True FTA and FTA on the same dish

[alex.1604]

I guys, so i have a modified dish 18inch with two circular LNB(One i use, the other one i dont), and a hh90 motor, which works amazing with all the sats that require circular lnb, but now i have installed an universal LNB beside the circular to see if i can get some true FTA sats, but the problem i have, yes i know the dish is too small for true FTA, but i heard there are some exceptions like SES 101W with an 18inch dish you could get some channels, but for me is not working, i live in Toronto, any idea of other sats i could point to? strong TP signal/strong signal? idk if my lnb isnt align properly or my setup isnt working, i have my circular lnb connected to 1 on the diseqc and universal lnb on 2, and then to the motor, and then from motor to the receiver, im only able to get my circular to work, my Universal doesnt work, anyone knows? even if the dish is too small, i should be able to get something with the strongest Sats and TPs, any suggestions?

Here are some pics:

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[Terryl]

Dump the universal and the other LNB's and go with one single LNB for both linear (true FTA satellites) and circular (for Dish and BEV) satellites.

A Invacom QPH -031 LNB would be the best for this setup.

However you will need a bigger dish, a 39" would be the best choice, as all the linear satellites show a minimum dish size of 33".


Now, where you have the universal mounted is a problem, it is out of the focal area for the satellite arc, it is too high, you wont track the arc of the Clark belt correctly.

You should remove one of the DP LNB's and mount the universal in it's place.

[ftastudent2]

Yes the Invacom works great. I have 2 setups. You need to be careful on max dish size in the GTA though as Brampton bylaws limit max size to 1 meter and other GTA areas will have similar bylaws.

[Costactc]

A 1m dish is exactly what you need and it will keep you legally ok. If you want to do both then I have to agree that an invacom quad with 2 circular and 2 linear ports is the best way to go, I used to have that setup as well. Are your settings correct for your universal lnb? 9750/10600 is what's required and just out of curiosity, what receiver are you using for true fta?

[jvvh5897]

For best results put any linear LNBs at the focal point of the dish (or as near as you can get them) and any circular (if you have separate LNBs) off-axis (ie away from center focal point). Circular polarized signals are far more forgiving in off-axis use than linear (off axis focal points have out-of-phase contributions and those are real problems for linear pol signals). The arc of where to put LNBs to get the Clark belt sats is most easily visualized if you think of the parabolic dish as a spherical reflector--the focal points are half-way between the sphere center and the surface of the dish on the line pointing to the satellite and going through the focal point as well as the sphere center (so picture an arc midway between center and surface--all LNBs have to be on that arc).

[jvvh5897]

BTW a quick calculation can give you the place to put an LNB. If the difference between the center azmuth of the dish and the one you want is "a" then the distance from the 0,0,0 point to the LNB is found from:
z = f * (1 + 0.5 * (tan(a))^2) where f is the focal length of the dish and it is on the z axis.
y = z * tan(a) where y is the distance to one side of the LNB--I typically just use f * tan(a) rather than figure z. You can do a similar equation for x = z * tan(e) where e is the elevation diff from elevation you are at to the elev you want to point to. For circular pol signals the Q readings should drop off as cos(a) (so if you are 45 degrees to one side of center you would only drop to Q of 0.7 of the center for the same sat), for linear it is not just the cosine drop that geometry requires but the phase problem too--I have not had good luck doing linear sats beyond 15 degrees or so off-axis, but have measured circular signals to 45 degree off center.

The calcs are really just an approx as the "focal" point off-axis are an arc rather than a single point--if you twist the LNB just right it will intercept most all the contributions of the dish, but not all of them will be in-phase and as you increase the angle it gets harder and harder to find a "sweet" spot. For any that might be interested I have a couple of qbasic programs to ray trace the parabola in 2D or sort of 3D by having a "viewing plane" of where rays end up from an array of points on a mathematical surface behind the viewing plane (it is easy to see the coma distortion on the viewing plane and tilt it to see how well you might correct that distortion).

An 18 inch dish at Ku band will have a beam width of about 57/18--about 3 degree or so (the dish 500 is 20.75 inches across--so 57/20.75 is about 2.75 degrees beamwidth. That dish is 22.5 inches high and f looks to be 12 inches.) Gain is a function of dish area, so if you wish to compare two dishes, just make a ratio of the square of the radii (and if you wish to know the dB diff just take ten times the log of that ratio).