More on Mr. Dongras 2.4m Offset Dish Fabrication
All,
This is additional information to the post by Dongra (
https://www.jamiiforums.com/tech-ga...lite-world-and-sat-gear-542.html#post11570281) on 5[SUP]th[/SUP] January 2015. Mr. Dongra had promised to share the measurements we used. Here they are.
The Dish from Mr. Subbi came without any of its metal parts (LNB arms, neck support structure, stand, etc). Hence we had an uphill task figuring out how to fabricate these. To start with, we used the following simple formula for calculating the Focal length (i.e., length of LNB arm) as:
Focal length = (Width x Width x Width) / (16 x Depth x Height)
In the case of the off-set dish we had: Height = 270 Cm; Width = 250 Cm; Depth = 22 Cm
Therefore, Focal length = (250 x 250 x 250) / (16 x 22 x 270) = 164.4 Cm
Hence the Focal length given by Mr. Subbi of 165Cm, was fairly accurate. We also decided to go by his LNB side support arm length of 197Cm.
CALCULATING THE OFFSET ANGLE:
According to the measurements on the dish:
AB = 270 Cm = distance across dish from top to bottom (HEIGHT)
BF = 165 Cm = distance of bottom of dish to focal point [i.e., size of LNB arm]
AF =
. = distance of top of dish to focal point
P = 22 Cm= maximum perpendicular distance from the chord to the dish (or DEPTH).
The offset angle theta can be calculated using the following formula:
Angle theta = (1/2) x asin(8P/AB) = 0.5 x asin[(8 x 22) / 270] = 20.68 Degrees
This angle of approximately 21 Degrees is comparable to that of the Eurostar 130Cm which is usually between 21 and 22 degrees.
Now that we had found the Offset angle (20.68 Degrees) above;
Offset Angle = asin [(AF - BF) / AB]
Now to calculate the distance AF = distance of top of dish to focal point ......................
From the formula: Angle theta = (1/2)*asin(8P/AB) = 20.68 Degrees,
Also:
20.68 Degrees = asin [(AF - BF) / AB] = asin [(AF 165)/270
Sin20.68 = (AF 165)/270 and therefore AF = 260.35
Ideally, this should be the correct length between the top of the Dish and the tip of the LNB focal arm (of 165Cm) in order to achieve an offset angle of 20.68 Degrees.
Therefore, we went ahead to use these measurements for the fabrication of the LNB arm and its support arms.
THEN CAME THE DILEMMA !!
Once this was done; after installing the dish, we started by tracking 5E and got a very strong quality of 85% for the DW TV Transponder. Next we tried 7East. Then we realized that the LNB was not in the right focal position since it was collecting little signal from 7 East.
To overcome this, we decided to bring down the LNB arm and the two side supports and make them adjustable so that we could play around with the arms during the tracking process! Also, thw wind factor was real trouble since the 100Kg or so dish structure plus its metal was shaking at slight provocation. Note that the 3-inch pipe stand structure (anchored by twelve M-10 Roll-bolts on the concrete base) was very firm.
SHOUTS OF JOY !!
Days later, after re-fabrication of the arms and making extra back supports to act as wind break, finally, we managed to track the 7East Europe beam using Focal length of 157Cm instead of 165Cm; and LNB side support arms of 196Cm instead of 197Cm. This effectively altered the Offset Angle.
However, all apart from three TPs are streaming in stable at quality of 78%. The 3 weak ones are scratching at 40% as indicated by Mr. Dongra (11534 V 30000; 11575 V 30000; 11617 V 30000). This means that further minor adjustments can be done to arrive at the original factory Focal position. Next we added other LNBs for 7W, 5E and Amos5.
Attached below find extra photos with measurements we used. Pardon for lots of mathematics above.
View attachment 216569 View attachment 216570 View attachment 216571 View attachment 216572 View attachment 216573 View attachment 216574 View attachment 216575 View attachment 216576 View attachment 216577 View attachment 216578