How much loss from LNB off focal point ?

[kstuart]

Hello,

I noticed that there are a number of devices, such as:

http://www.mxmart.com/info/bracketIII.htm

that attach additional LNBFs to a dish.

But all the LNBFs other than the central one are not at the designed focal point of the dish. This is different from a dish designed to use two LNBs, such as a Dish500, where both LNBs are offset from the center.

When mounting additional LNBFs, how much signal loss occurs for the LNBFs that are not in the designed focal point ?

(I'm not necessarily looking for a numerical answer...)

 

[Keith Brannen]

When mounting additional LNBFs, how much signal loss occurs for the LNBFs that are not in the designed focal point ?

(I'm not necessarily looking for a numerical answer...)

As you can see from my signature, I have multiple LNBs on all my dishes, with good success.

To give an example of some of the numbers I got in moving LNBs off-centre I found the following in my notes. At one time I had IA5 (97) at the centre and AMC3 (87) about 4 1/2" off-centre, and was not getting good signals for the PBS AC3 mux. By moving IA5 1 7/8" off-centre and AMC3 to 2 5/8" off-centre, I saw a maximum decrease in IA5 signal quality of about 15% on some of the transponders, whereas I saw a boost in signal quality of 20% and more on the AMC3 transponders. All transponders on IA5 still had very acceptable signal quality. (NOTE: I didn't move the dish at all, it's centre was still pointed at IA5.)

Of course, some satellites are better than others for being off-centre, depending on number of transponders/signal quality broadcasting FTA channels.

 

[caddata]

Keith:

I'm curious to know if you just moved the additional lnb's in an arc east or west thru the focal point or did you also tweak them for + or - elevation?

Many who add the mutiple lnb's fail to understand that the satellites change in elevation slightly east to west.

Thanks,
Harold

 

[Mr Tony]

Many who add the mutiple lnb's fail to understand that the satellites change in elevation slightly east to west.

depends on what satellites you are trying to get and how close to your true south they are. Here in MN, our true south is IA6 and from IA8-AMC1 there is minimal elevation differences (less than 1/2 a degree)

so I could do IA8/IA6/IA5 (89/93/97) without having to worry about elevation

 

[Keith Brannen]

I'm curious to know if you just moved the additional lnb's in an arc east or west thru the focal point or did you also tweak them for + or - elevation?

I always tweak to maximum signal quality using spacers (metal or wood) to adjust up or down. As well, the metal brackets I use are attached either to the top of the arm or to the bottom of the arm depending on the elevation change (and spacers are used there as well). As Iceberg notes, at the top of the arc the adjustment can be minimal, but even then I may throw in a 1/16" spacer to get that tiny bit extra! Even such a small adjustment may only help in reducing fluctuation, but is worth it!

 

[kstuart]

As you can see from my signature, I have multiple LNBs on all my dishes, with good success.

To give an example of some of the numbers I got in moving LNBs off-centre I found the following in my notes. At one time I had IA5 (97) at the centre and AMC3 (87) about 4 1/2" off-centre, and was not getting good signals for the PBS AC3 mux.

I'd be interested in what method or product you are using to mount multiple LNBs on your dishes - thanks.

 

[Keith Brannen]

I'd be interested in what method or product you are using to mount multiple LNBs on your dishes - thanks.

I have used two different types of brackets (some in modified form). The attachment shows an example of both when I had LNBs pointed at AMC3(87), IA5(97), and AMC4(101).

The big difference between the two was that the bracket on the left already had the tilt angle on the LNB holder, so the only adjustments to be made were up/down, side to side, and forward/back. The bracket on the right was harder to aim, as you also had to add the tilt angle to the LNB. As well, it was easier to make minor adjustments with the bracket on the left, whereas with the bracket on the right it was very easy to screw up the whole thing and have to start over! One advantage with the bracket on the right was that it could get closer to another LNB as the LNB holder was not as wide as the other one (hence its use for AMC4).

I have also used a LNB holder from the bracket on the left, and used a shorter and more lightweight piece of metal to mount it on. Just recently I have taken a LNB holder from the bracket on the right and made a holder for it (with the correct tilt angle) and mounted it on a piece of lightweight metal.

 

[rdel]

I use this on a 5ft dish, It used to be on a 30 wingard

http://cgi.ebay.com/Multi-Satellite...1QQihZ005QQcategoryZ11726QQrdZ1QQcmdZViewItem

 

[Keith Brannen]

I use this on a 5ft dish, It used to be on a 30 wingard

http://cgi.ebay.com/Multi-Satellite...1QQihZ005QQcategoryZ11726QQrdZ1QQcmdZViewItem

Looks to be the same one (or very similar) to part of the one I used on the right hand side of my above attachment.

What was your experience with aiming it and fine tuning? Also, was it easier to use on the 5ft dish vs. the 30in?