Fixed mount for Channel Master

[RimaNTSS]

One chap is interested in fixed mount for his Channel Master 1,2m dish. He is willing to install it for one satellite only (at least by now). And this satellite is ~ 24* out of true-South azimuth, so skew of the LNB should be more than 7*. His only request for the mount is that it should be very strong and stable. Polar mount is not an option for him (I do not understand why, maybe he thinks that PM is not stable enough). He did not ask for skew, but I think skewing of the dish gives better performance. Original CM fixed mount gives possibility to align dish on azimuth and elevation, no skew.
So, I am just thinking loud here: Is it only one member who needs stable fixed mount for CM dish or there could be other interested as well? What if we put some ideas together and generate proper way to go forward? When we finish with theoretical part and get common idea, I would try to do it practically reproduce this idea in metal.
Practical questions for the start would be:
- Is skewing of the dish is necessary at all?
- Is 25* skewing of the dish will be enough for majority of possible installations?
- What is proper diameter of the support pipe for such a mount?
Sketch of such a mount is very welcome.

 

[archive10]

Ehm - what's wrong with the original fixed pole mount?
It's quite simple, and does the job (although not as easy to adjust as the Az/El one).



Don't think they're made anymore.
But my distributor still has one left. I could send you one as a templateif you want (but it's more than EUR 15).

 

[RimaNTSS]

Thanks for the offer, but, as I've said before - original mount does not allow to skew whole antenna. Skewing of the LNB is proper thing to do but skewing of the dish should bring better performance as we know that for best performance axis of symmetry of antenna and LNB should be in transponder's polarization plane. And, again, from your words- they do not make mounts anymore, and there is only one left , so, why not create one.... better one?

 

[RimaNTSS]

Design of fixed SMW mount is crossing my mind right now. Perhaps we should not reinvent the bicycle and use it as a template? This fixed mount allows to skew whole antenna very gently depending on the range of the satellites you wish to receive.

 

[archive10]

Design of fixed SMW mount is crossing my mind right now. Perhaps we should not reinvent the bicycle and use it as a template? This fixed mount allows to skew whole antenna very gently depending on the range of the satellites you wish to receive.

I think that would be overkill for a CM120.
Perhaps something like the one for the wavefrontiers?

 

[RimaNTSS]

Perhaps something like the one for the wavefrontiers?

Yes, but maybe something even simpler?

 

[archive10]

- Is skewing of the dish is necessary at all?

Only if you want to hyper-optimise, and you've coverered all other bases such as super-optimal focal point (your turf), best LNB available, best cabling etc.
Then you go for skewing of dish, I would say.

- Is 25* skewing of the dish will be enough for majority of possible installations?

N/A

- What is proper diameter of the support pipe for such a mount?

3" is fine. 60 cm will probably be okay too. Less than that not ideal.

 

[archive10]

Yes, but maybe something even simpler?

Only simpler solution I can see is attach dish to disc-like structure with circular slots, than have normal pole mount as above attaching screws through these slots.
One disc, two side plates, one bracket. Doesn't get much simpler than that.

 

[RimaNTSS]

60 cm will probably be okay too

I would say that even 60mm will be enough

 

[RimaNTSS]

Slots are quiet difficult to make (at least for me), therefore I would go for discrete fix like SMW made, for example after each 5* of skew.

 

[archive10]

Actually, I think the Prodelin guys proved the concept with this mount:



which is taken from the installation manual for the type 1135 1.2m dish (Eutelsat approved):

 

[archive10]

Slots are quiet difficult to make (at least for me), therefore I would go for discrete fix like SMW made, for example after each 5* of skew.

Understood. But I'm sure something could be fabricated which relies on two plates and some stand-offs and a threaded rod for adjustment?
You know, much like the inclination adjustment on Gibertini dishes or even Azimut on CMs or your Prodelin 180s?

 

[RimaNTSS]

I am thinking of only one plate and proper elevation adjuster. Will leave it for thinking process for tomorrow.

 

[blademedia]

Is skewing of the dish is necessary at all?

Skewing a fixed dish ?

No need to skew a fixed dish, dish performs best full face.

 

[jeallen01]

FWIW

Most (all?) actuator/H-2-H mounts do slew the dish as they move from E to W to maximise signal pick-up.

