Correcting the dish's focus by bending LNBF's arm

[jorgek]

I think there would also be a manufacturing tolerance to allow for. Having said that, my 31" and 33" dish elevation markings are within about 1º accuracy.

The other possibily can be a mast not plumb or level. But perhaps we can discount that because a seasoned installer would insure the mast is perfectly plumb.

Iinteresting though, my Ariza dish has the same measurement of 33.5" from top of dish to tip of LNB bracket, and the elevation marking is correct. Yet Polygver's dish with the same measurement is out on the elevation marking.

Am I correct in assuming as long as all the mirror spots hit the LNB plastic cover that a tight grouping is not all that important since the beams are ducted to the antenna in the bottom of the LNB housing?

 

[polgyver]

I think there would also be a manufacturing tolerance to allow for. Having said that, my 31" and 33" dish elevation markings are within about 1º accuracy.

The other possibily can be a mast not plumb or level. But perhaps we can discount that because a seasoned installer would insure the mast is perfectly plumb.

Iinteresting though, my Ariza dish has the same measurement of 33.5" from top of dish to tip of LNB bracket, and the elevation marking is correct. Yet Polygver's dish with the same measurement is out on the elevation marking.

Am I correct in assuming as long as all the mirror spots hit the LNB plastic cover that a tight grouping is not all that important since the beams are ducted to the antenna in the bottom of the LNB housing?

Probably the focus should be 1/2 " under the plastic LNBF face, not that deep as to reach the probes...
My mast was plumb, and the markings were 4 degree off.
Today's trial showed, that by bending the arm the markings were repaired, but there was some signal ("Q") loss!
The loss was fixed by repositioning the LNBF holder, but the overall gain was negligible, 1 - 2 %.
A few pictures with text follow. Cheers, polgyver

 

[jorgek]

I did the same by placing 3 small mirrors, top middle and bottom of dish surface. At the Dollar store you can get one of those 6" party balls with tiny 1/2" square mirrors glued to it. They peel off easily and can be attached to the dish surface.

I aimed the dish at the sun and got 3 spots of light focused on the plastic cap of the LNB nicely centered. At this point I don't think any further tweaking of the dish arm or LNB require adjustment.
I believe that as long as the sun is shining this test can be done any time of day or year, as long as the dish is carefully aimed directly at the sun.

 

[polgyver]

I think that proper way to find LNB's position is to use already existing programs like "Porabola calculator" or "PARABOLA". With mirrors on surface of dish possible only to check if position is right and only twice a year when Sun copies Clarke-belt and in couple of minutes time gap.
Attached are pictures of calculations for Prodelin 1,8m offset dish.

Kindly disagree. The attachments are really impressive, but most likely not feasible for average user (needed exact measurements and access to computer program which is only for Windows). Many small (even 1/4 x 1/4") mirrors, attached randomly to dish's surface, can show instantly location of focal spot, without computer or math. The idea is to watch for a coma (not very popular term...), which shows as excessive spread of reflected light spots, and we need a tight grouping of spots. Another advantage of mirrors is that they can show if the dish is - even slightly - distorted, as is the case with my Ariza, where the mirrors on the lower left quadrant reflect the light aside of focal point. The faulty area can be easily detected by momentary obscuring consecutive mirrors. No program nor math could find such a fault. And, unless you are forbidden to undo the bolts for azimuth and elevation, the optical (mirrors) testing is available all year and hour, as long as the Sun is "visible" by the dish. Cheers, polgyver