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

[polgyver]

My Ariza dish, aimed at Hispasat, did not provide good signal quality, and its settings on the back mount were way off (by 4 degrees) from normal. Some trials with small mirrors and subsequent bending of LNBF's arm solved the problem. More description on pictures.

 

[attuco]

Nice..thanks. You can also buy plastic emergency repair mirror material at auto stores.

 

[toucan-man]

At the receiver, are you seeing a difference ?

 

[FaT Air]

Q 'numbers' before and after?

 

[FTAer0024]

Great trick, thank you for sharing. Can the same technique be applied to check if the arm is off center horizontally?

 

[polgyver]

Sorry, gents, somehow I failed to take pictures of TV before-after. For pointing dishes I use small cheap Sonic receiver, and I found two pictures made in August, when I experimented with telephone wire. Sonic's Q meter is rather lowering values. My main receiver is Captive Works and it shows higher values. I think I did not gain much, probably 3 - 5 percentage points. I also replaced cheap LNBF with a good one, Geosat, and this gave me instantly 4 - 6 percent more Q.

The last picture was taken today.
I have to add, that yesterday, when got prepared for taking more pictures of "obstacles", there was a period between 5.30 PM and 6.15 PM, when I observed almost complete loss of reception. The "Q" dropped to 3 - 7 % and it wasn't any program on Hispasat. Luckily, I restrained from attempt to tinker with altitude-azimuth, and, gradually, the "Q" returned to values above 50 %. I watched on the Eastern sky - it was absolutely no rain nor thunderstorm.Still puzzled.

 

[jorgek]

Great info. Any before and after Q readings?
I have the exact dish and its approximately within 1º of the 20 mark when pointed at Hispasat.

 

[polgyver]

Great trick, thank you for sharing. Can the same technique be applied to check if the arm is off center horizontally?

Theoretically, yes, but, in my opinion, not worth bothering, since arm "deviation" is so easily visible just by eyeballing or using a straight piece of wood and carpenter's square. Dishes are symmetrical in horizontal plane. There is no symmetry in vertical plane and here the small mirrors can help in finding the least "coma" in focal point. Cheers, polgyver

 

[avenger]

How did you determined where to specifically place the mirrows on the dish to obtain the proper readings or reflection on the LNBF face...Nice doing.

 

[jorgek]

Further to my previous post, my identical dish measured from top of dish to end of LNB arm is also 33.5 inches. Ariza must not been too particular with the calculations or testing of the final design.

 

[SatPhreak]

I should check this on my Ariza 90cm dish when I get around to adjusting to get the west sats better. Some times 125W comes in, most time not. I miss the west pbs feeds. 121W also only comes in occasionally.

 

[Conky]

did the dish have to be pointed directly at the sun? how long before it drifts off target?

 

[Pixl]

Sorry, gents, somehow I failed to take pictures of TV before-after.

Polgyver,

I'm sorry, but your experiment lacks very basic scientific facts such as before and after signal measurements.
Satellite dishes focus more of a cloud to the LNB rather than a precise spot beam you are doing with light and I would question the placement accuracy of your mirrors, let alone the accuracy of your pointing at the sun at a given time. Signal, not light beams are the important factor and you don't even offer that data.

 

[FaT Air]

The perpendicular of the dish face will be design offset degrees below the sun, or satellite, or anything else you want to aim at. This is assuming the dish is mounted 'normally'. Think a street lamp a block away would suffice. (for those that like playing in the dark)
Betcha the reflections would travel across the LNBF face from one side to the other in about 1 minute.
24x60/360 So 1 minute = 4 degrees.
Did I do that right?

 

[polgyver]

How did you determined where to specifically place the mirrows on the dish to obtain the proper readings or reflection on the LNBF face...Nice doing.

The paraboloid of rotation is such a beautiful surface, that it will reflect rays from ALL of its points to one point, the focal spot - providing the rays are parallel to the axis of rotation forming the paraboloid. So, any place on the dish is good for glueing the small mirror.
Practically is good to place a few on the vertical axis, some could go close to side edges.
?Some experimenters build solar cookers by covering all dish surface with mirrors - and, probably, such cookers would still receive sat signals, when pointed to sat, until Sun outage, which would melt/burn the LNBF.

 

[polgyver]

did the dish have to be pointed directly at the sun? how long before it drifts off target?

Yes, since the reflection spots are seen on LNBF face. And they move, because Sun "moves". FaT Air calculated it right...
Also I remember your info about location of the focal point - half inch under LNBF face (you wrote about it much earlier)

 

[polgyver]

Polgyver,

I'm sorry, but your experiment lacks very basic scientific facts such as before and after signal measurements.
Satellite dishes focus more of a cloud to the LNB rather than a precise spot beam you are doing with light and I would question the placement accuracy of your mirrors, let alone the accuracy of your pointing at the sun at a given time. Signal, not light beams are the important factor and you don't even offer that data.

You are right regarding lack of signal measurements. But mirror placement can be random, the only accuracy needed is to make sure the reflective area of a mirror is tight (snug) to dish, to represent this area for light beam. I used to glue small mirrors to small disc magnets, but found out that these magnets do not have really parallel surfaces, so I switched to thin two-sided tape.
My use of this Ariza dish was enabled by discovery that its markings for elevation were way off (by 4 degrees), and I found it out by using info from Naval Observatory about Sun data and these small mirrors. I will try to get the LNBF to its previous position (not by bending back the arm, but by raising the LNBF holder) to max the signal at previous elevation (24 deg) and write down the data. Cheers, polgyver

 

[polgyver]

The perpendicular of the dish face will be design offset degrees below the sun, or satellite, or anything else you want to aim at. This is assuming the dish is mounted 'normally'. Think a street lamp a block away would suffice. (for those that like playing in the dark)
Betcha the reflections would travel across the LNBF face from one side to the other in about 1 minute.
24x60/360 So 1 minute = 4 degrees.
Did I do that right?

That's the speed; small circle of light, brought to the centre of LNB by turning the dish, drifts to the edge in half minute...
I like the lamp idea, and I memorized it from your earlier post in other thread. I have 400 W mercury lamp, the only problem is to place it far enough to approximate its light rays as parallel. Maybe I should ask one of my neighbours to let me set up the lamp at his backyard...

 

[RimaNTSS]

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.

 

[Anole]

Tell tail sign

Whenever someone reports their elevation doesn't match the numbers on their mount, I wonder:
- are they reading the mount markings correctly?
- is the arm bent (out of focus)?
?.
Unfortunately, I don't always think to state (what I consider) the obvious.
http://www.satelliteguys.us/threads/321043-1M-Channel-Master-not-working?p=3271399#post3271399
.