Is two-string test a reliable method in assuring that dish is not warped?

[polgyver]

It seems that two-string test, in some particular situation, can mislead the dish owner, that the dish is OK. There are 3 photos with real dish, tested with a two thin flat rubber bands,(the bands, barely touching, "testify" that dish is OK) and two straight aluminum angles, showing warping.
Other five photos show a plastic model, twisted, and two strings, at 45 degree to the axis of twist, also barely touching, despite obvious distortion of this model.

 

[FaT Air]

Yes it is a reliable way to test.
If your initial test is OK, move your strings, straight edges, 45° about the edge. If bad then, it's bad.
Also, would think since your second example isn't a satellite dish, it wouldn't apply.

 

[Wescopc]

I have seen dishes pass the string test in every direction of string placement and still be substandard. But if it passes the string test and still doesn't preform well, it is usually a bent LNB support arm.
Bob

 

[polgyver]

Yes it is a reliable way to test.
If your initial test is OK, move your strings, straight edges, 45° about the edge. If bad then, it's bad.
Also, would think since your second example isn't a satellite dish, it wouldn't apply.

That's right, with two-step testing. Somehow I was not aware of this second step.

 

[SatelliteAV]

You are correct observe that there are certain points where the two strings could be placed at 90 degrees and the reflector is twisted rather than clam shelled. This type of reflector damage is very rare. Reflectors are usually damaged in a clam shell type failure as the opposing angles are the weakest points on the edge of a parabolic cutout.

When assembling a petal C-band dish multiple strings are strung or rotated in several positions around the reflector edge to check the relationship at all points. The two string test on smaller round or oval offset dishes will typically alert to a warped dish. Ideally the strings would be rotated around to several angles or the strings would be slid along the horizontal and vertical edge to check multiple points along the reflector edge.

It should be noted that the two string test does not work on many ellipical type reflectors as they are often designed for multiple point focusing.

 

[polgyver]

Reflectors are usually damaged in a clam shell type failure as the opposing angles are the weakest points on the edge of a parabolic cutout.
/QUOTE] If I understand correctly, is "clam shell type failure" a sort of slight folding of reflector along its vertical axis of symmetry?

 

[FaT Air]

Either vertical or horizontal**. Depending on how it was 'hit'. To get a 'double fold', as your second example tries to mimic***, I can't think of how that would happen 'naturally'.
**Taco shell - folded over. ***To turn it into a bowl requires a 'form'.

 

[polgyver]

Now I think that my simplified "model" of distorted dish introduces some confusion. I used a plastic, round microwave food cover, which was perfectly round and flat (its edges). For a demo, I had to distort it somehow, so I used a shoelace twisted with q-tip. Now the model's edges resemble oval, which deviates from a flat plane. But it is not "double fold", I believe, it is rather single fold, along the axis perpendicular to the shoelace.
I knew, that if I run the yellow and green testing strings parallel and perpendicular to the shoelace, they would be far apart, thus demonstrating - obviously - distortion of the model. But my purpose was to show, that there exists specific position, 45 degree to the shoelace, where the testing strings would slightly touch each other, and, by this, mislead the test. Real dish can be distorted along its vertical, or, horizontal, or - also - any axis. If this "any" axis is close to 45 degree, then vertical - horizontal test strings will not detect distortion. So, you rightly advised to move the strings to other positions. Probably running three strings in 60 degree spacing would suffice, but maybe this would be too complicated?

 

[tdti1]

My dealer told me to run two stings and go around at least 3 spots the same as I would with a level to check for a plumb pipe, he also told me I could run 3 stings. When I helped a buddy of mine put up the dish we ran two and hit a few spots circling on the dish and it was ok.

 

[AcWxRadar]

Another method, if you have the surface to use, is to place the dish face down on a pure flat surface and check to see if there are any open spaces around the rim of the dish.
I have a large glass-top table and for dishes 1.2 M and smaller, I can check it on this table. This test checks the dish rim all the way around the perimeter at one shot and as
long as you cannot slid a credit card under the rim at any one point, you can evaluate the outer rim for clam shell warpage or variances in the dish perimeter.

Many elliptical, multi-LNBF dishes may not be tested precisely this way as they may be constructed in a manner that intentionally produces an unusual
perimeter shape since they are designed to have multiple focal points, not just ONE single point. So these dishes naturally won't lay flat on such a table.

RADAR

 

[polgyver]

RADAR,
The method with flat glass table which you described is, of course, the best method, as it will show the places where the dish could be repaired and the progress of the fixing - returning it to its original shape. The only problem is, that not everybody has such reliable flat surface to test. But it is worth finding such a flat area, when the string test shows that the reflector needs to be repaired. Cheers, polgyver

 

[Anole]

But it is worth finding such a flat area, when the string test shows that the reflector needs to be repaired.

That would be up to you.
Sometimes a new dish is the best answer.

But if you are looking for flat surfaces, test these:
- hardwood floor
- vertical garage door
- cement driveway (not level, but might be flat)
- picture window
- the bed of your pickup truck

If the dish seems warped, rotate it 60-90• to verify.

 

[AcWxRadar]

If there is a machinist in your town (not a general mechanic, but a machinist) they may have a granite bed table that you can utilize. These are usually quite large (4 feet by 6 feet is typical).

RADAR

 

[polgyver]

That would be up to you.
Sometimes a new dish is the best answer.

But if you are looking for flat surfaces, test these:
- hardwood floor
- vertical garage door
- cement driveway (not level, but might be flat)
- picture window
- the bed of your pickup truck

If the dish seems warped, rotate it 60-90• to verify.

Sorry, Anole, I had something else in my mind, when I wrote "worth finding" : I thought about looking out for occasional sales in Home Depot or other big box store, where they sometimes offer offcuts of MDF board, or better still, double sided laminated board. Plywood would not be good, for its tendency to warp. Such a flat board allows for checking a dish - even with its LNBF bracket still connected. I got a laminated board 42" x 36" which is enough for medium dishes, which I occasionally buy from Kijiji or Craigslist.
Recently I bought Wavefrontier T90 which was dropped and bent before sale, the seller tried to straighten the edge, but I decided I would do it better. I did, but still need to check it. The price was 50, but, due to lack of LNB holders and this bent, was lowered to 30 bucks. So I am looking for bigger board...
It is good to have some reference surface in the household. Machinists spend big bucks for granite blocks. But small ones - 12"x18"x3" with flatness 0.0001"
are rather affordable at Busy Bee ($35). Not useful for us, though, too small...
Cheers, polgyver

 

[polgyver]

If there is a machinist in your town (not a general mechanic, but a machinist) they may have a granite bed table that you can utilize. These are usually quite large (4 feet by 6 feet is typical).

RADAR

Well, this would be the ultimate test for my T90 ! However, I usually try to use first the simplest means to check (and, probably, MDF board would suffice). Thanks for suggestion. Cheers, polgyver