Checking dish's focusing with mirrors and magnets - another version

[polgyver]

This could be useful for checking used or reclaimed dishes, big or small. String test is not reliable in some cases (for example, a steelpan would test perfect, despite having obvious "dents")

The previous version which I tried, involved glueing mirrors to dish using double-sided glue tape.
This could introduce some errors. Similarly, thin magnets between mini-mirror and dish could also deflect reflected light.
Present version assures that reflecting mirror stays strictly on dish's surface. Easy to move, too.
Mini-mirrors and magnets can be bought from craft stores. Handy for experimenters.
Five pictures follow.
Cheers, polgyver

 

[Wescopc]

Cool!
Bob

 

[lamp]

I tried this and it never seemed to work. The focus point I found using the mirrors was always about 3 inches off. I think my mistake was testing indoors with a flashlight about 20 feet from the dish. It might work better if done outdoors using the sun.

I corrected the error when using satellite signals and moving the LNB for maximum signal quality.

 

[polgyver]

You are right - paraboloid of rotation needs parallel rays to reflect and focus them in one small spot. Such are the rays from Sun or from satellites.
Assuming your dish is 30", it was "seen" by the flashlight - from distance 20 feet - at an angle about 7 degrees. The Sun is seen at an angle of 1/2 degree.
If you put the flashlight in dish's focal spot, it will reflect the light as parallel rays - converging at infinity...
Were the dish in shape of Ellipsoid Of Rotation, and the flashlight placed in one of its focal spots, it could focus the flashlight rays to the other focal spot.
Similar things happen with acoustic waves. See the photo :

 

[jorgek]

Same as Lamp, I first tried it with a flashlight in the basement. That was a waste of time. Worked fine once I pointed the dish at the sun. One small issue is you have to keep moving the dish to track the sun.

 

[bmk]

Another variation that I have used, but do not currently have pictures of, is to use clear packing tape to secure small pieces of aluminum foil to the dish. The foil is not a mirror perfect surface, but it is good enough to project clearly discernible spots and to determine the focal point (or warpedness) of a dish using the sun. Also works on non ferrous dishes as well. The only disadvantage is that the taped foil pieces are more difficult to move than magnets. Rather than move them, I just add a new one in the new desired location and remove them all in one go when I am finished.

Brian

 

[polgyver]

Sorry for delay, finally today it was possible to make some pictures.
Brian's foil with glue tape is most likely the easiest way to evaluate dishes.
However, new trick helps to locate distorted area.
Annotated pictures follow.

 

[KE4EST]

As always, love reading and looking at all the pictures from your tests.

 

[Wescopc]

Great write up - getting sun in Oregon can be a challenge!
Bob

 

[jorgek]

Same as Polgyver, I found that the mirrors along the edge of the dish pan do not focus to the center of the LNB. I wonder if losing that energy is really affecting the total gain of the dish?

 

[Titanium]

Most reflectors have significantly reduced efficiency near the edges. The higher quality reflectors have tighter tolerances across the entire surface.

I remember reviews back in the C-band hey-days that claimed 20-25% efficiency gains with hydroform and spun designs. One could debate the actual performance gains, but they were noticeably higher.