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Thread: T90 Geometry

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    Member Optima Collins's Avatar
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    Default T90 Geometry

    I was playing around with a mate's T90 last week just to see if that dish is right for my needs. Anyway to my surprise I picked up IS 8 and D1 on more than one position along the LNB bar and often with signal values exceeding 90% quality and strength. I'm assuming that the reflector bounces the sat signal back and forward so that it appears as multiple "peaks", particularly since I wasn't aligning the dish to any particular satellite, preferring to play it by ear. Nevertheless, one LNB, the same one on position 6 (on my right standing behind), was always pointing to D2 and the dish was skewed for a D2-C1-D1 configuration. I was simply sliding another along the arm and rescanning the box just to see what I would get. So, now I'm curious. Does anyone here know the actual geometry of the T90 and how it reflects signals or where I can find this info. I'm not talking about pointing the dish here gets you this or that, I'm talking about the actual optics of the design.

    Thanks
    OC



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    Default

    Quote Originally Posted by Optima Collins View Post
    So, now I'm curious. Does anyone here know the actual geometry of the T90 and how it reflects signals or where I can find this info. I'm not talking about pointing the dish here gets you this or that, I'm talking about the actual optics of the design.

    Thanks
    OC
    I think I read somewhere that it works like a Cassegrain telescope (only offset). Compare this to a regular dish which works like a Newtonian telescope, or an offset dish which is the same but offset!

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    Cool Toroidal principles and focusing.

    Quote Originally Posted by Optima Collins View Post
    I was playing around with a mate's T90 last week just to see if that dish is right for my needs. Anyway to my surprise I picked up IS 8 and D1 on more than one position along the LNB bar and often with signal values exceeding 90% quality and strength. I'm assuming that the reflector bounces the sat signal back and forward so that it appears as multiple "peaks", particularly since I wasn't aligning the dish to any particular satellite, preferring to play it by ear. Nevertheless, one LNB, the same one on position 6 (on my right standing behind), was always pointing to D2 and the dish was skewed for a D2-C1-D1 configuration. I was simply sliding another along the arm and rescanning the box just to see what I would get. So, now I'm curious. Does anyone here know the actual geometry of the T90 and how it reflects signals or where I can find this info. I'm not talking about pointing the dish here gets you this or that, I'm talking about the actual optics of the design.

    Thanks
    OC

    G'Day Cobber,
    When the T 90 (or T60) is correctly set up, as with any other multi-satellite dish, the elevation is set for True North. The LNBs are arranged along the bar, for the Southern Hemisphere, looking into the dish, with the Eastern most Satellite on the right and Western on the left.
    There is a double cross over of the line to the respective Satellites.
    There should only be ONE peak for each.
    Tracking of the Parabola of the Clarke Belt and relative Skew is accomplished with TILTING the dish along the azimuth.
    These may help you understand:-




    There is a plethora of information that others and I have posted on this forum. If you use the " Google search of Austech " at the top of every page, you should have no trouble finding them.

    Kindest Regards, " The Druid ".

    Last edited by beer4life; 06-11-09 at 05:59 PM.

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    Thanks beer4life. That explanation you provided confirms what I expected was the case with a normal setup. My experiment was slightly different. First of all, I skewed the dish according to the satlex calculator so that it should have been optimised for the reception of D2, C1 and D1 at positions +6, 0 and -6. I didn't expect to see IS8 at all, let alone in two places and at such high levels (like LNB A in your post). As I see it, the only explanation I have is that when I moved the left LNB it wasn't pointing at the correct azimuth. When I started moving LNB's I actually aligned the dish using D2 first instead of C1. That probably threw the alignment out a bit meaning that the signals were being bounced around between the dish and reflector like light bounces between two facing mirrors. If you look at one mirror through the other you can see an infinite array of reflections which tend to move as one mirror is skewed with repsect to the first. Perhaps that explains why I could pick up IS8 on position 19 and somewhere closer to D1 on the bar, where it shouldn't be, but I'm only guessing. The point is that had that been my dish and setup, I would have been tempted and quite happy leaving it as maladjusted as it was. The pictures were fine.

    OC
    Last edited by Optima Collins; 07-11-09 at 12:42 PM.

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    I followed Spencers setup many years ago, and at the time had everything tuned fine

    B3 is now C1

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    too interesting .. makes a bloke wanna get a toroidal
    Dm500, DM5620, DM600 x2, DM7000 x1, DM7020, DM7025, DM800, VU+DUO and a partridge in a pear tree
    All it takes for evil to prevail is for good men to do nothing

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    I found it very easy, as i said i set it up from the site above and had B3, B1, Pass2 Pass8 and i701 back in seca 1 days

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    Quote Originally Posted by Jaz808 View Post
    I found it very easy, as i said i set it up from the site above and had B3, B1, Pass2 Pass8 and i701 back in seca 1 days
    Thanks Jaz808. If I get a T90, I will certainly follow your advice and Beer's helpful post. Nevertheless, I got all of the sats from IS8 to D2, but I wasn't pointing to IS8 and got it twice. Again, my explanation might be that the LNB azimuth (not the dish) was out of alignment. Still, as nfnovice says, "it's too interesting" and half the fun is in playing with different configurations, right or wrong as they may be.

    OC

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