Polar mount stabilisation

[RimaNTSS]

Hi! I have mentioned this already couple of times but just saying sometimes is not enough. So, I decided to show how practically is possible to eliminate Polar Mount (PM) issues when it is hard for actuator to bring antenna from very edgy position. Of course everybody has heard about using of springs to help actuator or adding counterweight at the back of antenna. I would like to practically show here how to use antenna itself to work as counterweight. Will do that step by step. BTW, this method could also be applied not only to PMs but also to motors like Jaeger1224 and others and therefore remove unnecessary pressure to the motor.
First step- install antenna (in my case this is PFA, but could also be offset) on PM and observe that to move antenna from edge position we should apply some power. And if we leave antenna in the middle of the arc than it will fell to the edge position.

 

[RimaNTSS]

Forgot to tell that this antenna has no feedarms and no LNB holder, so I am going to make them

 

[Archived-1]

Hi Rimants, I am going to make something work, more for accuracy, probably from car tailgate gas struts.

 

[RimaNTSS]

Next step- take 2 pieces of metal angle (in my case it is INOX, but could also be usual metal). I took 2 pieces 110cm of 25mm angle and drilled some holes to fit them on PM.

 

[RimaNTSS]

STEP #3- attach 2 metal angles between PM and the dish. Find such a position of the dish that it goes out of edgy positions very easy. In fact, it is even possible to find such a position that PM will turn antenna to central stage without any power applied. Agree, it looks unusual and even strange, but it works.

 

[RimaNTSS]

And, yes, also made feedarms and LNB-holder. For feedarms used 10mm x 1mm x 950mm INOX pipes with welded to it's ends M6 nuts.

 

[RimaNTSS]

It is raining today Took some pictures of LNB-holder and feedarms. When rain stops have plan to catch some signals and tune PM.

 

[Captain Jack]

At least it won't rust...

 

[archive10]

STEP #3- attach 2 metal angles between PM and the dish. Find such a position of the dish that it goes out of edgy positions very easy.

Forgive me if I am being thick, but can you explain to me why this would work?

I have no mechanical engineering training or the like.
Just vague rememberings of high-school physics, and my "in-head 3D modeling" and intuition.
The latter two keep telling me that you gain little (well, nothing) from shifting the dish/weight down the polar axis.

My conclusion:

As far as I can figure it out, the amount of work (energy) required to move the dish on the axis of inclination (i.e. the axis of rotation of the polar mount) from one point to another on the arc should be the same, as it is about tipping fixed weight around the same central axis (which in principle can extend infinitely up or down).
The position on this axis the physical weight is attached only influences the load on the bearings in the points where the axis is actually mechanically hinged. Not the amount of work required to move dish.

Reasoning:

The work required to move the dish roughly speaking affects three components:

[gravity pulling the weight of the dish] + [inertia of accelerating the weight of the dish] + [mechanical friction of bearings & hinges]

Adding counter-weights to balance the dish on the axis of movement removes (as far as possible) the pure-gravity component of the resistance of the movement, whereas inertia due to mass-acceleration (of dish and counterweight) and bearing friction remains (and may actually increase due to the added weight).

The gravity induced component, as I see it, can only be lessened primarily by distributing the weight evenly (counter-weights), or secondarily by moving the actual dish face as close to the axis of inclination as mechanically possible.
Moving the dish closer to the axis of inclination is, again as I see it, why PF dishes are easier to drive than offset dishes, where the weight is further from the axis due to the necessary declination of the offset dish (due to the offset).

In fact, it is even possible to find such a position that PM will turn antenna to central stage without any power applied. Agree, it looks unusual and even strange, but it works.

Following the logic above, I find it hard to see how this can be achieved without counterweights that would not only balance the weigth of the dish, but actually weigh *more* than the dish. This would allow the whole thing to swing back into position if power was removed from system.

The ideal way to obtain either equilibrium, or even "return-to-center" behaviour would be if you moved the dish not just down, but also backwards.
If you can align the centre of gravity of the complete dish-assembly with the polar axis of the mount, then you can equilbirum. Move it even further back, and you get return-to-zero.

Mechanically, this would require you to "bend" your L-shaped beams to accomodate the dish beow or above the mechnical hinges.
It would look rather weird, though, and would required substantial bushings. I am guessing that it is (mechanically) far easier to add a counter-weight to the dish assembly.

But maybe I am wrong - after all, I've never fiddled with counterweights.
(Seems that all you need is a big enough actuator...)

I am more than happy to be proven wrong - feel free to have a go

 

[RimaNTSS]

@st1

Thanks for sharing your doubts and all the questions. I wold like to show you one picture. Why do you think this construction is stable? And even if we apply some power to one of the forks, whole construction goes to stable position afterwords. Right, that is because Center of Gravity (CG) of construction is below the turning axis. If we turn forks upside-down, CG will move upwards, construction will become unstable and fell down (that what our polar-mounted antennas are basically doing.
Just imagine that, in our case, turning axis is Polar axis and the construction (forks, cork and match) is our antenna system (dish, feedarms, LN. Perhaps it is not so easy to practically find out where antenna system has CG, but there is some for sure (I marked it as red dot #1). We see that, wherever CG #1 is located it is well above Polar axis, therefore system is unstable.
In your thoughts you forgot about one very important parameter of the PM system- that is Declination angle, and this angle for my location is almost 8*. On the drawing it is difference between Polar axis angle (blue line) and Declination from horizontal plane (green line). For sure, somewhere those lines will intersect each other. So, if we move antenna along green line, somewhere it's CG will touch blue line and even go under it (red dot #2)
As you rightly pointed out, perhaps it is also good idea to move antenna backwards. Yes, right, that will work, and will work even better (red dot #3) . But for this you need more mechanical work to be done. One day I will do that, of course.
Bigger actuator is not always good solution BTW.

