deviation factor' (BDF) into account, for the finding of the center of the offset focal cloud.
This BDF seems to be about 0.9 according to some literature on the internet; so an offset mounted LNB can be about 1.1 times further from the center than calculated with the rule 'angle of incidence = angle of reflection'.
I never tested this myself, but I believe it to be true.

By the way. For the angle between satellites, the difference of 'azimuth values' is not the proper value to calculate with, though some people/calculators do that.
This can be easily seen when you are near the equator, and when you calculate the angle between a satellite just east of your due south(north), and a satellite just west of your due south(north). Though the receiving angle between these two satellites is small, the azimuth difference is near 180 degrees....!
So that angle calculation should be done differently.

Greetz,
A33