Final Update on Mr. Dongra's 2.4m Offset Dish Fabrication
All,
This is the latest (and final) additional information to the two posts made on (
African Satellite World and Sat Gear) on 5[SUP]th[/SUP] January 2015 and (
https://www.jamiiforums.com/tech-gadgets-and-science-forum/388522-african-satellite-world-and-sat-gear-544.html#post11587473 ) on 7[SUP]th[/SUP] January 2015 regarding fabrication of arms and support structure for Mr. Dongra's 2.4m offset Dish.
Two days ago, on Saturday, 3Pm – 9m, Mr. Dongra and I decided to review the arm calculations and follow what ‘mathematics' was giving us. We got pleasant results.
WHAT DOES THEORY SAY?
According to the actual measurements on the dish:
AB = 270 Cm = distance across dish from top to bottom (HEIGHT)
Width = 250Cm
P = 22 Cm= maximum perpendicular distance from the chord to the dish (or DEPTH).
BF = ? = distance of bottom of dish to focal point [i.e., size of LNB arm]
AF = ? = distance of top of dish to focal point
According to our mathematics teacher:
Focal length (LNB arm) = (Width x Width x Width) / (16 x Depth x Height)
In the case of the off-set dish we had:
Focal length = (250 x 250 x 250) / (16 x 22 x 270) = 164.4 Cm
The offset angle ‘theta' is calculated as:
Angle theta = (1/2) x asin(8P/AB) = 0.5 x asin[(8 x 22) / 270] = 20.68 Degrees
Also:
Offset Angle = 20.68 Degrees = asin [(AF - BF) / AB]
We need to calculate the distance AF = distance of top of dish to focal point:
20.68 Degrees = asin [(AF - BF) / AB] = asin [(AF – 165)/270
Making AF the subject of the formula we got: AF = 260.35Cm
PUTTING THE MATHEMATICS TO PRACTICAL USE:
Now it was time to readjust the dish to reflect the above measurements namely:
AF = 260.35 Cm;
BF = 165 Cm = Length of LNB arm
Before today, we had used: AF = 266Cm; BF = 156.5Cm
First we set loose the existing LNB support arms and LNB arm itself.
Next: we adjusted the LNB arm to 165Cm.
Next, to adjust the dish to the top length AF=260Cm correctly, we first measured and marked with masking tape, a 260Cm piece of straight Coaxial cable (you may use a rope too). We then tied one end to the top of the dish, and the other end of 260Cm to the tip of the newly-adjusted LNB arm (165Cm).
With these two lengths (AF and BF) settled, the correct length of LNB support arms to "suspend" the AF and BF lengths correctly as given above was automatic. And this adjusted to 193.5Cm !
This means that although theoretically, mathematics gives us the offset angle to be 20.68 Degrees, there is no need to measure this angle on the dish practically. Just get AF and BF and ensure the LNB arms supporting this are equal in length (193.5Cm)!
FINAL TRACKING ….. ……. And ……….. SCREAMS OF JOY !!
Once this was done at 6Pm, it was time to track again! Need to battle with the wind, fix wind breaks, tweak and tweak until the sun goes down, looking for the torch, and all that …………
…. Then the team made the mighty shout from the roof-top …….. "We can't believe this! Mr. Dongra, are you seeing what I am seeing? History channel at 78% quality! ……….."
Those familiar with tracking this beam from Nairobi will agree that the TP 11617 V 30000 holding History and the two Nat-Geo channels (and a host of music channels) is the weakest on the beam!
After making final touches, we settled for a slightly shorter LNB arm of 162Cm (slightly shorter than what science / formula required). This enabled us to squeeze more gain from the monster dish.
Once this was done, on blind-scanning the SRT4950H, we were lost in excitement! Similar to the frenzy soccer lovers experience on the pitch when their international team wins the European Premier League!!
Believe it all not, the Monster Big-Beautiful-Dish (BBD) together with the Single black Inverto Ultra LNB were downloading all the TPs on the Europe-beam [TPs listed on:
Digiturk on Eutelsat 7A at 7.0°E - LyngSat ] at 78% quality and higher!!
Now, persons living further north who use a 90Cm and get all the goodies might say "….. Aaah! …. what's there to celebrate about? ……" But for enthusiasts further south who have been ‘deprived' of some of the 7E goodies (by virtual of them using a 130Cm offset) …….. the rest of the 2015 year is concluded!
Must be the best weekend we ever hand in the year AD2015 !
Finally, between 8 – 9Pm, we concentrated on re-adjusting the other three LNBs (for 7W, 5E and Amos5) since their reception had slightly changed after adjusting the LNB arm and other arms.
Attached below find a photo with new measurements we have used.
Friends, this concludes the story, " Mr. Dongra's 2.4m Offset Dish Fabrication". Thanks for your support in every way.
Regards,
Mr. Dongra and FTA-Nash.
View attachment 219774