 

[RimaNTSS]

So, I've played with 1m PFA Prodelin. Even it is PFA, it has 60* opening angle, that means usual LNB can be used, that is what I did. Polar-mount was desighned for offset antenna, that was an issue to solve. That was done by adding some nuts between mount and rails (red circles) anyway it was temporary solution. Got good following of arc from 45*E (further East neighbor's house does not allow) to 12,5*W and decent signals on all satellites. By now already removed 1m dish and installed 1,2m Prodelin. Actually this polar-mount came together with this dish. Got same arc with much better signals (of course it is bigger dish ). Instead of actuator I use special tool, with it I can move antenna H-H just in couple of seconds.

 

[archive10]

@st1

Thanks for sharing your doubts and all the questions. I wold like to show you one picture. View attachment 81265 Why do you think this construction is stable? And even if we apply some power to one of the forks, whole construction goes to stable position afterwords. Right, that is because Center of Gravity (CG) of construction is below the turning axis. If we turn forks upside-down, CG will move upwards, construction will become unstable and fell down (that what our polar-mounted antennas are basically doing.
Just imagine that, in our case, turning axis is Polar axis and the construction (forks, cork and match) is our antenna system (dish, feedarms, LN. Perhaps it is not so easy to practically find out where antenna system has CG, but there is some for sure (I marked it as red dot #1). We see that, wherever CG #1 is located it is well above Polar axis, therefore system is unstable.

Yes, I know.

In your thoughts you forgot about one very important parameter of the PM system- that is Declination angle, and this angle for my location is almost 8*. On the drawing it is difference between Polar axis angle (blue line) and Declination from horizontal plane (green line). For sure, somewhere those lines will intersect each other. So, if we move antenna along green line, somewhere it's CG will touch blue line and even go under it (red dot #2) View attachment 81266

Yes you are right I didn't Count this in - I expected you to put the L-brackets aligned with the polar axis. I didn't read from your post#5 that you intended to put the L-brackets directly behind dish. You wrote that you'd be using a prime focus dish - these normally have the mounting ring attached to the reflector in such a way that it is very difficult to move the dish anywhere in the plane described by the ring. I therefore took it that you would add the L-brackets in the mount on "the other side of the declination adjustment mechanism".

With the mount shown in the pictures (Prodelin/Echostar mount), you are right that you will be able to move the dish backwards slightly by moving is down. In fact you will achieve more than the 8 degrees - you will also add the around 23 degress of offset to the figure, so your angle would about 31 degrees.

But still the solution is a little impractical...

Even with 8 degrees (x), just sliding the dish would be a little impractical, as the distance (A) required to move the dish (and it's CG), say, 50 cms (O) backwards would require

A = O / tan(x) = 50 cm / 0,14054 = 356 cm

Firstly, this would require your dish being mounted very high.
Secondly the strain this would put on the hinged point (where the rotation around the polar axis is on the mount) would be quite substantial due to the levering effect of the is displaced dish. This would be most marked when in stormy conditions - your mount must be designed to be very strong.

With the offset dishes, however, this becomes more manageable, as the 31 degress would mean moving the dish 83 cms downwards to move the point 50 cms backwards. But still a bit impractical. I will still argue that a counterweight it probably a better mechanical design option that displacing the dish from the mount.

As you rightly pointed out, perhaps it is also good idea to move antenna backwards. Yes, right, that will work, and will work even better (red dot #3)View attachment 81267 . But for this you need more mechanical work to be done. One day I will do that, of course.
Bigger actuator is not always good solution BTW.

No, but it's a very widely used one - and keeps the mount mechanics simple.

I will still argue that a counterweight it probably a better mechanical design option that displacing the dish from the mount.

 

[archive10]

First step- install antenna (in my case this is PFA, but could also be offset) [...]

On different note, something else I have been wondering:
Why do you think that that is a PF reflector?
I'm reasonably certain that it is a Prodelin 198x cm offset antenna.
Used in different configurations for both Ku and Ka in professional installations.
But I couldn't find any drawings on the web to confirm if this might be the case.

Then purely by coincidence, I passed by a super-market in France yesterday, and immediately reckognised one that looked like the one you have.
I only had my phone with me, so no high-res close-ups, but I snapped a couple of pics.

As you can see from the dirt on the de-icing equipment, the ribbing in the back is extremely similar to yours.
It should also be apparent that the feed-arm configuration is off-set.

I noted in your Playground thread that you measured it at 978 x 978 mm.
Why they would make a round offset-reflector, I am not sure.
(But they have no references to PF reflector other than 2.4 meter ones.)
According to Prodelin web-site, it is a 0.8 f/D reflector.

You may want to reconsider the feed-arm design to get more mileage from the dish...

 

[RimaNTSS]

I will still argue that a counterweight it probably a better mechanical design option that displacing the dish from the mount.

Maybe maybe. That was just a try. Will do similar project after some time. My idea is to make universal polar-mount to hold any antenna and balanced in some way, counterweight or another way.

Why do you think that that is a PF reflector?

Mine is definitely PFA. I think it was designed to work in some kind of radio-link not for satellite reception.