Sponge as 45 degree reflector holder

polgyver

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Sep 21, 2010
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I was curious if I could check in easy way if a dish was still pointed correctly, (and its LNBF might be damaged), WITHOUT REMOVING (UNSCREWING)
THE EXISTING LNBF.
Assuming that I have another, tested LNBF and any small receiver.
Reflector (made of metallized paper) was glued to a piece of sponge which had 45 degree angle.
The sponge has a hole which fits snugly on existing LNBF.
Second LNBF was placed next to 45 degree "mirror" - and it received signal, unfortunately, much weaker, (Q = 30%).
Now, the existing LNBF cable can be unscrewed, and small testing receiver connected to existing LNBF.
If there is no program, it means the existing LNBF is damaged.
Maybe, if I used better reflector, not paper, but thin aluminum plate, the reflected signal could be better... I have to test it later.
If there is no reflected signal on added LNBF, it could indicate that the dish has moved from its original position.
Probably, moving the added LNBF around a little, might catch the program and thus indicate in which direction the dish shifted.
This will not work if the dish has moved more than a few degrees.
10 photos, some annotated, follow.
Cheers, polgyver
 

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a33

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I was curious if I could check in easy way if a dish was still pointed correctly,
Well, you indeed chose the easiest way! ;) ;)

I once read about a case where someone received satellite signal, that was reflected of a glass-faced building, so where the glass acted as a mirror.
Really fun, when this works!
It seems the reflecting surface must indeed be very 'smooth', if that is the right english word for it, so do you have a plate of polished metal lying around? That might increase signal quality even more.....

greetz,
thanks for sharing,
A33
 

polgyver

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Yes, I found an aluminum plate and attached it to the sponge holder. Also I tried it on a larger dish, as today's weather was better (yesterday's rain forced me to continue on a porch).
It seems that the reflection "costs" some losses - direct reception had Q = 62, reflected reception was at Q= 43.
And, finally, I showed the testing receiver - Freesat V8, which was more optimistic Q-vise : it showed Q=46.
The sponge is bulky and is not worth carrying in a tool box. I see some possibility of improvement : it could be made as a set of 2 rings, one for existing LNBF and other for added testing LNBF, such a set should be folded (with hinges), it could save time for unnecessary
loosening of bolts holding dish - or, removing existing LNBF.
7 photos follow.
Cheers, polgyver
thumb_IMG_5527_1024.jpg thumb_IMG_5528_1024.jpg thumb_IMG_5529_1024.jpg thumb_IMG_5530_1024.jpg thumb_IMG_5531_1024.jpg thumb_IMG_5532_1024.jpg thumb_IMG_5534_1024.jpg
 
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a33

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Mmm, I think Keith Brannen has a point there.
It's not for nothing that the cassegrain and gregorian dishes have a curved secondary mirror.
Is that paraboloid, though, or hyperboloid?
But I'm no expert on that.

Another question, inspired by the post from clucas:
Did you keep signal way from dish to LNB exactly the same? So that focal length is kept steady? It looks like the distance is now greater, but that might be an optical illusion.

However, for a quick check if dish is OK, when you have a reception problem, this is a brilliant thought. :) :)

Greetz,
A33
 

polgyver

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RAVEN transmitting dish with secondary reflector
Finally, I found this thread, posted 4 years ago.
The secondary reflector on Wildblue dish - or, other name Raven, is for sure flat.
The approximate 30% loss of Q could be due to 45 degree, maybe having something to do with half of square root of 2?
But, even with 30% loss, the signal - if present - still saves time with diagnosis regarding the dish correct position or old LNBF disfuntion.
Cheers, polgyver
 

a33

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Feb 4, 2015
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The secondary reflector on Wildblue dish - or, other name Raven, is for sure flat.
Well, that's amazing! I didn't know it existed!

I found it is also in this post (interesting thread!): Look what I dug up this weekend - Wildblue dish (you have to open the cited text, to read the post itself).

I would guess (but it really is just a guess), that the nearer the focal point you put the secondary reflector, the less important the shape of the reflector is.
Still, that thread is about gregorian, polgyver in this thread does it cassegrain.

Fascinating stuff, this! :)

Greetz,
A33
 
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