Re: Request for advice: dish wobbling in the wind, alignment & signal quality


Peter Novak
 

Hello Ernst, group,
thanks a for your response. It's indeed very useful and informative.

I looked up the mechanical specs of your SAB Satellite (S120/M120) dish here:

https://www.sabsatellite.nl/upload/files/1452181362-SAB%202014%20dishes.pdf

It's 108.2 x 120.6cm in 0.9mm steel. Mast diameter possible 34 - 60mm.

Already the mast diameter span is close to unacceptable for a 120cm dish.
It's 42mm diameter, 2mm steel. From your comment I understand a thicker
one would be better. That is indeed an option.

In fact, we originally designed the setup for a 96cm dish, but that
turned out to produce too weak signal, so we replaced it later on
without changing the mast, hence the diameter.

Also the long pillar for the LNB does not look very stable for a 120cm dish.
Stabilisation of the arm seems like a good idea too.

Your problem seems to be a resonance effect with the natural
mechanical oscillation frequency of the dish. The steel material works
like a spring with little attenuation and the frequency is low because
the M120 mount is not wide enough.
Thanks. That makes sense indeed. Regarding the not enough wide mount you
mean the mast diameter, or generally construction of the dish mount at
the back of it?

I attach two images of my dish. Please note the stable construction of
the LNB support. But also note the mass of concrete plates used to
keep my homebrew tripod struts on the ground. YOUR CONSTRUCTION LOOKS
TOO LIGHTWEIGHT FOR A 120cm DISH!
Those are 2 layers of 4 concrete tiles, each 22.5kg heavy. That makes
180kg anchor weight on the flat roof stand construction. Do you think
it's not enough? I mean, another layer of tiles would make +90kg, that's
easy to do, but will it help?

2) Try to stabilize the dish with 4 aluminum stripe struts to the pole.
The downside is this makes repointing close to mission impossible.
Also it's possible that the dish can be distorted by temperature effects.
We had something like that in mind indeed. It seems this is indeed the
factor which would help most.

4) You can bring down the natural oscillation frequency by attaching left
and right a 1kg screw clamp to the brim (on your own risk and peril!).
As a quick fix that might be a good idea too. From your comments, I also
understand that I need to speak to a mechanical engineer for advice - I
am a software guy :-).

5) Buy a more professional dish and a solid/heavy 80mm mast construction.
That's indeed what I understand from your comment above. Just to
understand it clearly. It seems you think that large part of the problem
is actually the mast bending like a spring and the dish contributes only
in part. Is that correct?

Actually, another idea could be to weld a tripod structure between the
mast top and the stand below similarly as you have it on the high pole
you have there.

Also, I checked out the Gibertini dish you mentioned, but I see it's
also steel material of 1mm thickness, while the one we use is 0.9mm.
Does it make a difference? Apart from a better connected arm, could you
please explain how is that Gibertini dish actually significantly better
than the one we have? As I said, I am not a mechanical engineer, so I am
learning along the way here. To my layman eyes, these are bent metal
plates one like the other :-).

Thanks a lot for your help, I really appreciate.

Best,

Peter.

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