OTOH, if a fixed dish is circular, rather than elliptical, and large enough to compensate for the signal lost caused by not skewing it then that "should probably" work. Certainly, on a much smaller dish on a sat with strong signals, skewing the dish itself is not normal, or even possible, but the LNB may be skewed if the "target" sat is not directly South of the dish (Sky LNBs on Zone 1 dishes in the UK, in particular, are a good example, and on my Raven Gemini 90 with the dish aimed at 19E then I do that for 13E & 28E, but the effect is marginal).

 

[RimaNTSS]

Skewing of prime-focus dish is not necessary at all as it is symmetrical. Skewing of the offset dish will provide better performance on inclined orbit satellites. All the polar-mounts skew the dish, some of the fixed mounts skew the dish. I will quote myself

for best performance, axis of symmetry of antenna and LNB should be in transponder's polarization plane.

, lets take it as an axiom. Of course, on the very small dish we, probably, can not feel the difference, but for bigger offset dishes it is possible.

 

[blademedia]

Skewing of prime-focus dish is not necessary at all as it is symmetrical. Skewing of the offset dish will provide better performance on inclined orbit satellites. All the polar-mounts skew the dish, some of the fixed mounts skew the dish. I will quote myself , lets take it as an axiom. Of course, on the very small dish we, probably, can not feel the difference, but for bigger offset dishes it is possible.

Your barking up the wrong tree, no need to skew a fixed dish, look at nelsons 2.4 its AZ/EL the most accurate pointing mount you can buy, what you see on bigger dishes is fine tuning mot skew ;)

Put your post on ricks satellite and see what answers you get

 

[a33]

... but skewing of the dish should bring better performance as we know that for best performance axis of symmetry of antenna and LNB should be in transponder's polarization plane.

The dish itself is just a reflector (a "mirror"), unsensitive to polarization, reflecting the signals to the focal point.
So with an LNB at the focal point, skewing of the dish has no benefits, as far as I know. Skewing of the LNB is the only thing that is needed.
Skewing of the dish is only beneficial for the byrider LNBs ("in the passenger seats") on multifeed brackets for non-prime-focus dishes, as the form of the offset parabola creates abberations for these side-LNBs that you want to minimize.

You also mentioned inclined orbit satellites. I guess that if you want to have a correction mechanism to follow these, skewing the dish makes this simpeler, because that way you can create a 'one-dimensional' correction mechanism for it.
I have no further knowledge of that, though.

greetz,
A33

 

[a33]

Thinking further along these lines:

If people rapport better signal when skewing the dish than when skewing the LNB, that would suggest to me that the LNB encounters abberations, due to probably not being exactly in the focal point.

In that case, skewing the dish could mean that you find a LNB-spot that has less abberations than with the unskewed dish with suboptimal LNB-placing.
Better solution would probably be to find the proper focal point, and place the LNB there. (That solution is something that certainly would appeal to you, @RimaNTSS ?)

I must add this is (just) a continued line of thinking; I have no practical experience or evidence to its right-ness.

Greetz,
A33

 

[archive10]

The dish itself is just a reflector (a "mirror"), unsensitive to polarization, reflecting the signals to the focal point.
So with an LNB at the focal point, skewing of the dish has no benefits, as far as I know.

I'm not entirely sure I agree with you.
AFAIU, the paraboloid reflection causes phase distortions (polarisations changes) in the reflected wavefront, meaning that the result is not optimally reflected as it would be from a flat mirror.
It is for this reason that the Gregorian dishes has a seond (or sub) reflector in the signal path before the LNB.
Apart from getting the LNB and possible transmit electronics closer to the centre of gravity, I believe that it actually corrects the phase abberations introduced by the main reflector.

What is not clear to me is if the abberations are different in an off-set configuration, where the mirror is not physcially symmetrical - only optically when seen from the offset angle. One could speculate that the larger physical surface might diffract the signal more in the top half of the dish.
Or, the wavefront is slightly phase-shifted due to the difference in physical distance between top and bottom of reflector, meaning that some signal is lost as the reflecte waves interfere.

This might just have an influence when you have H and V polarised signals, where the abberations may be kept to a minimum when the H axis is aligned with the H axis of the feed attached to the LNB.
"Well", you could say, "doesn't that mean that the distortion is greatest in the H aligned direction?"
Possibly, yes, but I am speculating this may have be a reason for the apparent mis-alignment of the LNB that RimaNTSS has documented so diligently in the past.

But then again, I am only speculating. And perhaps the effect is so small, that it is overshadowed by other factors on smaller dishes (say, <1.5m)